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-
-
-
-Serial HOWTO
-
-
-
-----
-
-!!! Serial HOWTO
-
-!!David S.Lawyer
-
-dave@lafn.org
-original by Greg Hankins v2.16 March 2002
-
-
-----
-'' This document describes serial port features other than those which
-should be covered by Modem-HOWTO, PPP-HOWTO,
-Serial-Programming-HOWTO, or Text-Terminal-HOWTO. It does not cover
-the Universal Serial Bus (see the kernel documentation for USB).
-It lists info on multiport serial cards. It contains technical info
-about the serial port itself in more detail than found in the above
-HOWTOs and should be best for troubleshooting when the problem is
-the serial port itself. If you are dealing with a Modem, PPP (used
-for Internet access on a phone line), or a Text-Terminal, those
-HOWTOs should be consulted first. ''
-----
-
-
-
-
-!!1. Introduction
-
-
-*1.1 Copyright, Disclaimer, & Credits
-
-*1.2 New Versions of this Serial-HOWTO
-
-*1.3 Related HOWTO's re the Serial Port
-
-*1.4 Feedback
-
-*1.5 What is a Serial Port?
-
-
-
-
-
-!!2. How the Hardware Transfers Bytes
-
-
-*2.1 Transmitting
-
-*2.2 Receiving
-
-*2.3 The Large Serial Buffers
-
-
-
-
-
-!!3. Serial Port Basics
-
-
-*3.1 What is a Serial Port ?
-
-*3.2 IO Address & IRQ
-
-*3.3 Names: ttyS0, ttyS1, etc.
-
-*3.4 Interrupts
-
-*3.5 Data Flow (Speeds)
-
-*3.6 Flow Control
-
-*3.7 Data Flow Path; Buffers
-
-*3.8 Complex Flow Control Example
-
-*3.9 Serial Driver Module
-
-
-
-
-
-!!4. Is the Serial Port Obsolete?
-
-
-*4.1 Introduction
-
-*4.2 EIA-232 Cable Is Low Speed & Short Distance
-
-*4.3 Inefficient Interface to the Computer
-
-
-
-
-
-!!5. Multiport Serial Boards/Cards/Adapters
-
-
-*5.1 Intro to Multiport Serial
-
-*5.2 Modem Limitations
-
-*5.3 Dumb vs. Smart Cards
-
-*5.4 Getting/Enabling a Driver
-
-*5.5 Making multiport devices in the /dev directory
-
-*5.6 Standard PC Serial Cards
-
-*5.7 Dumb Multiport Serial Boards (with standard UART chips)
-
-*5.8 Intelligent Multiport Serial Boards
-
-*5.9 Unsupported Multiport Boards
-
-
-
-
-
-!!6. Configuring Overview
-
-
-
-
-!!7. Locating the Serial Port: IO address, IRQs
-
-
-*7.1 IO & IRQ Overview
-
-*7.2 PCI Bus Support
-
-*7.3 Common mistakes made re low-level configuring
-
-*7.4 IRQ & IO Address Must be Correct
-
-*7.5 What is the IO Address and IRQ per the driver ?
-
-*7.6 What is the IO Address & IRQ of my Serial Port Hardware?
-
-*7.7 Choosing Serial IRQs
-
-*7.8 Choosing Addresses --Video card conflict with ttyS3
-
-*7.9 Set IO Address & IRQ in the hardware (mostly for PnP)
-
-*7.10 Giving the IRQ and IO Address to Setserial
-
-
-
-
-
-!!8. Configuring the Serial Driver (high-level) "stty"
-
-
-*8.1 Overview
-
-*8.2 Flow Control
-
-
-
-
-
-!!9. Serial Port Devices /dev/ttyS2, etc.
-
-
-*9.1 Devfs (The new Device File System)
-
-*9.2 Serial Port Device Names & Numbers
-
-*9.3 USB (Universal Serial Bus) Ports
-
-*9.4 Link ttySN to /dev/modem
-
-*9.5 Which Connector on the Back of my PC is ttyS1, etc?
-
-*9.6 Creating Devices In the /dev directory
-
-
-
-
-
-!!10. Interesting Programs You Should Know About
-
-
-*10.1 Serial Monitoring/Diagnostics Programs
-
-*10.2 Changing Interrupt Priority
-
-*10.3 What is Setserial ?
-
-*10.4 Stty
-
-*10.5 What is isapnp ?
-
-*10.6 What is slattach?
-
-
-
-
-
-!!11. Speed (Flow Rate)
-
-
-*11.1 Can't Set a High Enough Speed
-
-*11.2 Higher Serial Throughput
-
-
-
-
-
-!!12. Locking Out Others
-
-
-*12.1 Introduction
-
-*12.2 Lock-Files
-
-*12.3 Lock-Files and devfs Problems
-
-*12.4 Change Owners, Groups, and/or Permissions of Device Files
-
-
-
-
-
-!!13. Communications Programs And Utilities
-
-
-*13.1 List of Software
-
-*13.2 kermit and zmodem
-
-
-
-
-
-!!14. Serial Tips And Miscellany
-
-
-*14.1 Serial Module
-
-*14.2 Serial Console (console on the serial port)
-
-*14.3 Line Drivers
-
-*14.4 Stopping the Data Flow when Printing, etc.
-
-*14.5 Known Defective Hardware
-
-
-
-
-
-!!15. Troubleshooting
-
-
-*15.1 Serial Electrical Test Equipment
-
-*15.2 Serial Monitoring/Diagnostics
-
-*15.3 (The following subsections are in both the Serial and Modem HOWTOs)
-
-*15.4 My Serial Port is Physically There but Can't be Found
-
-*15.5 Extremely Slow: Text appears on the screen slowly after long delays
-
-*15.6 Somewhat Slow: I expected it to be a few times faster
-
-*15.7 The Startup Screen Show Wrong IRQs for the Serial Ports.
-
-*15.8 "Cannot open /dev/ttyS?: Permission denied"
-
-*15.9 "Operation not supported by device" for ttyS?
-
-*15.10 "Cannot create lockfile. Sorry"
-
-*15.11 "Device /dev/ttyS? is locked."
-
-*15.12 "/dev/tty? Device or resource busy"
-
-*15.13 "Input/output error" from setserial or stty
-
-*15.14 Overrun errors on serial port
-
-*15.15 Port get characters only sporadically
-
-*15.16 Troubleshooting Tools
-
-
-
-
-
-!!16. Interrupt Problem Details
-
-
-*16.1 Types of interrupt problems
-
-*16.2 Symptoms of Mis-set or Conflicting Interrupts
-
-*16.3 Mis-set Interrupts
-
-*16.4 Interrupt Conflicts
-
-*16.5 Resolving Interrupt Problems
-
-
-
-
-
-!!17. What Are UARTs? How Do They Affect Performance?
-
-
-*17.1 Introduction to UARTS
-
-*17.2 Two Types of UARTs
-
-*17.3 FIFOs
-
-*17.4 Why FIFO Buffers are Small
-
-*17.5 UART Model Numbers
-
-
-
-
-
-!!18. Pinout and Signals
-
-
-*18.1 Pinout
-
-*18.2 Signals May Have No Fixed Meaning
-
-*18.3 Cabling Between Serial Ports
-
-*18.4 RTS/CTS and DTR/DSR Flow Control
-
-*18.5 Preventing a Port From Opening
-
-
-
-
-
-!!19. Voltage Waveshapes
-
-
-*19.1 Voltage for a Bit
-
-*19.2 Voltage Sequence for a Byte
-
-*19.3 Parity Explained
-
-*19.4 Forming a Byte (Framing)
-
-*19.5 How "Asynchronous" is Synchronized
-
-
-
-
-
-!!20. Other Serial Devices (not async EIA-232)
-
-
-*20.1 Successors to EIA-232
-
-*20.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
-
-*20.3 EIA-485
-
-*20.4 EIA-530
-
-*20.5 EIA-612/613
-
-*20.6 The Universal Serial Bus (USB)
-
-*20.7 Firewire
-
-*20.8 MIDI
-
-*20.9 Synchronization & Synchronous
-
-
-
-
-
-!!21. Other Sources of Information
-
-
-*21.1 Books
-
-*21.2 Serial Software
-
-*21.3 Related Linux Documents
-
-*21.4 Usenet newsgroups:
-
-*21.5 Serial Mailing List
-
-*21.6 Internet
-
-
-
-
-
-!!22. Appendix: Obsolete Hardware (prior to 1990) Info
-
-
-*22.1 Replacing obsolete UARTS
-
-----
-
-!!1. Introduction
-
-
- This HOWTO covers basic info on the Serial Port and multiport
-serial cards. Information specific to modems and text-terminals has
-been moved to Modem-HOWTO and Text-Terminal-HOWTO. Info on getty (the
-program that runs the login process or the like) has been also moved
-to these HOWTOs since mgetty and uugetty are best for modems while
-agetty is best for text-terminals. If you are dealing with a modem,
-text terminal, or printer, then you may not need to consult this
-HOWTO. But if you are using the serial port for some other device,
-using a multiport serial card, trouble-shooting the serial port
-itself, or want to understand more technical details of the serial
-port, then you may want to use this HOWTO as well as some of the other
-HOWTOs. (See
-Related HOWTO's) This
-HOWTO lists info on various multiport serial cards since they may be
-used for either modems or text-terminals. This HOWTO addresses Linux
-running on PCs (ISA or PCI buses), although it might be valid for
-other architectures.
-
-
-
-
-!!1.1 Copyright, Disclaimer, & Credits
-
-
-!Copyright
-
-
-Copyright (c) 1993-1997 by Greg Hankins, (c) 1998-2001 by David S.
-Lawyer
-mailto:dave@lafn.org
-
-Please freely copy and distribute (sell or give away) this document
-in any format. Send any corrections and comments to the document
-maintainer. You may create a derivative work and distribute it
-provided that you:
-
-
-
-
-
-# If it's not a translation: Email a copy of your derivative work
-(in a format LDP accepts) to the author(s) and maintainer (could be
-the same person). If you don't get a response then email the LDP
-(Linux Documentation Project): submit@linuxdoc.org.
-#
-
-#License the derivative work in the spirit of this license or use
-GPL. Include a copyright notice and at least a pointer to the
-license used.
-#
-
-#Give due credit to previous authors and major contributors.
-#
-
-
-
-If you're considering making a derived work other than a
-translation, it's requested that you discuss your plans with the
-current maintainer.
-
-
-
-
-!Disclaimer
-
-
- While I haven't intentionally tried to mislead you, there are
-likely a number of errors in this document. Please let me know about
-them. Since this is free documentation, it should be obvious that I
-cannot be held legally responsible for any errors.
-
-
-
-
-!Trademarks.
-
-
- Any brand names (starts with a capital letter) should be assumed to
-be a trademark). Such trademarks belong to their respective owners.
-
-
-
-
-
-
-
-!Credits
-
-
- Most of the original Serial-HOWTO was written by Greg Hankins.
-mailto:gregh@twoguys.org
-He also rewrote many contributions by others in order to maintain
-continuity in the writing style and flow. He wrote: ``Thanks to
-everyone who has contributed or commented, the list of people has
-gotten too long to list (somewhere over one hundred). Special thanks
-to Ted Ts'o for answering questions about the serial drivers.''
-Approximately half of v2.00 was from Greg Hankins HOWTO and the other
-half is by David Lawyer. Ted Ts'o has continued to be helpful.
-
-
-
-
-!!1.2 New Versions of this Serial-HOWTO
-
-
-
- New versions of the Serial-HOWTO will be available to
-browse and/or download at LDP mirror sites. For a list of mirror
-sites see:
-http://www.linuxdoc.org/mirrors.html.
-Various formats are available. If you only want to quickly check the
-date of the latest version look at
-http://www.linuxdoc.org/HOWTO/Serial-HOWTO.html and compare
-it to this version: v2.16 March 2002 . New in recent versions:
-v2.16 March 2002 fixed a few broken links.
-v2.15 November 2001: mention of MIDI ports, problems with lockfiles
-for devfs
-v2.14 August 2001: major revision of Configuring the Serial Port
-v2.13 August 2001: fixed typos: done->down and "is is", USRT chip,
-synchronous defined better
-v2.12 July 2001: serial printing under LPRng
-v2.11 May 2001: stty 0 => hangup (was ok in v2.08. )
-v2.10 EIA-485, frame errors on networks, gkermit, firewire
-
-
-
-
-!! 1.3 Related HOWTO's re the Serial Port
-
-
-
- Modems, Text-Terminals, some printers, and other peripherals often
-use the serial port. Get these HOWTOs from the nearest mirror site as
-explained above.
-
-
-
-
-
-*Modem-HOWTO is about installing and configuring modems
-*
-
-*Printing-HOWTO has info for serial printers using old
-lpr command
-*
-
-*LPRng-HOWTO (not a LDP HOWTO, may come with software)
-has info for serial printing for "Next Generation" lpr
-*
-
-*Serial-Programming-HOWTO helps you write
-C programs (or parts of them) that read and write to the serial port
-and/or check/set its state. A new version has been written by Vern
-Hoxie but not submitted. A copy is at
-Internet.
-*
-
-*Text-Terminal-HOWTO is about how they work, how to install
-configure, and repair them. It includes a section on "Make a
-Terminal the Console" which is useful for using a remote terminal to
-control a server (via the serial port).
-*
-
-
-
-
-
-!!1.4 Feedback
-
-
-
-Please send me any questions, comments, suggestions, or additional
-material. I'm always eager to hear about what you think about this
-HOWTO. I'm also always on the lookout for improvements! Tell me
-exactly what you don't understand, or what could be clearer. You can
-reach me via email at
-mailto:dave@lafn.org (David
-Lawyer).
-
-
-
-
-!!1.5 What is a Serial Port?
-
-
-
- The conventional serial port (not the newer USB port, or HSSI
-port) is a very old I/O port. Almost all PC's have them. But Macs
-(Apple Computer) after mid 1998 (with colored cases) only have the USB
-port. It's possible, however, to put a conventional serial port
-device on the USB.
-
-
-The common specification for the conventional serial port is RS-232
-(or EIA-232). The connector for the serial port is often seen as one
-or two 9-pin connectors (in some cases 25-pin) on the back of a PC.
-But the serial port is more than just that. It includes the
-associated electronics which must produce signals conforming to the
-EIA-232 specification. See
-Voltage Waveshapes. One pin is used to send out data bytes and another to
-receive data bytes. Another pin is a common signal ground. The other
-"useful" pins are used mainly for signalling purposes with a steady
-negative voltage meaning "off" and a steady positive voltage meaning
-"on".
-
-
-The UART (Universal Asynchronous Receiver-Transmitter) chip does most
-of the work. Today, the functionality of this chip is usually built
-into another chip. See
-What Are UARTs? These
-have improved over time and old models (several years old) are now
-obsolete.
-
-
-The serial port was originally designed for connecting modems but it's
-used to connect many other devices also such as mice, text-terminals, some
-printers, etc. to a computer. You just plug these devices into the
-serial port using the correct cable. Many internal modem cards have a
-built-in serial port so when you install one inside your PC it's as if
-you just installed another serial port in your PC.
-
-
-
-----
-
-!! 2. How the Hardware Transfers Bytes
-
-
- Below is an introduction to the topic, but for a more advanced
-treatment of it see
-FIFOs.
-
-
-
-
-!!2.1 Transmitting
-
-
-
- Transmitting is sending bytes out of the serial port away from the
-computer. Once you understand transmitting, receiving is easy to
-understand since it's similar. The first explanation given
here will
-be grossly oversimplified. Then more detail will be added in later
-explanations. When the computer wants to send a byte out the serial
-port (to the external cable) the CPU sends the byte on the bus inside
-the computer to the I/O address of the serial port. The serial port
-takes the byte, and sends it out one bit at a time (a serial
-bit-stream) on the transmit pin of the serial cable connector. For what a
-bit (and byte) look like electrically see
-Voltage Waveshapes.
-
-
-Here's a replay of the above in a little more detail (but still very
-incomplete). Most of the work at the serial port is done by the UART
-chip (or the like). To transmit a byte, the serial device driver
-program (running on the CPU) sends a byte to the serial port"s I/O
-address. This byte gets into a 1-byte "transmit shift register" in
-the serial port. From this shift register bits are taken from the
-byte one-by-one and sent out bit-by-bit on the serial line. Then when
-the last bit has been sent and the shift register needs another byte
-to send it could just ask the CPU to send it another byte. Thus would
-be simple but it would likely introduce delays since the CPU might not
-be able to get the byte immediately. After all, the CPU is usually
-doing other things besides just handling the serial port.
-
-
-A way to eliminate such delays is to arrange things so that the CPU
-gets the byte before the shift register needs it and stores it in a
-serial port buffer (in hardware). Then when the shift register has
-sent out its byte and needs a new byte immediately, the serial port
-hardware just transfers the next byte from its own buffer to the shift
-register. No need to call the CPU to fetch a new byte.
-
-
-The size of this serial port buffer was originally only one byte, but
-today it is usually 16 bytes (more in higher priced serial ports).
-Now there is still the problem of keeping this buffer sufficiently
-supplied with bytes so that when the shift register needs a byte to
-transmit it will always find one there (unless there are no more bytes
-to send). This is done by contacting the CPU using an interrupt.
-
-
-First we'll explain the case of the old fashioned one-byte buffer,
-since 16-byte buffers work similarly (but are more complex). When the
-shift register grabs the byte out of the buffer and the buffer needs
-another byte, it sends an interrupt to the CPU by putting a voltage on
-a dedicated wire on the computer bus. Unless the CPU is doing
-something very important, the interrupt forces it to stop what it was
-doing and start running a program which will supply another byte to
-the port's buffer. The purpose of this buffer is to keep an extra
-byte (waiting to be sent) queued in hardware so that there will be no
-gaps in the transmission of bytes out the serial port cable.
-
-
-Once the CPU gets the interrupt, it will know who sent the interrupt
-since there is a dedicated interrupt wire for each serial port (unless
-interrupts are shared). Then the CPU will start running the serial
-device driver which checks registers at I/0 addresses to find out what
-has happened. It finds out that the serial's transmit buffer is empty
-and waiting for another byte. So if there are more bytes to send, it
-sends the next byte to the serial port's I/0 address. This next byte
-should arrive when the previous byte is still in the transmit shift
-register and is still being transmitted bit-by-bit.
-
-
-In review, when a byte has been fully transmitted out the transmit
-wire of the serial port and the shift register is now empty the
-following 3 things happen almost simultaneously:
-
-
-
-
-
-# The next byte is moved from the transmit buffer into
-the transmit shift register
-#
-
-# The transmission of this new byte (bit-by-bit) begins
-#
-
-# Another interrupt is issued to tell the device driver to send
-yet another byte to the now empty transmit buffer
-#
-
-
-
-Thus we say that the serial port is interrupt driven. Each time the
-serial port issues an interrupt, the CPU sends it another byte. Once
-a byte has been sent to the transmit buffer by the CPU, then the CPU
-is free to pursue some other activity until it gets the next
-interrupt. The serial port transmits bits at a fixed rate which is
-selected by the user (or an application program). It's sometimes
-called the baud rate. The serial port also adds extra bits to each
-byte (start, stop and perhaps parity bits) so there are often 10 bits
-sent per byte. At a rate (also called speed) of 19,200 bits per
-second (bps), there are thus 1,920 bytes/sec (and also 1,920
-interrupts/sec).
-
-
-Doing all this is a lot of work for the CPU. This is true for many
-reasons. First, just sending one 8-bit byte at a time over a 32-bit
-data bus (or even 64-bit) is not a very efficient use of bus width.
-Also, there is a lot of overhead in handing each interrupt. When the
-interrupt is received, the device driver only knows that something
-caused an interrupt at the serial port but doesn't know that it's
-because a character has been sent. The device driver has to make
-various checks to find out what happened. The same interrupt could
-mean that a character was received, one of the control lines changed
-state, etc.
-
-
-A major improvement has been the enlargement of the buffer size of the
-serial port from 1-byte to 16-bytes. This means that when the CPU
-gets an interrupt it gives the serial port up to 16 new bytes to
-transmit. This is fewer interrupts to service but data must still be
-transferred one byte at a time over a wide bus. The 16-byte buffer is
-actually a FIFO (First In First Out) queue and is often called a FIFO.
-See
-FIFOs for details about the FIFO along
-with a repeat of some of the above info.
-
-
-
-
-!!2.2 Receiving
-
-
-
- Receiving bytes by a serial port is similar to sending them only
-it's in the opposite direction. It's also interrupt driven. For the
-obsolete type of serial port with 1-byte buffers, when a byte is fully
-received from the external cable it goes into the 1-byte receive
-buffer. Then the port gives the CPU an interrupt to tell it to pick
-up that byte so that the serial port will have room for storing the
-next byte which is currently being received. For newer serial ports
-with 16-byte buffers, this interrupt (to fetch the bytes) may be sent
-after 14 bytes are in the receive buffer. The CPU then stops what it
-was doing, runs the interrupt service routine, and picks up 14 to 16
-bytes from the port. For an interrupt sent when the 14th byte has
-been received, there could be 16 bytes to get if 2 more bytes have
-arrived since the interrupt. But if 3 more bytes should arrive
-(instead of 2), then the 16-byte buffer will overrun. It also may
-pick up less than 14 bytes by setting it that way or due to timeouts.
-See
-FIFOs for more details.
-
-
-
-
-!!2.3 The Large Serial Buffers
-
-
-
- We've talked about small 16-byte serial port hardware
-buffers but there are also much larger buffers in main memory. When
-the CPU takes some bytes out of the receive buffer of the hardware, it
-puts them into a much larger (say 8k-byte) receive buffer in main
-memory. Then a program that is getting bytes from the serial port
-takes the bytes it's receiving out of that large buffer (using a
-"read" statement in the program). A similar situation exists for
-bytes that are to be transmitted. When the CPU needs to fetch some
-bytes to be transmitted it takes them out of a large (8k-byte)
-transmit buffer in main memory and puts them into the small 16-byte
-transmit buffer in the hardware.
-
-
-
-----
-
-!! 3. Serial Port Basics
-
-
- You don't have to understand the basics to use the serial port But
-understanding it may help to determine what is wrong if you run into
-problems. This section not only presents new topics but also repeats
-some of what was said in the previous section
-How the Hardware Transfers Bytes but in greater detail.
-
-
-
-
-!!3.1 What is a Serial Port ?
-
-
-!Intro to Serial
-
-
- The serial port is an I/O (Input/Output) device.
-
-
-An I/O device is just a way to get data into and out of a computer.
-There are many types of I/O devices such as serial ports, parallel
-ports, disk drive controllers, ethernet boards, universal serial
-buses, etc.
-Most PC's have one or two serial ports. Each has a 9-pin connector
-(sometimes 25-pin) on the back of the computer. Computer programs can
-send data (bytes) to the transmit pin (output) and receive bytes from
-the receive pin (input). The other pins are for control purposes and
-ground.
-
-
-The serial port is much more than just a connector. It converts the
-data from parallel to serial and changes the electrical representation
-of the data. Inside the computer, data bits flow in parallel (using
-many wires at the same time). Serial flow is a stream of bits over a
-single wire (such as on the transmit or receive pin of the serial
-connector). For the serial port to create such a flow, it must
-convert data from parallel (inside the computer) to serial on the
-transmit pin (and conversely).
-
-
-Most of the electronics of the serial port is found in a computer chip
-(or a part of a chip) known as a UART. For more details on UARTs
-see the section
-
-
-
-What Are UARTS?
-But you may want to finish this section first so that you will
-hopefully understand how the UART fits into the overall scheme of
-things.
-
-
-
-
-!Pins and Wires
-
-
- Old PC's used 25 pin connectors but only about 9 pins were
-actually used so today most connectors are only 9-pin. Each of the 9
-pins usually connects to a wire. Besides the two wires used for
-transmitting and receiving data, another pin (wire) is signal ground.
-The voltage on any wire is measured with respect to this ground. Thus
-the minimum number of wires to use for 2-way transmission of data is
-3. Except that it has been known to work with no signal ground wire
-but with degraded performance and sometimes with errors.
-
-
-There are still more wires which are for control purposes (signalling)
-only and not for sending bytes. All of these signals could have been
-shared on a single wire, but instead, there is a separate dedicated
-wire for every type of signal. Some (or all) of these control wires
-are called "modem control lines". Modem control wires are either in
-the asserted state (on) of +12 volts or in the negated state (off) of
--12 volts. One of these wires is to signal the computer to stop
-sending bytes out the serial port cable. Conversely, another wire
-signals the device attached to the serial port to stop sending bytes
-to the computer. If the attached device is a modem, other wires may
-tell the modem to hang up the telephone line or tell the computer that
-a connection has been made or that the telephone line is ringing
-(someone is attempting to call in). See section
-Pinout and Signals for more details.
-
-
-
-
-
-
-
-
-
-
-!RS-232 or EIA-232, etc.
-
-
- The serial port (not the USB) is usually a RS-232-C, EIA-232-D, or
-EIA-232-E. These three are almost the same thing. The original RS
-(Recommended Standard) prefix became EIA (Electronics Industries
-Association) and later EIA/TIA after EIA merged with TIA
-(Telecommunications Industries Association). The EIA-232 spec
-provides also for synchronous (sync) communication but the hardware to
-support sync is almost always missing on PC's. The RS designation is
-obsolete but is still widely used. EIA will be used in this howto.
-Some documents use the full EIA/TIA designation. For info on other
-(non-EIA-232) serial ports see the section
-Other Serial Devices (not async EIA-232)
-
-
-
-!!3.2 IO Address & IRQ
-
-
-
- Since the computer needs to communicate with each serial port, the
-operating system must know that each serial port exists and where it
-is (its I/O address). It also needs to know which wire (IRQ number)
-the serial port must use to request service from the computer's CPU.
-It requests service by sending an interrupt on this wire. Thus every
-serial port device must store in its non-volatile memory both its I/O
-address and its Interrupt !ReQuest number: IRQ. See
-Interrupts. For the PCI bus it doesn't work
-exactly this way since the PCI bus has its own system of interrupts.
-But since the PCI-aware BIOS sets up chips to map these PCI interrupts
-to IRQs, it seemingly behaves just as described above except that
-sharing of interrupts is allowed (2 or more devices may use the same
-IRQ number).
-
-
-I/O addresses are not the same as memory addresses. When an I/O
-addresses is put onto the computer's address bus, another wire is
-energized. This both tells main memory to ignore the address and
-tells all devices which have I/O addresses (such as the serial port)
-to listen to the address to see if it matches the device's. If the
-address matches, then the I/O device reads the data on the data bus.
-
-
-
-
-!!3.3 Names: ttyS0, ttyS1, etc.
-
-
-
- The serial ports are named ttyS0, ttyS1, etc. (and usually
-correspond respectively to COM1, COM2, etc. in DOS/Windows). The /dev
-directory has a special file for each port. Type "ls /dev/ttyS*" to
-see them. Just because there may be (for example) a ttyS3 file,
-doesn't necessarily mean that there exists a physical serial port
-there.
-
-
-Which one of these names (ttyS0, ttyS1, etc.) refers to which
-physical serial port is determined as follows. The serial driver
-(software) maintains a table showing which I/O address corresponds to
-which ttyS. This mapping of names (such as ttyS1) to I/O addresses
-(and IRQ's) may be both set and viewed by the "setserial" command.
-See
-What is Setserial. This does
-not set the I/O address and IRQ in the hardware itself (which is
-set by jumpers or by plug-and-play software). Thus what physical port
-corresponds to say ttyS1 depends both on what the serial driver thinks
-(per setserial) and what is set in the hardware. If a mistake has
-been made, the physical port may not correspond to any name (such as
-ttyS2) and thus it can't be used. See
-Serial Port Devices /dev/ttyS2, etc. for more details>
-
-
-
-
-!! 3.4 Interrupts
-
-
-
-
-
-
-When the serial port receives a number of bytes (may be set to 1, 4,
-8, or 14) into its FIFO buffer, it signals the CPU to fetch them by
-sending an electrical signal known as an interrupt on a certain wire
-normally used only by that port. Thus the FIFO waits for a number of
-bytes and then issues an interrupt.
-
-
-However, this interrupt will also be sent if there is an unexpected
-delay while waiting for the next byte to arrive (known as a timeout).
-Thus if the bytes are being received slowly (such as someone typing on
-a terminal keyboard) there may be an interrupt issued for every byte
-received. For some UART chips the rule is like this: If 4 bytes in a
-row could have been received, but none of these 4 show up, then the
-port gives up waiting for more bytes and issues an interrupt to fetch
-the bytes currently in the FIFO. Of course, if the FIFO is empty,
-no interrupt will be issued.
-
-
-Each interrupt conductor (inside the computer) has a number (IRQ) and
-the serial port must know which conductor to use to signal on. For
-example, ttyS0 normally uses IRQ number 4 known as IRQ4 (or IRQ 4). A
-list of them and more will be found in "man setserial" (search for
-"Configuring Serial Ports"). Interrupts are issued whenever the
-serial port needs to get the CPU's attention. It's important to do
-this in a timely manner since the buffer inside the serial port can
-hold only 16 (1 in old serial ports) incoming bytes. If the CPU fails
-to remove such received bytes promptly, then there will not be any
-space left for any more incoming bytes and the small buffer may
-overflow (overrun) resulting in a loss of data bytes.
-There is no
-Flow Control to prevent
-this.
-
-
-Interrupts are also issued when the serial port has just sent out all
-16 of its bytes from its small transmit buffer out the external cable.
-It then has space for 16 more outgoing bytes. The interrupt is to
-notify the CPU of that fact so that it may put more bytes in the small
-transmit buffer to be transmitted. Also, when a modem control line
-changes state an interrupt is issued.
-
-
-The buffers mentioned above are all hardware buffers. The serial port
-also has large buffers in main memory. This will be explained later
-
-
-Interrupts convey a lot of information but only indirectly. The
-interrupt itself just tells a chip called the interrupt controller
-that a certain serial port needs attention. The interrupt controller
-then signals the CPU. The CPU then runs a special program to service
-the serial port. That program is called an interrupt service routine
-(part of the serial driver software). It tries to find out what has
-happened at the serial port and then deals with the problem such a
-transferring bytes from (or to) the serial port's hardware buffer.
-This program can easily find out what has happened since the serial
-port has registers at IO addresses known to the the serial driver
-software. These registers contain status information about the serial
-port. The software reads these registers and by inspecting the
-contents, finds out what has happened and takes appropriate action.
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-!!3.5 Data Flow (Speeds)
-
-
-
- Data (bytes representing letters, pictures, etc.) flows into and
-out of your serial port. Flow rates (such as 56k (56000) bits/sec)
-are (incorrectly) called "speed". But almost everyone says "speed"
-instead of "flow rate".
-
-
-It's important to understand that the average speed is often less than
-the specified speed. Waits (or idle time) result in a lower average
-speed. These waits may include long waits of perhaps a second due to
-Flow Control. At the other extreme
-there may be very short waits (idle time) of several micro-seconds
-between bytes. If the device on the serial port (such as a modem)
-can't accept the full serial port speed, then the average speed must
-be reduced.
-
-
-
-
-!! 3.6 Flow Control
-
-
-
- Flow control means the ability to slow down the flow of bytes in a
-wire. For serial ports this means the ability to stop and then
-restart the flow without any loss of bytes. Flow control is needed
-for modems to allow a jump in instantaneous flow rates.
-
-
-
-
-!Example of Flow Control
-
-
- For example, consider the case where you connect a 33.6k external
-modem via a short cable to your serial port. The modem sends and
-receives bytes over the phone line at 33.6k bits per second (bps).
-Assume it's not doing any data compression or error correction. You
-have set the serial port speed to 115,200 bits/sec (bps), and you are
-sending data from your computer to the phone line. Then the flow from
-the your computer to your modem over the short cable is at 115.2k bps.
-However the flow from your modem out the phone line is only 33.6k bps.
-Since a faster flow (115.2k) is going into your modem than is coming
-out of it, the modem is storing the excess flow (115.2k -33.6k = 81.6k
-bps) in one of its buffers. This buffer would soon overrun (run out
-of free storage space) unless the high 115.2k flow is stopped.
-
-
-But now flow control comes to the rescue. When the modem's buffer is
-almost full, the modem sends a stop signal to the serial port. The
-serial port passes on the stop signal on to the device driver and the
-115.2k bps flow is halted. Then the modem continues to send out data
-at 33.6k bps drawing on the data it previous accumulated in its
-buffer. Since nothing is coming into the buffer, the level of bytes
-in it starts to drop. When almost no bytes are left in the buffer,
-the modem sends a start signal to the serial port and the 115.2k flow
-from the computer to the modem resumes. In effect, flow control
-creates an average flow rate in the short cable (in this case 33.6k)
-which is significantly less than the "on" flow rate of 115.2k bps.
-This is "start-stop" flow control.
-
-
-In the above simple example it was assumed that the modem did no data
-compression. This would be true when the modem is sending a file
-which is already compressed and can't be compressed further. Now
-let's consider the opposite extreme where the modem is compressing the
-data with a high compression ratio. In such a case the modem might
-need an input flow rate of say 115.2k bps to provide an output (to the
-phone line) of 33.6k bps (compressed data). The compression ratio is
-3.43 (115.2/33.6) which is much higher than average. In this case the
-modem is able to compress and the 115.2 bps PC-to-modem flow and send
-the same data out on the phone line at 33.6bps. There's no need for
-flow control here. But such a high compression ratio rarely happens
-so that most of the time flow control is needed to slow down the flow
-on the 115.2 bps PC-to-modem cable. The flow is stopped and started
-so that the average flow is usually well under the "on" flow of 115.2
-bps.
-
-
-In the above example the modem was an external modem. But the same
-situation exists (as of late 2000) for most internal modems. There is
-still a speed limit on the PC-to-modem speed even though this flow
-doesn't take place over an external cable. This makes the internal
-modems compatible with the external modems.
-
-
-In the above example of flow control the flow was from the computer to
-a modem. But there is also flow control which is used for the
-opposite direction of flow: from a modem (or other device) to a
-computer. Each direction of flow involve 3 buffers: 1. in the modem
-2. in the UART chip (called FIFOs) 3. in main memory managed by the
-serial driver. Flow control protects certain buffers from
-overflowing. The small UART FIFO buffers are not protected in this
-way but rely instead on a fast response to the interrupts they issue.
-FIFO stand for "First In, First Out" which is the way it handles
-bytes. All the 3 buffers use the FIFO rule but only one of them also
-uses it as a name. This is the essence of flow control but there are
-still some more details.
-
-
-
-
-
-
-
-
-
-
-!Symptoms of No Flow Control
-
-
- Understanding flow-control theory can be of practical use. The
-symptom of no flow control is chunks of data missing from files sent
-without the benefit of flow control. This is because when overflow
-happens, it's usually more than just a few bytes that overflow and are
-lost. Often hundreds or even thousands of bytes get lost, and all in
-contiguous chunks.
-
-
-
-
-!Hardware vs. Software Flow Control
-
-
- If feasible it's best to use "hardware" flow control that uses two
-dedicated "modem control" wires to send the "stop" and "start"
-signals.
-
-
-
-
-
-Software flow control uses the main receive and transmit wires to send
-the start and stop signals. It uses the ASCII control characters DC1
-(start) and DC3 (stop) for this purpose. They are just inserted into
-the regular stream of data. Software flow control is not only slower
-in reacting but also does not allow the sending of binary data unless
-special precautions are taken. Since binary data will likely contain
-DC1 and DC3, special means must be taken to distinguish between a DC3
-that means a flow control stop and a DC3 that is part of the binary
-code. Likewise for DC1.
-
-
-
-
-
-
-
-!!3.7 Data Flow Path; Buffers
-
-
-
- Much has been explained about this including flow
-control, a pair of 16-byte FIFO buffers (in the UART), and a pair of
-larger buffers inside a device connected to the serial port (such as a
-modem. But there is still another pair of buffers. These are large
-buffers (perhaps 4k) in main memory also known as serial port buffers.
-When an application program sends bytes to the serial port
-
-
-they first get stashed in the the transmit serial port buffer in main
-memory. The pair consists of both this transmit buffer and a receive
-buffer for the opposite direction of byte-flow. Here's an example
-diagram for the case of browsing the Internet with a browser.
-Transmit data flow is left to right while receive flow is right to
-left.
-
-
-
-
-application 4k-byte 16-byte 1k-byte tele-
-BROWSER ------- MEMORY -------- UART --------- MODEM -------- phone
-program buffer buffer buffer line
-
-
-
-The serial device driver takes out say 16 bytes from this transmit buffer,
-one byte at a time and puts them into the 16-byte transmit buffer in the
-serial UART for transmission. Once in that transmit buffer, there
-is no way to stop them from being transmitted. They are then
-transmitted to the modem or other device connected to the serial port
-which also has a fair sized (say 1k) buffer. When the device driver
-(on orders from flow control) stops the flow of outgoing bytes from
-the computer, what it actually stops is the flow of outgoing bytes
-from the large transmit buffer in main memory. Even after this has
-happened and the flow to the device
-connected to the serial port has stopped, an application program
-may keep sending bytes to the 4k transmit buffer until it becomes
-fill.
-
-
-When it gets fill, the application program can't send any more bytes
-to it (a "write" statement in a C_program blocks) and the application
-program temporarily stops running and waits until some buffer space
-becomes available. Thus a flow control "stop" is ultimately able to
-stop the program that is sending the bytes. Even though this program
-stops, the computer does not necessarily stop computing. It may
-switch to running other processes while it's waiting at a flow control
-stop. The above was a little oversimplified since there is another
-alternative of having the application program itself do something else
-while it is waiting to "write".
-
-
-
-
-
-
-
-
-
-
-!!3.8 Complex Flow Control Example
-
-
-
- For many situations, there is a transmit path involving several
-links, each with its own flow control. For example, I type at a
-text-terminal connected to a PC with a modem to access a BBS. For
-this I use the application program "minicom" which deals with 2 serial
-ports: one connected to a modem and another connected to the
-text-terminal. What I type at the text terminal goes into the first
-serial port to minicom, then from minicom out the second serial port
-to the modem, and then onto the telephone line to the BBS. The
-text-terminal has a limit to the speed at which bytes can be displayed
-on its screen and issues a flow control "stop" from time to time to
-slow down the flow. What happens when such a "stop" is issued? Let's
-consider a case where the "stop" is long enough to get thru to the BBS
-and stop the program at the BBS which is sending out the bytes.
-
-
-Let's trace out the flow of this "stop" (which may be "hardware" on
-some links and "software" on others). First, suppose I'm "capturing"
-a long file from the BBS which is being sent simultaneously to both my
-text-terminal and a to file on my hard-disk. The bytes are coming in
-faster than the terminal can handle them so it sends a "stop" out its
-serial port to the first serial port on my PC. The device driver
-detects it and stops sending bytes from the 4k serial buffer (in main
-memory) to the terminal. Now minicom still keeps sending out bytes for
-the terminal into this 4k buffer.
-
-
-When this 4k transmit buffer (on the first serial port) is full,
-minicom must stop writing to it. Minicom stops and waits. But this
-also causes minicom to stop reading from the 4k receive buffer on the
-2nd serial port connected to the modem. Flow from the modem continues
-until this 4k buffer too fills up and sends a different "stop" to the
-modem. Now the modem's buffer ceases to send to the serial port and
-also fills up. The modem (assuming error correction is enabled) sends
-a "stop signal" to the other modem at the BBS. This modem stops
-sending bytes out of its buffer and when its buffer gets fill, another
-stop signal is sent to the serial port of the BBS. At the BBS, the
-8-k (or whatever) buffer fills up and the program at the BBS can't
-write to it anymore and thus temporarily halts.
-
-
-Thus a stop signal from a text terminal has halted a programs on a BBS
-computer. What a complex sequence of events! Note that the stop
-signal passed thru 4 serial ports, 2 modems, and one application
-program (minicom). Each serial port has 2 buffers (in one direction
-of flow): the 4k one and the hardware 16-byte one. The application
-program may have a buffer in its C_code. This adds up to 11 different
-buffers the data is passing thru. Note that the small serial hardware
-buffers do not participate directly in flow control.
-
-
-If the terminal speed limitation is the bottleneck in the flow from
-the BBS to the terminal, then its flow control "stop" is actually
-stopping the program that is sending from the BBS as explained above.
-But you may ask: How can a "stop" last so long that 11 buffers (some
-of them large) all get filled up? It can actually happen this way if
-all the buffers were near their upper limits when the terminal sent
-out the "stop".
-
-
-But if you were to run a simulation on it you would discover that it's
-usually more complicated than this. At an instant of time some links
-are flowing and others are stopped (due to flow control). A "stop"
-from the terminal seldom propagates back to the BBS neatly as
-described above. It may take a few "stops" from the terminal to
-result in one "stop" at the BBS. To understand what is going on you
-really need to observe a simulation which can be done for a simple
-case with coins on a table. Use only a few buffers and set the upper
-level for each buffer at only a few coins.
-
-
-Does one really need to understand all this? Well, understanding this
-explained to me why capturing text from a BBS was loosing text. The
-situation was exactly the above example but modem-to-modem flow
-control was disabled. Chunks of captured text that were supposed to
-also get to my hard-disk never got there because of an overflow at my
-modem buffer due to flow control "stops" from the terminal. Even
-though the BBS had a flow path to the hard-disk without bottlenecks,
-the overflow due to the terminal happened on this path and chunks of
-text were lost and never even made it to the hard-disk. Note that the
-flow to the hard-disk passed thru my modem and since the overflow
-happened there, bytes intended for the hard-disk were lost.
-
-
-
-
-!!3.9 Serial Driver Module
-
-
-
- The device driver for the serial port is the software that
-operates the serial port. It is now provided as a serial module.
->From kernel 2.2 on, this module will normally get loaded automatically
-if it's needed. In earlier kernels, you had to have kerneld
-running in order to do auto-load modules on demand. Otherwise the
-serial module needed to be explicitly listed in /etc/modules. Before
-modules became popular with Linux, the serial driver was usually built
-into the kernel (and sometimes still is). If it's built-in don't let
-the serial module load or else you will have two serial drivers
-running at the same time. With 2 drivers there are all sorts of
-errors including a possible "I/O error" when attempting to open a
-serial port. Use "lsmod" to see if the module is loaded.
-
-
-When the serial module is loaded it displays a message on the screen
-about the existing serial ports (often showing a wrong IRQ). But once
-the module is used by setserial to tell the device driver the
-(hopefully) correct IRQ then you should see a second display similar
-to the first but with the correct IRQ, etc. See
-Serial Module
-See
-What is Setserial for more info on
-setserial.
-
-
-
-
-
-
-
-
-
-----
-
-!!4. Is the Serial Port Obsolete?
-
-!!4.1 Introduction
-
-
-
- The answer is yes, but ... The serial port is somewhat obsolete
-but it's still needed, especially for Linux. The serial port has many
-shortcomings but almost all new PC's seem to come with them. Linux
-supports ordinary telephone modems only if they work thru a serial
-port (although the port may be built into the modem).
-
-
-The serial port must pass data between the computer and the external
-cable. Thus it has two interfaces: the serial-port-to cable and the
-serial-port-to-computer-bus. Both of these interfaces are slow.
-First we'll consider the interface via external cable to the outside
-world.
-
-
-
-
-!!4.2 EIA-232 Cable Is Low Speed & Short Distance
-
-
-
- The conventional EIA-232 serial port is inherently low speed and
-is severely limited in distance. Ads often read "high speed" but it
-can only work at "high speed" over very short distances such as to a
-modem located right next to the computer. Compared to a network card,
-even this "high speed" is low speed. All of the EIA-232 serial cable
-wires use a common ground return wire so that twisted-pair technology
-(needed for high speeds) can't be used without additional hardware.
-More modern interfaces for serial ports exist but they are not
-standard on PC's like the EIA-232 is. See
-Successors to EIA-232. Some multiport serial cards support
-them.
-
-
-It is somewhat tragic that the RS-232 standard from 1969 did not use
-twisted pair technology which could operate about a hundred times
-faster. Twisted pairs have been used in telephone cables since the
-late 1800's. In 1888 (over 110 years ago) the "Cable Conference"
-reported its support of twisted-pair (for telephone systems) and
-pointed out its advantages. But over 80 years after this approval by
-the "Cable Conference", RS-232 failed to utilize it. Since RS-232
-was originally designed for connecting a terminal to a low speed modem
-located nearby, the need for high speed and longer distance
-transmission was apparently not recognized.
-
-
-
-
-!!4.3 Inefficient Interface to the Computer
-
-
-
- To communicate with the computer, any I/O device needs to have
-an address so that the computer can write to it and read from it. For
-this purpose many I/O devices (such as serial ports) use a special
-type of address known as an I/O addresses (sometimes called an I/O
-port). It's actually a range of addresses and the lower address in
-this range is the base address. If someone only says (or writes)
-"address" it likely really means "base address"
-
-
-Instead of using I/O addresses, some I/O devices read and write
-directly from/to main memory. This provides more bandwidth since the
-conventional serial I/O system only moves a byte at a time. There are
-various ways to read/write directly to main memory. One way is called
-shared memory I/O (where the shared memory is usually on the same card
-as the I/O device). Other methods are DMA (direct memory access) on
-the ISA bus and what is about the same as DMA (only much faster):
-"bus mastering" on the PCI bus. These methods are a lot faster than
-those used for the serial port. Thus the conventional serial port
-with its interrupt driven (every 14 bytes) interface and single bytes
-transfers on a bus which could accommodate 4 (or 8) bytes at a time is
-not suited for very high speed I/O.
-
-
-
-----
-
-!!5. Multiport Serial Boards/Cards/Adapters
-
-!!5.1 Intro to Multiport Serial
-
-
-
- Multiport serial cards install in slots in a PC on the ISA or PCI
-bus. Instead of being called "... cards" they are also called "...
-adapters" or "... boards". Each such card provides you with many
-serial ports. Today they are commonly used for the control of
-external devices (including automation for both industry and the
-home). They can connect to computer servers for the purpose of
-monitoring/controlling the server from a remote location. They were
-once mainly used for connecting up many dumb terminals and/or modems
-to serial ports. Today, use of dumb terminals has declined, and
-several modems (or digital modems) can now be built into
-an internal card. So multiport serial cards are not as significant as
-they once were.
-
-
-Each multiport card has a number of external connecters (DB-25 or
-RJ45) so that one may connect up a number of devices (modems,
-terminals, etc.). Each such physical device would then be connected
-to its own serial port. Since the space on the external-facing part
-of the card is limited there is often not enough room for all the
-serial port connectors. To solve this problem, the connectors may be
-on the ends of cables which come out (externally) from the card
-(octopus cable). Or they may be on an external box (possibly rack
-mountable) which is connected by a cable to a multiport card.
-
-
-
-
-!!5.2 Modem Limitations
-
-
-
-For a modem to transmit at nearly 56k requires that it be a special
-digital modem and have a digital connection to a digital phone line
-(such as a T1 line). Modem banks that connect to multiport cards do
-exist, and some have a card that can access multiplexed digital phone
-lines. Thus one can use a multiport card with a few 56k digital
-modems.
-
-
-For both analog and digital modem there is one modem on each serial
-port so there needs to be an external cable (modem bank to multiport)
-for each modem. This can lead to a large number of cables. So it's
-less clutter (and cheaper) to use internal modems without a multiport
-card. It's somewhat analogous to the lower cost of an internal modem
-for a desktop PC as compared to the higher cost (and more cabling) for
-an external modem. See Modem-HOWTO: Modem Pools, Digital Modems.
-
-
-
-
-!!5.3 Dumb vs. Smart Cards
-
-
-
-Dumb multiport cards are not too much different than ordinary serial
-ports. They are interrupt driven and the CPU of the computer does
-most all the work servicing them. They usually have a system of
-sharing a single interrupt for all the ports. This doesn't decrease
-the load on the CPU since the single interrupt will be sent to the CPU
-each time any one port needs servicing. Such devices usually require
-special drivers that you must put into the kernel or activate by
-putting a #define in the source code (or the like).
-
-
-Smart boards may use ordinary UARTs but handle most interrupts from
-the UARTs internally within the board. This frees the CPU from the
-burden of handling all these interrupts. The board may save up bytes
-in its large internal FIFOs and transfer perhaps 1k bytes at a time to
-the serial buffer in main memory. It may use the full bus width of 32
-bits for making data transfers to main memory (instead of transferring
-only 8-bit bytes like dumb serial cards do). Not all "smart" boards
-are equally efficient. Many boards today are Plug-and-Play.
-
-
-
-
-!!5.4 Getting/Enabling a Driver
-
-
-!Introduction
-
-
-For a multiport board to work, a special driver for it must be used.
-This driver may either be built into the kernel source code or
-supplied as a module. Support for dumb boards is likely to the built
-into the kernel while smart boards usually need a module.
-
-
-
-
-!Driver is built into the kernel (mostly dumb boards)
-
-
-A pre-compiled kernel is not likely to have multiport support built
-in. So you probably need to compile it yourself. In kernel 2.4 you
-should select "CONFIG_SERIAL_EXTENDED when configuring the kernel
-(just before you compile). If you select this there will be still
-more choices presented to you. Even after you do this you may need to
-edit the resulting source code a little (depending on the card).
-
-
-
-
-! Modules (mostly for smart boards)
-
-
-A pre-compiled kernel may come with a pre-compiled module for the
-board so that you don't need to recompile the kernel. This module
-must be loaded in order to use it, but the kernel may automatically do
-this for you if a program is trying to use a device on the smart board
-(provided there exists a table showing which module to load for the
-device). This table may be in /etc/modules.conf and/or be internal to
-the kernel. Also certain parameters may need to be passed to the
-driver (via lilo's append command or via /etc/modules.conf). For
-kernel 2.4 the modules should be found in
-/lib/modules/.../kernel/drivers/char.
-
-
-
-
-!Getting info on multiport boards
-
-
- The board's manufacturer should have info on their website.
-Unfortunately, info for old boards is sometimes not there but might be
-found somewhere else on the Internet (including discussion groups).
-You might also want to look at the kernel documentation in
-/usr/share/kernel-doc... For configuring the kernel or modules prior
-to compiling see: Configure.help and search for "serial", etc. There
-are also kernel documentation files for certain boards including
-computone, hayes-esp, moxa-smartio, riscom8, specialix, stallion, and
-sx (specialix).
-
-
-
-
-!! 5.5 Making multiport devices in the /dev directory
-
-
-
-The serial ports your multiport board uses depends on what kind of board
-you have. Some have their own device names like /dev/ttyE27 or
-/dev/ttyD2, etc. Ones that use the standard names like /dev/ttyS14
-may be listed in detail in rc.serial or in
-0setserial. These files may be included in a setserial or
-serial package. You may need to create these devices (unless an
-installation script does it for you). Either use the mknod
-command, or the MAKEDEV script. Devices (in the /dev directory)
-for ttyS type serial ports are made by adding ``64 + port number''.
-So, if you wanted to create devices for ttyS17, you would
-type:
-
-
-
-
-
-linux# mknod -m 666 /dev/ttyS17 c 4 81
-
-
-Note the "major" number is always 4 for ttyS devices (and 5 for the
-obsolete cua devices). Also ``64 + 17 = 81''. Using the MAKEDEV
-script, you would type:
-
-
-
-
-
-linux# cd /dev
-linux# ./MAKEDEV ttyS17
-
-
-
-
-For the names and numbers of other types of serial ports other than
-ttyS.. see devices.txt in the kernel documentation. Besides the
-listing of various brands of multiports found in this HOWTO there is
-Gary's Encyclopedia - Serial Cards. It's not as complete, but may have
-some different links.
-
-
-
-
-!!5.6 Standard PC Serial Cards
-
-
-
- In olden days PCs came with a serial card installed. Later on the
-serial function was put on the hard-drive interface card. Today, one
-or two serial ports are usually built into the motherboard. Most of
-them (as of 2001) use a 16550 but some use 16650 (32-byte FIFOs).
-But one may still buy the old PC serial cards if they need 1-4 more
-serial ports. These are for ttyS0-ttyS3 (COM1 - COM4). They can be
-used to connect external serial devices (modems, serial mice, etc...).
-Only a tiny percentage of retail computer stores carry such cards.
-But one can purchase them on the Internet. Before getting a PCI one,
-make sure Linux supports it.
-
-
-Here's a list of a few popular brands:
-
-
-*Byte Runner (may order directly, shows prices)
-http://www.byterunner.com
-*
-
-* SIIG
-http://www.siig.com/products/io/
-*
-
-* Dolphin
-http://www.dolphinfast.com/sersol.html
-*
-
-
-
-Note: due to address conflicts, you may not be able to use COM4 and
-IBM8514 video card (or some others) simultaneously. See
-Avoiding IO Address Conflicts with Certain Video Boards
-
-
-
-!!5.7 Dumb Multiport Serial Boards (with standard UART chips)
-
-
-
- They are also called "serial adapters". They often have a
-special method of sharing interrupts which requires that you compile
-support for them into the kernel.
-
-
-* => The file that ran setserial in Debian shows some details of configuring
-# => See note below for this board
-
-
-*AST !FourPort and clones (4 ports) * #
-*
-
-*Accent Async-4 (4 ports) *
-*
-
-*Arnet Multiport-8 (8 ports)
-*
-
-*Bell Technologies HUB6 (6 ports)
-*
-
-*Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports;
-See the Boca mini-howto revised in 2001) * #
-*
-
-*Boca IOAT66 or? ATIO66 (6 ports, Linux doesn't support its IRQ
-sharing ?? Uses odd-ball 10-cond RJ45-like connectors)
-*
-
-*Boca 2by4 (4 serial ports, 2 parallel ports)
-*
-
-*Byte Runner
-http://www.byterunner.com
-*
-
-*Computone !ValuePort V4-ISA (AST !FourPort compatible) *
-*
-
-*Digi PC/8 (8 ports) #
-*
-
-*Dolphin
-http://www.dolphinfast.com/sersol/
-*
-
-*Globetek
-http://www.globetek.com/
-*
-
-*GTEK BBS-550 (8 ports; See the mini-howto)
-*
-
-*Hayes ESP (after kernel 2.1.15)
-*
-
-*HUB-6 See Bell Technologies.
-*
-
-*Longshine LCS-8880, Longshine LCS-8880+ (AST !FourPort compatible) *
-*
-
-*Moxa C104, Moxa C104+ (AST !FourPort compatible) *
-*
-
-*
-NI-SERIAL by National Instruments
-*
-
-*PC-COMM (4 ports)
-*
-
-*
-Sealevel Systems
-COMM-2 (2 ports), COMM-4 (4 ports) and COMM-8 (8 ports)
-*
-
-*SIIG I/O Expander 2S IO1812 (4 ports) #
-*
-
-*STB-4COM (4 ports)
-*
-
-*Twincom ACI/550
-*
-
-*Usenet Serial Board II (4 ports) *
-*
-
-*VScom (uses same driver as !ByteRunner)
-*
-
-
-
-In general, Linux will support any serial board which uses a 8250,
-16450, 16550, 16550A, 16650, etc. UART. See the latest man page for
-"setserial" for a more complete list.
-
-
-Notes:
-
-
-AST Fourport: You might need to specify skip_test in rc.serial.
-
-
-BB-1004 and BB-1008 do not support DCD and RI lines, and thus are not
-usable for dialin modems. They will work fine for all other purposes.
-
-
-Digi PC/8 Interrupt Status Register is at 0x140.
-
-
-SIIG IO1812 manual for the listing for COM5-COM8 is
-wrong. They should be COM5=0x250, COM6=0x258, COM7=0x260, and
-COM8=0x268.
-
-
-
-
-!!5.8 Intelligent Multiport Serial Boards
-
-
-
-Make sure that a Linux-compatible driver is available and read the
-information that comes with it. These boards use special devices (in
-the /dev directory), and not the standard ttyS ones. This information
-varies depending on your hardware. If you have updated info which
-should be shown here please email it to me.
-
-
-Names of Linux driver modules are *.o but these may not work for all
-models shown. See
-Modules (mostly for smart boards) The needed module may have been supplied with your Linux
-distribution. Also, parameters (such as the io and irq often need to
-be given to the module so you need to find instructions on this
-(possibly in the source code tree).
-
-
-There are many different brands, each of which often offers many
-different cards. No attempt is currently being made to list all the
-cards here (and many listed are obsolete). But all major brands and
-websites should be shown here so it something is missing let me know.
-Go the the webpage shown for more information. These websites often
-also have info (ads) on related hardware such as modem pools, remote
-access servers (RASs), and terminal servers. Where there is no
-webpage, the cards are likely obsolete. If you would like to put
-together a better list, let me know.
-
-
-
-
-
-*Chase Research (UK based, ISA/PCI cards)
-webpage:
-www.chaser.com
-driver status: for 2.2 kernel. Supported by Chase.
-
-*
-
-*Comtrol !RocketPort (36MHz ASIC; 4, 8, 16, 32, up to 128 ports)
-webpage:
-http://www.comtrol.com
-driver status: supported by Comtrol. rocket.o
-driver location:
-ftp://tsx-11.mit.edu/pub/linux/packages/comtrol
-
-*
-
-*Computone !IntelliPort II (ISA, PCI and EISA busses up to 64
-ports)
-webpage:
-http://www.computone.com
-driver location: old patch at
-http://www.wittsend.com/computone/linux-2.2.10-ctone.patch.gz
-mailing list:
-mailto:majordomo@lazuli.wittsend.com with
-"subscribe linux-computone" in body
-note: Old ATvantage and Intelliport cards are not supported by Computone
-
-*
-
-* Connecttech
-website:
-http://www.connecttech.com/
-driver location:
-ftp://ftp.connecttech.com/pub/linux/
-
-*
-
-*Cyclades
-Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),
-Cyclom-Z (MIPS R3000; 8 - 64 ports)
-website:
-http://www.cyclades.com/products/svrbas/zseries.php
-driver status: supported by Cyclades
-driver location:
-ftp://ftp.cyclades.com/pub/cyclades and included in Linux
-kernel since version 1.1.75: cyclades.o
-
-*
-
-*Decision PCCOM (2-8 ports; ISA and PCI; aka PC COM)
-ISA:
-contact:
-mailto:info@cendio.se
-driver location: (dead link)
-ftp://ftp.signum.se/pub/pccom8
-PCI:
-drivers:
-http://www.decision.com.tw
-driver status: Support in serial driver 5.03. For an earlier driver,
-there exists a patch for kernel 2.2.16 at
-http://www.qualica.com/serial/ and for kernels 2.2.14-2.2.17
-at
-htp://www.pccompci.com/mains/installing_pci_linux1.html
-
-*
-
-*Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),
-PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),
-PC/Xr (16MHz IDT3041; 4 or 8 ports),
-PC/Xem (20MHz IDT3051; 8 - 64 ports)
-website:
-http://www.dgii.com
-driver status: supported by Digi
-driver location:
-ftp://ftp.dgii.com/drivers/linux and
-included in Linux kernel since version 2.. epca.o
-
-*
-
-*Digi COM/Xi (10MHz 80188; 4 or 8 ports)
-contact: Simon Park,
-si@wimpol.demon.co.uk
-driver status: ?
-note: Simon is often away from email for months at a time due to
-his job. Mark Hatle,
-mailto:fray@krypton.mankato.msus.edu
-has graciously volunteered to make the driver available if you need
-it. Mark is not maintaining or supporting the driver.
-
-*
-
-*Equinox !SuperSerial Technology (30MHz ASIC; 2 - 128 ports)
-website:
-http://www.equinox.com
-driver status: supported by Equinox
-driver location:
-ftp://ftp.equinox.com/library/sst
-
-*
-
-*Globetek
-website:
-http://www.globetek.com/products.shtml
-driver location:
-http://www.globetek.com/media/files/linux.tar.gz
-
-*
-
-*GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),
-!SmartCard (24MHz Dallas DS80C320; 8 ports),
-!BlackBoard-8A (16C654 UARTs; 8 ports),
-PCSS (15/24MHz 8032; 8 ports)
-website:
-http://www.gtek.com
-driver status: supported by GTEK
-driver location:
-ftp://ftp.gtek.com/pub
-
-*
-
-*Hayes ESP (COM-bic; 1 - 8 ports)
-website:
-http://www.nyx.net/~arobinso
-driver status: Supported by Linux kernel (1998) since v. 2.1.15.
-esp.o. Setserial 2.15+ supports. Also supported by author
-driver location:
-http://www.nyx.net/~arobinso
-
-*
-
-*Intelligent Serial Interface by Multi-Tech Systems
-PCI: 4 or 8 port. ISA 8 port. DTE speed 460.8k
-webpage:
-http://www.multitech.com/products/
-
-*
-
-*Maxpeed SS (Toshiba; 4, 8 and 16 ports)
-website:
-http://www.maxpeed.com
-driver status: supported by Maxpeed
-driver location:
-ftp://maxpeed.com/pub/ss
-
-*
-
-* Microgate !SyncLink ISA and PCI high speed multiprotocol
-serial. Intended for synchronous HDLC.
-website:
-http://ww/microgate.com/products/sllinux/hdlcapi.htm
-driver status: supported by Microgate: synclink.o
-
-*
-
-*Moxa C218 (12MHz 80286; 8 ports),
-Moxa C320 (40MHz TMS320; 8 - 32 ports)
-website:
-http://www.moxa.com
-driver status: supported by Moxa
-driver locations:
-">http://www.moxa.com/support/download/download.php3>
-
-ftp://ftp.moxa.com/drivers/linux
-(also from Taiwan at www.moxa.com.tw/...) where ... is the same as
-above)
-
-*
-
-*SDL RISCom/8 (Cirrus Logic CD180; 8 ports)
-website:
-http://www.sdlcomm.com
-driver status: supported by SDL
-driver location:
-ftp://ftp.sdlcomm.com/pub/drivers
-
-*
-
-* Specialix SX (25MHz T225; 8? - 32 ports),
-SIO/XIO (20 MHz Zilog Z280; 4 - 32 ports)
-webpage:
-www.specialix.com/products/io/serialio.htm
-driver status: Supported by Specialix
-driver location:
-http://www.!BitWizard.nl/specialix/
-old driver location:
-ftp://metalab.unc.edu/pub/Linux/kernel/patches/serial
-
-*
-
-*Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and
-!EasyConnection (8 - 32 ports) - each with
-Cirrus Logic CD1400 UARTs,
-Stallion (8MHz 80186 CPU; 8 or 16 ports),
-Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),
-ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),
-!EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)
-contact:
-sales@stallion.com or
-
-http://www.stallion.com
-driver status: supported by Stallion
-driver location:
-ftp://ftp.stallion.com/drivers/ata5/Linux and
-included in linux kernel since 1.3.27
-
-*
-
-*System Base
-website:
-http://www.sysbas.com/
-*
-
-
-
-
-
-
-A review of Comtrol, Cyclades, Digi, and Stallion products was printed
-in the June 1995 issue of the ''Linux Journal''. The article is
-available at
-http://www.ssc.com/lj/issue14.
-
-
-
-
-!!5.9 Unsupported Multiport Boards
-
-
-
- The following boards don't mention any Linux support as of 1 Jan.
-2000. Let me know if this changes.
-
-
-* Aurora (PCI only)
-www.auroratech.com
-*
-
-
-
-
-----
-
-!!6. Configuring Overview
-
-
- The first part (locating the hardware or low-level configuring) is
-assigning each port an IO address, IRQ, and name (such as ttyS2). This
-IO-IRQ pair must be set in both the hardware and told to the serial
-driver. We might just call this "io-irq" configuring for short. The
-"setserial" program is sometimes used to tell the driver. PnP
-methods, jumpers, etc, are used to set the hardware. Details will be
-supplied later. If you need to configure but don't understand certain
-details it's easy to get into trouble.
-See
-Locating the Serial Port: IO address IRQs
-What is Setserial
-
-The second part (high-level configuring) is assigning it a speed (such
-as 115.2k bits/sec), selecting flow control, etc. This is often done
-by communication programs such as PPP, minicom, or by getty (which you
-may run on the port so that others may log into your computer).
-However you will need to tell these programs what speed you want, etc.
-by using a menu or a configuration file. This high-level configuring
-may also be done with the stty program. stty is also useful
-to view the current status if you're having problems.
-See the section
-Stty
-
-
-----
-
-!! 7. Locating the Serial Port: IO address, IRQs
-
-!!7.1 IO & IRQ Overview
-
-
-
- For a serial port to work properly, it must have both an IRQ and
-an IO address. Without an IO address, it can't be located and will not
-work at all. Without an IRQ it will need to use inefficient polling
-methods for which one must set the IRQ to . So every serial port
-needs an IO address and IRQ. In olden days this was set by jumpers on
-a serial port card. Today it's set by digital signals sent to the
-hardware and this is part of "Plug-and-Play (PnP).
-
-
-The driver must also know both the IO address and IRQ so that it can
-locate the card. Modern drivers (kernel 2.4) determine this by PnP
-methods so one doesn't need to tell them (by using "setserial"). The
-driver uses PnP functions provided by Linux to determine what the IO
-and IRQ are and to set them if necessary. The driver also probes
-possible serial port addresses to see if there are any serial ports
-there. This works for the case of jumpers and sometimes works for a
-PnP port when the driver doesn't do PnP.
-
-
-Locating the serial port by giving it an IRQ and IO address is
-low-level configuring. What follows repeats what was said above in
-more detail. This low-level configuring consists of assigning an IO
-address, IRQ, and name (such as ttyS2). This IO-IRQ pair must be set
-in both the hardware and told to the serial driver. We could call
-this "io-irq" configuring for short. The "setserial" program is one
-way to tell the driver. The other way is for the driver to use PnP
-methods to determine/set the IO/IRQ and then remember what it did.
-For jumpers you must always use "setserial". If you need to configure
-but don't understand certain details it's easy to get into trouble.
-
-
-When Linux starts, some effort is made to detect and configure
-(low-level) a few serial ports. Exactly what happens depends on your
-BIOS, hardware, Linux distribution, etc. If the serial ports work OK,
-there may be no need for you to do any more low-level configuring.
-
-
-If you're having problems with the serial ports, then you may need to
-do low-level configuring. If you have kernel 2.2 or lower,
-then you need to do it if you:
-
-
-
-
-
-* Plan to use more than 2 ISA serial ports
-*
-
-* Are installing a new serial port (such as an internal modem)
-*
-
-
-
-For kernel 2.2+ you may be able to use more that 2 serial ports
-without doing any low-level configuring by sharing interrupts. All
-PCI cards should support this but for ISA it only works for some
-hardware. It may be just as easy to give each port a unique interrupt
-if they is available. See
-Interrupt sharing and Kernels 2.2+
-
-The low-level configuring (setting the IRQ and IO address) seems to
-cause people more trouble than the high-level, although for many it's
-fully automatic and there is no configuring to be done. Until the
-serial driver knows the correct IRQ and IO address, the port will not
-usually not work at all. Also, PnP ports can be disabled so that they
-can't be found (except with PnP tools). Applications, and utilities
-such as "setserial" and "scanport" don't use PnP tools and thus can't
-detect disabled ports. For example, an IO enabling bit must be set in
-PCI serial port hardware by PnP so that it's IO address may be used.
-
-
-Even if the port can be found by normal (non-PnP) software, it may
-work extremely slow if the IRQ is wrong. See
-Extremely Slow: Text appears on the screen slowly after long delays.
-
-
-In the Wintel world, the IO address and IRQ are called "resources" and
-we are thus configuring certain resources. But there are many other
-types of "resources" so the term has many other meanings. In summary,
-the low-level configuring consists of enabling the device, giving it a
-name (ttyS2 for example) and putting two values (an IRQ number and IO
-address) into two places:
-
-
-
-
-
-# the device driver (often by running "setserial" at
-boot-time)
-#
-
-# memory registers of the serial port hardware itself
-#
-
-
-
-You may watch the start-up (= boot-time) messages. They are usually
-correct. But if you're having problems, there's a good chance that
-the ones that look like "setserial" output don't show the true
-configuration of the hardware (and they are not necessarily supposed
-to). See
-I/O Address & IRQ: Boot-time messages.
-
-
-
-
-!! 7.2 PCI Bus Support
-
-
-
-
-
-
-
-
-
-If you have kernel 2.4, then there should be support for PnP (either
-built-in or by modules). Some PCI serial cards can be automatically
-detected and low-level configured by the serial driver. Others will
-not be.
-Kernel 2.2 had no support for PCI serial ports (although some people
-got them working anyway). The 2.4 serial driver will read the id
-number digitally stored on the serial hardware to determine how to
-support it (if it knows how). It should assign the I/O address to it,
-determine it's IRQ, etc. So you don't need to use "setserial" for it
-??
-
-
-If you have a
-
-
-PCI modem card you should be looking at Modem-HOWTO and not this
-Serial-HOWTO. If you just have a PCI serial port card (with no modem
-on the card) but it will not work because the latest serial driver doesn't support
-it, you can help in attempting to create a driver for it. To do this
-you'll need to contact the maintainer of the serial driver, Theodore
-(Ted) Y. Ts'o.
-
-
-
-
-
-Look at
-Ted Ts'o's site for the details of what you need to do. Here's a summary of
-what you need to do to help him. You will need to email Ted Ts'o a
-copy of the output of "lspci -vv" with full information about the
-model and manufacturer of the PCI modem (or serial port). Then he
-will try to point you to a test driver which might work for it. You
-will then need to get it, compile it and possibly recompile your
-kernel. Then you will test the driver to see if it works OK for you
-and report the results to Ted Ts'o. If you are willing to do all the
-above (and this is the latest version of this HOWTO) then email the
-needed info to him at:
-mailto:tytso@mit.edu.
-
-
-PCI ports are not well standardized. Some use main memory for
-communication with the PC. Some require special enabling of the IRQ.
-The output of "lspci -vv" can help determine if one can be supported.
-If you see a 4-digit IO port, the port might work by just telling
-"setserial" the IO port and the IRQ.
-For example, if lspci shows IRQ 10, I/O at 0xecb8 and you decide to
-name it ttyS2 then the command is:
-
-
-setserial /dev/ttyS2 irq 10 port 0xecb8 autoconfig
-
-
-Note that the boot-time message "Probing PCI hardware" means reading
-the PnP configuration registers in the PCI cards which reveals the IO
-addresses and IRQs. This is different that the probing of IO
-addresses by the serial driver which means reading certain IO
-addresses to see if what's read looks like there's a serial port at
-that address.
-
-
-
-
-!!7.3 Common mistakes made re low-level configuring
-
-
-
- Here are some common mistakes people make:
-
-
-*setserial command: They run it (without the "autoconfig" and
-auto_irq options) and think it has checked the hardware to see if
-what it shows is correct (it hasn't).
-*
-
-*setserial messages: They see them displayed on the screen at
-boot-time (or by giving the setserial command) and erroneously think
-that the result always shows how their hardware is actually
-configured.
-*
-
-*/proc/interrupts: When their serial device isn't in use they
-don't see its interrupt there, and erroneously conclude that their
-serial port can't be found (or doesn't have an interrupt set).
-*
-
-*/proc/ioports and /proc/tty/driver/serial: People think this
-shows the actual hardware configuration when it only shows about the
-same info (possibly erroneous) as setserial.
-*
-
-
-
-
-
-!! 7.4 IRQ & IO Address Must be Correct
-
-
-
-There are really two answers to the question "What is my IO and
-IRQ?" 1. What the device driver thinks has been set (This is what
-setserial usually sets and shows.). 2. What is actually set in the
-hardware. Both 1. and 2. above should be the same. If they're not it
-spells trouble since the driver has incorrect info on the physical
-serial port. In some cases the hardware is disabled so it has no IO
-address or IRQ.
-
-
-If the driver has the wrong IO address it will try to send data to a
-non-existing serial port --or even worse, to some other device. If it
-has the wrong IRQ the driver will not get interrupt service requests
-from the serial port, resulting in a very slow or no response. See
-Extremely Slow: Text appears on the screen slowly after long delays. If it has the wrong model of UART there
-is also apt to be trouble. To determine if both I0-IRQ pairs are
-identical you must find out how they are set in both the driver and
-the hardware.
-
-
-
-
-!!7.5 What is the IO Address and IRQ per the driver ?
-
-
-!Introduction
-
-
-What the driver thinks is not necessarily how the hardware is
-actually set. If everything works OK then what the driver thinks is
-likely correct (set in the hardware) and you don't need to investigate
-(unless you're curious or want to become a guru). Ways to determine
-what the driver thinks include: boot-time messages
-I/O Address & IRQ: Boot-time messages, the
-/proc directory "files"
-The /proc directory and setserial, and the "setserial" command.
-
-
-
-
-
-
-
-! I/O Address & IRQ: Boot-time messages
-
-
- In many cases your ports will automatically get low-level
-configured at boot-time (but not always correctly). To see what is
-happening, look at the start-up messages on the screen. Don't neglect
-to check the messages from the BIOS before Linux is loaded (no
-examples shown here). These BIOS messages may be frozen by pressing
-the Pause key. Use Shift-!PageUp to scroll back to the messages after
-they have flashed by. Shift-!PageDown will scroll in the opposite
-direction. The dmesg command (or looking at logs in /var/log)
-will show some of the messages but they seem to miss important ones
-from setserial. Here's an example of the start-up messages (as of mid
-1999). Note that ttyS00 is the same as /dev/ttyS0.
-
-
-
-
-
-At first you see what was detected (but the irq is only a wild guess):
-Serial driver version 4.27 with no serial options enabled
-ttyS00 at 0x03f8 (irq = 4) is a 16550A
-ttyS01 at 0x02f8 (irq = 3) is a 16550A
-ttyS02 at 0x03e8 (irq = 4) is a 16550A
-Later setserial shows you what was saved, but it's not necessarily
-correct either:
-Loading the saved-state of the serial devices...
-/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
-/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
-/dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A
-
-
-
-
-Note that there is a slight disagreement: The first message shows
-ttyS2 at irq=4 while the second shows it at irq=5. dmesg may not
-display the second message. In most cases the second message is the
-correct one. But if your having trouble it may be misleading. Before
-reading the explanation of all of this complexity in the rest of this
-section, you might just try using your serial port and see if it works
-OK. If so it may not be essential to read further.
-
-
-The second message is from the setserial program being run at
-boot-time. It shows what the device driver thinks is the correct
-configuration. But this too could be wrong. For example, the irq
-could actually be set to irq=8 in the hardware (both messages wrong).
-The irq=5 could be there because someone incorrectly put this into a
-configuration file (or the like). The fact that Linux sometimes gets
-IRQs wrong is because it doesn't by default probe for IRQs. It just
-assumes the "standard" ones (first message) or accepts what you told
-it when you configured it (second message). Neither of these is
-necessarily correct. If the serial driver has the wrong IRQ, the
-serial port is very slow or doesn't seem to work at all.
-
-
-The first message is a result of Linux probing the serial port
-addresses but it doesn't probe for IRQs. If a port shows up here it
-exists but the IRQ may be wrong. Linux doesn't check IRQs because
-doing so is not foolproof. It just assumes the IRQs are as shown
-because they are the "standard" values. Your may check them manually
-with setserial using the autoconfig and auto_irq
-options but this isn't guaranteed to be correct either.
-
-
-The data shown by the BIOS messages (which you see at first before
-Linux is booted) is what is initially set in the hardware. If your
-serial port is Plug-and-Play (PnP) then it's possible that "isapnp" or
-"setpci" will run and change these settings. Look for messages about
-this after Linux starts. The last serial port message shown in the
-example above should agree with the BIOS messages (as possibly
-modified by isapnp or setpci). If they don't agree then you either
-need to change the setting in the port hardware or use setserial to
-tell the driver what is actually set in the hardware.
-
-
-Also, if you have Plug-and-Play (PnP) serial ports, they can only be
-found by PnP software unless the IRQ and IO has been set inside the
-hardware by Plug-and-Play software. Prior to kernel 2.4 this was a
-common reason why the start-up messages did not show a serial port
-that physically exists. A PnP BIOS may automatically low-level
-configure them. PnP configuring will be explained later.
-
-
-
-
-! The /proc directory and setserial
-
-
- Type "setserial -g /dev/ttyS*". There are some other
-ways to find this info by looking at "files" in the /proc directory.
-Be warned that there is no guarantee that the same is set in the
-hardware.
-
-
-/proc/ioports will show the IO addresses that the drivers are using.
-/proc/interrupts shows the IRQs that are used by drivers of
-currently running processes (that have devices open). It shows how
-many interrupts have actually be issued.
-/proc/tty/driver/serial shows much of the above, plus the
-number of bytes that have been received and sent (even if the device
-is not now open).
-
-
-Note that for the IO addresses and IRQ assignments, you are only seeing
-what the driver thinks and not necessarily what is actually set in the
-hardware. The data on the actual number of interrupts issued and
-bytes processed is real however. If you see a large number of
-interrupts and/or bytes then it probably means that the device is (or
-was) working. But the interrupts might be from another device. If
-there are no bytes received (rx:) but bytes were transmitted (tx:3749
-for example), then only one direction of flow is working (or being
-utilized).
-
-
-Sometimes a showing of just a few interrupts doesn't mean that the
-interrupt is actually being physically generated by any serial port.
-Thus if you see almost no interrupts for a port that you're trying to
-use, that interrupt might not be set in the hardware. To view
-/proc/interrupts to check on a program that you're currently running
-(such as "minicom") you need to keep the program running while you
-view it.
-
-
-
-
-!! 7.6 What is the IO Address & IRQ of my Serial Port Hardware?
-
-
-!Introduction
-
-
-If it's PCI or ISA PnP then what's set in the hardware has been done
-by PnP methods. Even if nothing has been set or the port disabled,
-PnP ports may still be found by using "lspci -v" or "isapnp
---dumpregs". Ports disabled by jumpers (or hardware failures) are
-lost. See
-ISA PnP ports,
-PCI: What IOs and IRQs have been set?,
-PCI: Is the port enabled?
-
-PnP ports don't store their configuration in the hardware when the
-power is turned off. This is in contrast to Jumpers (non-PnP) which
-remain the same with the power off. That's why a PnP port is more
-likely to be found in a disabled state than an old non-PnP one.
-
-
-
-
-! PCI: What IOs and IRQs have been set?
-
-
- For PCI, the BIOS almost always sets the IRQ and may set the IO
-address as well. To see how it's set use "lspci -vv" (best) or look
-in /proc/bus/pci (or for kernels <2.2 /proc/pci). The modem's
-serial port is often called a "Communication controller". If more
-than one IO address is shown, the first one is more likely to be it.
-You can't change the IRQ (at least not with "setpci") This is
-because if one writes it in it's hardware register no action is taken
-on it. It's the BIOS that should actually set up the IRQs and then
-write the correct value to this register for lspci to view. If you
-must, change the IO address with "setpci" by changing the
-BASE_ADDRESS_0 or the like. The _0 (or _1) after BASE_ADDRESS must be
-the correct register.
-
-
-
-
-! PCI: Is the port enabled?
-
-
-If the port communicates via an IO address "then lspci -vv" should
-show "Control: I/O+ ..." with + meaning that the IO address is
-enabled. If it shows "I/O-" then you may need to use the setpci
-command to enable it. For example "setpci -d 151f:000 command=101".
-The "command" means the command register which is the same as the
-"Control" register displayed by lspci. The 101h sets two bits: the 1
-sets I/O to + and the 100 part keeps SERR# set to +. In this case
-only the SERR# bit of the Control register was initially observed to
-be + when the lspci command was run. So we kept it enabled to + by
-setting bit 8 (where bit 0 is I/O) to 1 by the first 1 in 101. My
-apologies if setting bits is confusing. Bit 8 is actually the 9th bit
-since we started counting bits from .
-
-
-
-
-! ISA PnP ports
-
-
-For an ISA Plug-and-Play (PnP) port one may try the pnpdump
-program (part of isapnptools). If you use the --dumpregs option
-then it should tell you the actual IO address and IRQ set in the port.
-It should also find an ISA PnP port that is disabled. The address it
-"trys" is not the device's IO address, but a special address used for
-communicating with PnP cards.
-
-
-
-
-!Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)
-
-
- Perhaps the BIOS messages will tell you some info before Linux
-starts booting. Use the shift-!PageUp key to step back thru the
-boot-time messages and look at the very first ones which are from the
-BIOS. This is how it was before Linux started. Setserial can't
-change it but isapnp or setpci can. Starting with kernel 2.4, these
-are built into the serial driver.
-
-
-Using "scanport" will probe all I/O ports and will indicate what it
-thinks may be serial port. After this you could try probing with
-setserial using the "autoconfig" option. You'll need to guess the
-addresses to probe at (using clues from "scanport"). See
-What is Setserial.
-
-
-For a port set with jumpers, the IO ports and IRQs are set per the
-jumpers. If the port is not Plug-and-Play (PnP) but has been setup by
-using a DOS program, then it's set at whatever the person who ran that
-program set it to.
-
-
-
-
-!Exploring via MS Windows (a last resort)
-
-
-For PnP ports, checking on how it's configured under DOS/Windows
-may (or may not) imply how it's under Linux. MS Windows stores its
-configuration info in its Registry which is not used by Linux so they
-are not necessarily configured the same. If you let a PnP BIOS
-automatically do the configuring when you start Linux (and have told
-the BIOS that you don't have a PnP operating system when starting
-Linux) then Linux should use whatever configuration is in the BIOS's
-non-volatile memory. Windows also makes use of the same non-volatile
-memory but doesn't necessarily configure it that way.
-
-
-
-
-!! 7.7 Choosing Serial IRQs
-
-
-
- If you have Plug-and-Play ports then either a PnP BIOS or a
-serial driver may configure all your devices for you so then you may
-not need to choose any IRQs. PnP software determines what it thinks
-is best and assigns them (but it's not always best). But if you
-directly use isapnp (ISA bus) or jumpers then you have to choose. If
-you already know what IRQ you want to use you could skip this section
-except that you may want to know that IRQ 0 has a special use (see the
-following paragraph).
-
-
-
-
-!IRQ 0 is not an IRQ
-
-
- While IRQ 0 is actually the timer (in hardware) it has a special
-meaning for setting a serial port with setserial. It tells the driver
-that there is no interrupt for the port and the driver then will use
-polling methods. Such polling puts more load on the CPU but can be
-tried if there is an interrupt conflict or mis-set interrupt. The
-advantage of assigning IRQ 0 is that you don't need to know what
-interrupt is set in the hardware. It should be used only as a
-temporary expedient until you are able to find a real interrupt to
-use.
-
-
-
-
-! Interrupt sharing, Kernels 2.2+
-
-
- Sharing of IRQs is where two devices use the same IRQ. As a
-general rule, this wasn't allowed for the ISA bus. The PCI bus may
-share IRQs but one can't share the same IRQ between the ISA and the
-PCI bus. Most multi-port boards may share IRQs. Sharing is not as
-efficient since every time a shared interrupt is given a check must be
-made to determine where it came from. Thus if it's feasible, it's
-nicer to allocate every device its own interrupt.
-
-
-Prior to kernel 2.2, serial IRQs could not be shared with each other
-except for most multiport boards. Starting with kernel 2.2 serial
-IRQs may be sometimes shared between serial ports. In order for
-sharing to work in 2.2 the kernel must have been compiled with
-CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support
-sharing (so that if two serial cards put different voltages on the
-same interrupt wire, only the voltage that means "this is an
-interrupt" will prevail). Since the PCI bus specs permit sharing, any
-PCI card should allow sharing.
-
-
-
-
-!What IRQs to choose?
-
-
- The serial hardware often has only a limited number of IRQs. Also
-you don't want IRQ conflicts. So there may not be much of a choice.
-Your PC may normally come with ttyS0 and ttyS2 at IRQ 4, and
-ttyS1 and ttyS3 at IRQ 3. Looking at
-/proc/interrupts will show which IRQs are being used by
-programs currently running. You likely don't want to use one of
-these. Before IRQ 5 was used for sound cards, it was often used for a
-serial port.
-
-
-Here is how Greg (original author of Serial-HOWTO) set his up in
-/etc/rc.d/rc.serial. rc.serial is a file (shell script) which runs at
-start-up (it may have a different name or location). For versions of
-"setserial" after 2.15 it's not always done this way anymore but this
-example does show the choice of IRQs.
-
-
-
-
-
-/sbin/setserial /dev/ttyS0 irq 3 # my serial mouse
-/sbin/setserial /dev/ttyS1 irq 4 # my Wyse dumb terminal
-/sbin/setserial /dev/ttyS2 irq 5 # my Zoom modem
-/sbin/setserial /dev/ttyS3 irq 9 # my USR modem
-
-
-
-
-Standard IRQ assignments:
-
-IRQ 0 Timer channel 0 (May mean "no interrupt". See below.)
-IRQ 1 Keyboard
-IRQ 2 Cascade for controller 2
-IRQ 3 Serial port 2
-IRQ 4 Serial port 1
-IRQ 5 Parallel port 2, Sound card
-IRQ 6 Floppy diskette
-IRQ 7 Parallel port 1
-IRQ 8 Real-time clock
-IRQ 9 Redirected to IRQ2
-IRQ 10 not assigned
-IRQ 11 not assigned
-IRQ 12 not assigned
-IRQ 13 Math coprocessor
-IRQ 14 Hard disk controller 1
-IRQ 15 Hard disk controller 2
-
-
-
-
-
-
-There is really no Right Thing to do when choosing interrupts. Try to
-find one that isn't being used by the motherboard, or any other
-boards. 2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices. Note
-that IRQ 2 is the same as IRQ 9. You can call it either 2 or 9, the
-serial driver is very understanding. If you have a very old serial
-board it may not be able to use IRQs 8 and above.
-
-
-Make sure you don't use IRQs 1, 6, 8, 13 or 14! These are used by
-your motherboard. You will make her very unhappy by taking her IRQs.
-When you are done you might want to double-check
-/proc/interrupts when programs that use interrupts are being
-run and make sure there are no conflicts.
-
-
-
-
-!! 7.8 Choosing Addresses --Video card conflict with ttyS3
-
-
-
- Here's a problem with some old serial cards. The IO address of
-the IBM 8514 video board (and others like it) is allegedly 0x?2e8
-where ? is 2, 4, 8, or 9. This may conflict with the IO address of
-ttyS3 at 0x02e8. Your may think that this shouldn't happen since
-the addresses are different in the high order digit (the leading 0 in
-02e8). You're right, but a poorly designed serial port may ignore the
-high order digit and respond to any address that ends in 2e8. That is
-bad news if you try to use ttyS3 (ISA bus) at this IO address.
-
-
-For the ISA bus you should try to use the default addresses shown
-below. PCI cards use different addresses so as not to conflict with
-ISA addresses. The addresses shown below represent the first address
-of an 8-byte range. For example 3f8 is really the range 3f8-3ff.
-Each serial device (as well as other types of devices that use IO
-addresses) needs its own unique address range. There should be no
-overlaps (conflicts). Here are the default addresses for commonly
-used serial ports on the ISA bus:
-
-
-
-
-
-ttyS0 address 0x3f8
-ttyS1 address 0x2f8
-ttyS2 address 0x3e8
-ttyS3 address 0x2e8
-
-
-
-
-Suppose there is an address conflict (as reported by setserial -g
-/dev/ttyS*) between a real serial port and another port which
-does not physically exist (and shows UART: unknown). Such a conflict
-shouldn't cause problems but it sometimes does in older kernels. To
-avoid this problem don't permit such address conflicts or delete
-/dev/ttySx if it doesn't physically exist.
-
-
-
-
-!! 7.9 Set IO Address & IRQ in the hardware (mostly for PnP)
-
-
-
- After it's set in the hardware don't forget to insure that it also
-gets set in the driver by using setserial. For non-PnP serial
-ports they are either set in hardware by jumpers or by running a DOS
-program ("jumperless") to set them (it may disable PnP). The rest of
-this subsection is only for PnP serial ports. Here's a list of the
-possible methods of configuring PnP serial ports:
-
-
-
-
-
-* Using a PnP BIOS CMOS setup menu
-(usually only for external
-devices
-on ttyS0 (Com1) and ttyS1 (Com2))
-*
-
-* Letting a PnP BIOS automatically configure a PnP serial port
-See
-Using a PnP BIOS to I0-IRQ Configure
-*
-
-* Doing nothing if the serial driver recognized your card OK
-*
-
-* Using isapnp for a PnP serial port non-PCI)
-*
-
-* Using setpci (pciutils or pcitools) for the PCI bus
-*
-
-
-
-The IO address and IRQ must be set (by PnP) in their registers each
-time the system is powered on since PnP hardware doesn't remember how
-it was set when the power is shut off. A simple way to do this is to
-let a PnP BIOS know that you don't have a PnP OS and the BIOS will
-automatically do this each time you start. This might cause problems
-in Windows (which is a PnP OS) if you start Windows with the BIOS
-thinking that Windows is not a PnP OS. See Plug-and-Play-HOWTO.
-
-
-Plug-and-Play (PnP) was designed to automate this io-irq configuring,
-but for Linux it initially made life much more complicated. In modern
-Linux (2.4 kernels --partially in 2.2 kernels), each device driver has
-to do it's own PnP (using supplied software which it may utilize).
-There is unfortunately no centralized planning for assigning IO
-addresses and IRQs as there is in MS Windows. But it usually works
-out OK in Linux anyway.
-
-
-
-
-! Using a PnP BIOS to I0-IRQ Configure
-
-
- While the explanation of how to use setpci or isapnp for io-irq
-configuring should come with such software, this is not the case if
-you want to let a PnP BIOS do such configuring. Not all PnP BIOS can
-do this. The BIOS usually has a CMOS menu for setting up the first
-two serial ports. This menu may be hard to find. For an "Award"
-BIOS it was found under "chipset features setup" There is often
-little to choose from. Unless otherwise indicated in a menu, these
-first two ports normally get set at the standard IO addresses and
-IRQs. See
-Serial Port Device Names & Numbers
-
-Whether you like it or not, when you start up a PC a PnP BIOS starts
-to do PnP (io-irq) configuring of hardware devices. It may do the job
-partially and turn the rest over to a PnP OS (which Linux is in some
-sense) or if thinks you don't have a PnP OS it may fully configure all
-the PnP devices but not configure the device drivers. This is what
-you want but it's not always easy to figure out exactly what the PnP
-BIOS has done.
-
-
-If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS
-should do the configuring of all PnP serial ports --not just the first
-two. An indirect way to control what the BIOS does (if you have
-Windows 9x on the same PC) is to "force" a configuration under
-Windows. See Plug-and-Play-HOWTO and search for "forced". It's
-easier to use the CMOS BIOS menu which may override what you
-"forced" under Windows. There could be a BIOS option that can set or
-disable this "override" capability.
-
-
-If you add a new PnP device, the BIOS should PnP configure it. It
-could even change the io-irq of existing devices if required to avoid
-any conflicts. For this purpose, it keeps a list of non-PnP devices
-provided that you have told the BIOS how these non-PnP devices are
-io-irq configured. One way to tell the BIOS this is by running a
-program called ICU under DOS/Windows.
-
-
-But how do you find out what the BIOS has done so that you set up the
-device drivers with this info? The BIOS itself may provide some info,
-either in its setup menus of via messages on the screen when you turn
-on your computer. See
-What is set in my serial port hardware?. Other ways of finding out is to use lspci for
-the PCI bus or isapnp --dumpregs for the ISA bus. The cryptic results
-it shows you may not be clear to a novice.
-
-
-
-
-!!7.10 Giving the IRQ and IO Address to Setserial
-
-
-
- Once you've set the IRQ and IO address in the hardware (or arranged
-for it to be done by PnP) you also need to insure that the "setserial"
-command is run each time you start Linux. See the subsection
-Boot-time Configuration
-
-
-
-
-
-----
-
-!! 8. Configuring the Serial Driver (high-level) "stty"
-
-!!8.1 Overview
-
-
-
-See the section
-Stty. The "stty" command
-sets many things such as flow control, speed, and parity. The only
-one discussed in this section is flow control.
-
-
-
-
-!!8.2 Flow Control
-
-
-
- Configuring Flow Control: Hardware Flow Control is Usually Best
-See
-Flow Control for an explanation of
-it. It's usually better to use hardware flow control rather than
-software flow control using Xon/Xoff. To use full hardware flow
-control you must normally have two wires for it in the cable between
-the serial port and the device. If the device is on a card or the
-motherboard, then it should always be possible to use hardware flow
-control.
-
-
-Many applications (and the getty program) give you an option
-regarding flow control and will set it for you. It might even set
-hardware flow control by default. It must be set both in the serial
-driver and in the hardware connected to the serial port. How it's set
-into the hardware is hardware dependent. Sometimes there is a certain
-"init string" you send to the hardware device via the serial port from
-your PC. For a modem, the communication program should set it in both
-places.
-
-
-If a program you use doesn't set flow control in the serial driver,
-then you may do it yourself using the stty command. Since the
-driver doesn't remember the setting after you stop Linux, you could
-put the stty command in a file that runs at start-up or when you login
-(such as /etc/profile for the bash shell). Here's what you would add
-for hardware flow control for port ttyS2:
-
-
-
-
-
-stty crtscts < /dev/ttyS2
-or for stty version >= 1.17:
-stty -F /dev/ttyS2 crtscts
-
-
-
-
-crtscts stands for a Control setting to use the RTS and CTS pins of
-the serial port for hardware flow control. Note that RTS+CTS almost
-spells: crtscts.
-
-
-
-----
-
-!! 9. Serial Port Devices /dev/ttyS2, etc.
-
-
- For creating devices in the device directory see:
-
-
-
-Creating Devices In the /dev directory
-
-
-
-!!9.1 Devfs (The new Device File System)
-
-
-
- This is a new type of device interface to Linux. It's optional
-starting with kernel 2.4. It's more efficient than the conventional
-interface and makes it easy to deal with a huge number of devices.
-The device names have all changed as well. But there's an option to
-continue using the old names. For a detailed description of it see:
-http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html
-
-The name changes (if used) are: ttyS2 becomes tts/2 (Serial port),
-tty3 becomes vc/3 (Virtual Console), ptyp1 becomes pty/m1 (PTY
-master), ttyp2 becomes pty/s2 (PTY slave). "tts" looks like a
-directory which contains devices "files": , 1, 2, etc. All of these
-new names should still be in the /dev directory although optionally
-one may put them elsewhere.
-
-
-
-
-!! 9.2 Serial Port Device Names & Numbers
-
-
-
- Devices in Linux have major and minor numbers (unless you use the
-new devfs). The serial port ttySx (x=,1,2, etc.) has major number 4.
-You may see this (and the minor numbers too) by typing: "ls -l ttyS*"
-in the /dev directory.
-
-
-There formerly was a "cua" name for each serial port and it behaved
-just a little differently. For example, ttyS2 would correspond to
-cua2. The cua major number was 5 and minor numbers started at 64.
-You may still have the cua devices in your /dev directory but they are
-now deprecated. Their drivers behave slightly different than for the
-ttyS ones. name="cua Device Obsolete">."') See the Modem-HOWTO section: "cua Device
-Obsolete">.
-
-
-Dos/Windows use the COM name while the setserial program uses
-tty00, tty01, etc. Don't confuse these with dev/tty0, dev/tty1, etc.
-which are used for the console (your PC monitor) but are not serial
-ports. The table below is for the "standard" case (but yours could be
-different). The major/minor numbers don't exist with the devfs.
-
-
-
-
-
-ISA IO devfs usb
-dos major minor address devfs devfs usb acm modem
-COM1 /dev/ttyS0 4, 64; 3F8 /dev/tts/0 /dev/usb/tts/0 /dev/usb/acm/
-COM2 /dev/ttyS1 4, 65; 2F8 /dev/tts/1 /dev/usb/tts/1 /dev/usb/acm/1
-COM3 /dev/ttyS2 4, 66; 3E8 /dev/tts/2 /dev/usb/tts/2 /dev/usb/acm/2
-COM4 /dev/ttyS3 4, 67; 2E8 /dev/tts/3 /dev/usb/tts/3 /dev/usb/acm/3
-
-
-
-
-
-
-!!9.3 USB (Universal Serial Bus) Ports
-
-
-
-The serial ports on the USB are: /dev/ttyUSB0, /dev/ttyUSB1, etc.
-The devfs names for these are: /dev/usb/tts/, /dev/usb/tts/1, etc.
-For many modems they are /dev/usb/acm/, etc. (in devfs notation).
-For more info see the usb subdirectory in the kernel documentation
-directory for files: acm and usb-serial.
-
-
-
-
-!!9.4 Link ttySN to /dev/modem
-
-
-
- On some installations, two extra devices will be created,
-/dev/modem for your modem and /dev/mouse for a
-mouse. Both of these are symbolic links to the appropriate serial
-device in /dev which you specified during the installation
-Except if you have a bus mouse, then /dev/mouse will point to
-the bus mouse device).
-
-
-Formerly (in the 1990s) the use of /dev/modem was discouraged
-since lock files might not realize that it was really say
-/dev/ttyS2. The newer lock file system doesn't fall into
-this trap so it's now OK to use such links.
-
-
-
-
-
-
-
-
-
-
-!!9.5 Which Connector on the Back of my PC is ttyS1, etc?
-
-
-!Inspect the connectors
-
-
- Inspecting the connectors may give some clues but is often not
-definitive. The serial connectors on the back side of a PC are
-usually DB connectors with male pins. 9-pin is the most common but
-some are 25-pin (especially older PCs like 486s). There may be one
-9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??). For two
-9-pin ones the top one might be ttyS0.
-
-
-If you only have one serial port connector on the back of your PC,
-this may be easy. If you also have an internal modem, a program like
-wvdial may be able to tell you what port it's on (unless it's a PnP
-that hasn't been PnP configured yet). A report from setserial (at
-boot-time or run by you from the command line) should help you
-identify the non-modem port.
-
-
-If you have two serial connectors it may be more difficult.
-First check manuals (if any) for your computer. Look at the
-connectors for meaningful labels. You might even want to take off the
-PC's cover and see if there are any meaningful labels on the card
-where the internal ribbon cables plug in. Labels (if any) are likely
-to say something like "serial 1", "serial 2" or A, B. Which com port it
-actually is will depend on jumper or PnP settings (sometimes shown in
-a CMOS setup menu). But 1 or A are more likely to be ttyS0 with 2 or
-B ttyS1.
-
-
-
-
-!Send bytes to the port
-
-
- Labels are not apt to be definitive so here's another method. If
-the serial ports have been configured correctly per setserial, then
-you may send some bytes out a port and try to detect which connector
-(if any) it's coming out of. One way to send such a signal is to copy
-a long text file to the port using a command like: cp my_file_name
-/dev/ttyS1. A voltmeter connected to the DTR pin (see Serial-HOWTO
-for Pinout) will display positive as soon as you give the copy
-command.
-
-
-The transmit pin should go from several volts negative to a voltage
-fluctuating around zero after you start sending the bytes. If it doesn't
-(but the DTR went positive) then you've got the right port but it's
-blocked from sending. This may be due to a wrong IRQ, -clocal being
-set, etc. The command "stty -F /dev/ttyS1 -a" should show
-clocal (and not -clocal). If not, change it to clocal.
-
-
-Another test is to jumper the transmit and receive pins (pins 2 and 3
-of either the 25-pin or 9-pin connector) of a test serial port. Then
-send something to each port (from the PCs keyboard) and see if it gets
-sent back. If it does it's likely the port with the jumper on it.
-Then remove the jumper and verify that nothing gets sent back. Note
-that if "echo" is set (per stty) then a jumper creates an infinite
-loop. Bytes that pass thru the jumper go into the port and come right
-back out of the other pin back to the jumper. Then they go back in
-and out again and again. Whatever you send to the port repeats itself
-forever (until you interrupt it by removing the jumper, etc.). This
-may be a good way to test it as the repeating test messages halt when
-the jumper is removed.
-
-
-As a jumper you could use a mini (or micro) jumper cable and perhaps
-use a scrap of paper to prevent the mini clips from accidentally
-touching the metal of the connector. Whatever you use as a jumper
-take care not to bend or excessively scratch the pins. To receive
-something from a port, you can go to a virtual terminal (Alt-F2 for
-example) and type something like "cp /dev/ttyS2 /dev/tty". Then at
-another virtual terminal you may send something to ttyS2 (or whatever)
-by "echo test_message > /dev/ttyS2". Then go back to the receive
-virtual terminal and look for the test_message. See
-Serial Electrical Test Equipment for
-more info.
-
-
-
-
-!Connect a device to the connector
-
-
- Another way to try to identify a serial port is to connect some
-physical serial device to it and see if it works. But a problem here
-is that it might not work because it's not configured right. A serial
-mouse might get detected if connected.
-
-
-
-
-!Missing connectors
-
-
- If the software shows that you have more serial ports than you
-have connectors for (including an internal modem which counts as a
-serial port) then you may have a serial port that has no connector.
-Some motherboards come with a serial port with no cable or serial DB
-connector. Someone may build a PC from this and omit the connector.
-There may be a "serial" label on the motherboard but no ribbon cable
-connects to the pins next to this label. To use this port you must
-get a ribbon cable/connector. I've seen different wiring arrangements
-for such ribbon cables so beware.
-
-
-
-
-!! 9.6 Creating Devices In the /dev directory
-
-
-
-If you don't have a device "file" that you need, you will have to
-create it with the mknod command or with the MAKEDEV shell
-script. Example, suppose you needed to create ttyS0:
-
-
-linux# mknod -m 666 /dev/ttyS0 c 4 64
-
-
-You can use the MAKEDEV script, which lives in /dev.
-See the man page for it. This simplifies the making of devices. For
-example, if you needed to make the devices for ttyS0 you
-would type:
-
-
-linux# cd /dev
-linux# ./MAKEDEV ttyS0
-
-
-This handles the devices creation and should set the correct permissions.
-For making multiport devices see
-Making multiport devices in the /dev directory.
-
-
-
-----
-
-!!10. Interesting Programs You Should Know About
-
-
- Most info on getty has been moved to Modem-HOWTO with a little info on
-the use of getty with directly connected terminals now found in
-Text-Terminal-HOWTO.
-
-
-
-
-!! 10.1 Serial Monitoring/Diagnostics Programs
-
-
-
- A few Linux programs (and one "file") will monitor various modem
-control lines and indicate if they are positive (1 or green) or
-negative (0 or red).
-
-
-* The "file": /proc/tty/driver/serial lists those that are positive
-*
-
-* modemstat (Only works correctly on Linux PC consoles. Status
-monitored in a tiny window. Color-coded and compact. Must kill
-it (a process) to quit.
-*
-
-* statserial (Info displayed on entire screen)
-*
-
-* serialmon (Doesn't monitor RTS, CTS, DSR but logs other
-functions)
-*
-
-You may already have them. If not, download them from
-Serial Software. As of June 1998, I know of no diagnostic program in Linux
-for the serial port.
-
-
-
-
-!!10.2 Changing Interrupt Priority
-
-
-
-
-
-
-* irqtune will give serial port interrupts higher
-priority to improve performance.
-*
-
-* hdparm for hard-disk tuning may help some more.
-*
-
-
-
-
-
-!! 10.3 What is Setserial ?
-
-
-
- This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal. There
-are some minor differences, depending on which HOWTO it appears in.
-
-
-
-
-!Introduction
-
-
- If you have a Laptop (PCMCIA) don't use setserial until you
-read
-Laptops: PCMCIA. setserial is a
-program which allows you to tell the device driver software the I/O
-address of the serial port, which interrupt (IRQ) is set in the port's
-hardware, what type of UART you have, etc. Since there's a good
-chance that the serial ports will be automatically detected and set,
-many people never need to use setserial. In any case setserial
-will not work without either serial support built into the kernel or
-loaded as a module. The module may get loaded automatically if you
-(or a script) tries to use setserial.
-
-
-Setserial can also show how the driver is currently set. In addition,
-it can be made to probe the hardware I0 port addresses to try to
-determine the UART type and IRQ, but this has severe limitations. See
-Probing. Note that it can't set the IRQ
-or the port address in the hardware of PnP serial ports (but the
-plug-and-play features of the serial driver may do this). It can't
-read the PnP data stored in configuration registers in the hardware.
-
-
-If you only have one or two built-in serial ports, they will usually
-get set up correctly without using setserial. Otherwise, if you add
-more serial ports (such as a modem card) you may need to deal with
-setserial. Besides the man page for setserial, check out info in
-/usr/doc/setserial.../ or /usr/share/doc/setserial.
-It should tell you how setserial is handled in your distribution of
-Linux.
-
-
-Setserial is often run automatically at boot-time by a start-up
-shell-script for the purpose of assigning IRQs, etc. to the driver.
-Setserial will only work if the serial module is loaded (or if the
-equivalent was compiled into your kernel). If the serial module gets
-unloaded later on, the changes previously made by setserial will
-be forgotten by the kernel. But recent (2000) distributions may
-contain scripts that save and restore this. If not, then
-setserial must be run again to reestablish them. In addition to
-running via a start-up script, something akin to setserial also
-runs earlier when the serial module is loaded (or the like). Thus
-when you watch the start-up messages on the screen it may look like it
-ran twice, and in fact it has.
-
-
-Setserial can set the time that the port will keep operating after
-it's closed (in order to output any characters still in its buffer in
-main RAM). This is needed at slow baud rates of 1200 or lower. It's
-also needed at higher speeds if there are a lot of "flow control"
-waits. See "closing_wait" in the setserial man page.
-
-
-
-
-
-Setserial does not set either IRQ's nor I/O addresses in the serial
-port hardware itself. That is done either by jumpers or by
-plug-and-play. You must tell setserial the identical values that have
-been set in the hardware. Do not just invent some values that you
-think would be nice to use and then tell them to setserial. However,
-if you know the I/O address but don't know the IRQ you may command
-setserial to attempt to determine the IRQ.
-
-
-You can see a list of possible commands by just typing setserial
-with no arguments. This fails to show you the one-letter options such
-as -v for verbose which you should normally use when troubleshooting.
-Note that setserial calls an IO address a "port". If you type:
-
-
-setserial -g /dev/ttyS*
-
-
-you'll see some info about how that device driver is configured for
-your ports. Note that where it says "UART: unknown" it
-probably means that no uart exists. In other words you probably have
-no such serial port and the other info shown about the port is
-meaningless and should be ignored. If you really do have such a
-serial port, setserial doesn't recognize it and that needs to be
-fixed.
-
-
-If you add -a to the option -g you will see more info although few
-people need to deal with (or understand) this additional info since
-the default settings you see usually work fine. In normal cases the
-hardware is set up the same way as "setserial" reports, but if you are
-having problems there is a good chance that "setserial" has it wrong.
-In fact, you can run "setserial" and assign a purely fictitious I/O
-port address, any IRQ, and whatever uart type you would like to have.
-Then the next time you type "setserial ..." it will display these
-bogus values without complaint. They will also be officially
-registered with the kernel as displayed (at the top of the screen) by
-the "scanport" command. Of course the serial port driver will not
-work correctly (if at all) if you attempt to use such a port. Thus
-when giving parameters to "setserial" anything goes. Well almost. If
-you assign one port a base address that is already assigned (such as
-3e8) it will not accept it. But if you use 3e9 it will accept it.
-Unfortunately 3e9 is already assigned since it is within the range
-starting at base address 3e8. Thus the moral of the story is to make
-sure your data is correct before assigning resources with setserial.
-
-
-While assignments made by setserial are lost when the PC is powered
-off, a configuration file may restore them (or a previous
-configuration) when the PC is started up again. In newer versions,
-what you change by setserial may get automatically saved to a
-configuration file. In older versions, the configuration file only
-changes if you edit it manually so the configuration always remains
-the same from boot to boot. See
-Configuration Scripts/Files
-
-
-
-! Probing
-
-
-Prior to probing with "setserial", one may run the "scanport"
-command to check all possible ports in one scan. It makes crude
-guesses as to what is on some ports but doesn't determine the IRQ. But
-it's a fast first start. It may hang your PC but so far it's worked
-fine for me.
-
-
-With appropriate options, setserial can probe (at a given I/O
-address) for a serial port but you must guess the I/O address. If you
-ask it to probe for /dev/ttyS2 for example, it will only probe at the
-address it thinks ttyS2 is at (2F8). If you tell setserial that ttyS2
-is at a different address, then it will probe at that address, etc.
-See
-Probing
-
-The purpose of this is to see if there is a uart there, and if so,
-what its IRQ is. Use "setserial" mainly as a last resort as there are
-faster ways to attempt it such as wvdialconf to detect modems, looking
-at very early boot-time messages, or using pnpdump
---dumpregs. To try to detect the physical hardware use for
-example :
-setserial /dev/ttyS2 -v autoconfig
-If the resulting message shows a uart type such as 16550A, then you're
-OK. If instead it shows "unknown" for the uart type, then there
-is supposedly no serial port at all at that I/O address. Some cheap
-serial ports don't identify themselves correctly so if you see
-"unknown" you still might have a serial port there.
-
-
-Besides auto-probing for a uart type, setserial can auto-probe for
-IRQ's but this doesn't always work right either. In one case it first
-gave the wrong irq but when the command was repeated it found the
-correct irq. In versions of setserial >= 2.15, the results of your
-last probe test could be automatically saved and put into the
-configuration file /etc/serial.conf which will be used next
-time you start Linux. At boot-time when the serial module loads (or
-the like), a probe for UARTs is made automatically and reported on the
-screen. But the IRQs shown may be wrong. The second report of the
-same is the result of a script which usually does no probing and thus
-provides no reliable information as to how the hardware is actually
-set. It only shows configuration data someone wrote into the script
-or data that got saved in /etc/serial.conf.
-
-
-It may be that two serial ports both have the same IO address set in
-the hardware. Of course this is not permitted but it sometimes
-happens anyway. Probing detects one serial port when actually there
-are two. However if they have different IRQs, then the probe for IRQs
-may show IRQ = . For me it only did this if I first used
-setserial to give the IRQ a fictitious value.
-
-
-
-
-
-
-
-! Boot-time Configuration
-
-
- When the kernel loads the serial module (or if the "module
-equivalent" is built into the kernel) then only ttyS{-3} are
-auto-detected and the driver is set to use only IRQs 4 and 3
-(regardless of what IRQs are actually set in the hardware). You see
-this as a boot-time message just like as if setserial had been
-run.
-
-
-To correct possible errors in IRQs (or for other reasons) there may be
-a file somewhere that runs setserial again. Unfortunately, if
-this file has some IRQs wrong, the kernel will still have incorrect
-info about the IRQs. This file should run early at boot-time before
-any process uses the serial port. In fact, your distribution may have
-set things up so that the setserial program runs automatically from a
-start-up script at boot-time. More info about how to handle this
-situation for your particular distribution might be found in file
-named "setserial..." or the like located in directory /usr/doc/ or
-/usr/share/doc/.
-
-
-Before modifying a configuration file, you can test out a "proposed"
-setserial command by just typing it on the command line. In some
-cases the results of this use of setserial will automatically get
-saved in /etc/serial.conf when you shutdown. So if it worked OK (and
-solved your problem) then there's no need to modify any configuration
-file. See
-New configuration method using /etc/serial.conf.
-
-
-
-
-! Configuration Scripts/Files
-
-
- Your objective is to modify (or create) a script file in the /etc
-tree that runs setserial at boot-time. Most distributions provide
-such a file (but it may not initially reside in the /etc tree). In
-addition, setserial 2.15 and higher often have an /etc/serial.conf
-file that is used by the above script so that you don't need to
-directly edit the script that runs setserial. In addition just using
-setserial on the command line (2.15+) may ultimately alter this
-configuration file.
-
-
-So prior to version 2.15 all you do is edit a script. After 2.15 you
-may need to either do one of three things: 1. edit a script. 2. edit
-/etc/serial.conf or 3. run "setserial" on the command line
-which may result in /etc/serial.conf automatically being
-edited. Which one of these you need to do depends on both your
-particular distribution, and how you have set it up.
-
-
-
-
-! Edit a script (required prior to version 2.15)
-
-
- Prior to setserial 2.15 (1999) there was no /etc/serial.conf file
-to configure setserial. Thus you need to find the file that runs
-"setserial" at boot time and edit it. If it doesn't exist, you need
-to create one (or place the commands in a file that runs early at
-boot-time). If such a file is currently being used it's likely
-somewhere in the /etc directory-tree. But Redhat <6.0 has supplied it
-in /usr/doc/setserial/ but you need to move it to the /etc tree before
-using it. You might use "locate" to try to find such a file. For
-example, you could type: locate "*serial*".
-
-
-The script /etc/rc.d/rc.serial was commonly used in the past.
-The Debian distribution used /etc/rc.boot/0setserial.
-Another file once used was /etc/rc.d/rc.local but it's
-not a good idea to use this since it may not be run early enough.
-It's been reported that other processes may try to open the serial
-port before rc.local runs resulting in serial communication failure.
-Today it's most likely in /etc/init.d/ but it isn't normally intended
-to be edited.
-
-
-If such a file is supplied, it should contain a number of
-commented-out examples. By uncommenting some of these and/or
-modifying them, you should be able to set things up correctly. Make
-sure that you are using a valid path for setserial, and a valid
-device name. You could do a test by executing this file manually
-(just type its name as the super-user) to see if it works right.
-Testing like this is a lot faster than doing repeated reboots to get
-it right.
-
-
-For versions >= 2.15 (provided your distribution implemented the
-change, Redhat didn't) it may be more tricky to do since the file that
-runs setserial on startup, /etc/init.d/setserial or the like was not
-intended to be edited by the user. See
-New configuration method using /etc/serial.conf.
-
-
-If you want setserial to automatically determine the uart and the IRQ
-for ttyS3 you would add something like:
-
-
-
-
-
-/sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig
-
-
-Do this for every serial port you want to auto configure. Be sure to
-give a device name that really does exist on your machine. In some
-cases this will not work right due to the hardware. If you know what
-the uart and irq actually are, you may want to assign them explicitly
-with "setserial". For example:
-
-
-
-
-
-/sbin/setserial /dev/ttyS3 irq 5 uart 16550A skip_test
-
-
-
-
-
-
-
-
-
-! New configuration method using /etc/serial.conf
-
-
- Prior to setserial version 2.15, the way to configure setserial
-was to manually edit the shell-script that ran setserial at boot-time.
-See
-Edit a script (after version 2.15: perhaps not). Starting with version 2.15 (1999) of setserial
-this shell-script is not edited but instead gets its data from a
-configuration file: /etc/serial.conf. Furthermore you may
-not even need to edit serial.conf because using the "setserial"
-command on the command line may automatically cause serial.conf to be
-edited appropriately.
-
-
-This was intended so that you don't need to edit any file in order to
-set up (or change) what setserial does each time that Linux is booted.
-But there are serious pitfalls because it's not really "setserial"
-that edits serial.conf. Confusion is compounded because different
-distributions handle this differently. In addition, you may modify it
-so that it works differently.
-
-
-What often happens is this: When you shut down your PC the script
-that runs "setserial" at boot-time is run again, but this time it only
-does what the part for the "stop" case says to do: It uses
-"setserial" to find out what the current state of "setserial" is, and
-it puts that info into the serial.conf file. Thus when you
-run "setserial" to change the serial.conf file, it doesn't get changed
-immediately but only when and if you shut down normally.
-
-
-Now you can perhaps guess what problems might occur. Suppose you
-don't shut down normally (someone turns the power off, etc.) and the
-changes don't get saved. Suppose you experiment with "setserial" and
-forget to run it a final time to restore the original state (or make a
-mistake in restoring the original state). Then your "experimental"
-settings are saved.
-
-
-If you manually edit serial.conf, then your editing is destroyed when
-you shut down because it gets changed back to the state of setserial
-at shutdown. There is a way to disable the changing of serial.conf at
-shutdown and that is to remove "###AUTOSAVE###" or the like from first
-line of serial.conf. In at least one distribution, the removal of
-"###AUTOSAVE###" from the first line is automatically done after the
-first time you shutdown just after installation. The serial.conf file
-should contain some comments to explain this.
-
-
-The file most commonly used to run setserial at boot-time (in
-conformance with the configuration file) is now /etc/init.d/setserial
-(Debian) or /etc/init.d/serial (Redhat), or etc., but it should not
-normally be edited. For 2.15, Redhat 6.0 just had a file
-/usr/doc/setserial-2.15/rc.serial which you have to move to
-/etc/init.d/ if you want setserial to run at boot-time.
-
-
-To disable a port, use setserial to set it to
-"uart none". The format of /etc/serial.conf appears to be just like
-that of the parameters placed after "setserial" on the command line
-with one line for each port. If you don't use autosave, you may edit
-/etc/serial.conf manually.
-
-
-BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE###
-only the setserial parameters displayed by "setserial -Gg /dev/ttyS*"
-get saved but the other parameters don't get saved. Use the -a flag
-to "setserial" to see all parameters. This will only affect a small
-minority of users since the defaults for the parameters not saved are
-usually OK for most situations. It's been reported as a bug and may
-be fixed by now.
-
-
-In order to force the current settings set by setserial to be saved to
-the configuration file (serial.conf) without shutting down, do what
-normally happens when you shutdown: Run the shell-script
-/etc/init.d/{set}serial stop. The "stop" command will save
-the current configuration but the serial ports still keep working OK.
-
-
-In some cases you may wind up with both the old and new configuration
-methods installed but hopefully only one of them runs at boot-time.
-Debian labeled obsolete files with "...pre-2.15".
-
-
-
-
-!IRQs
-
-
- By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
-ttyS3 share IRQ 3. But actually sharing serial interrupts (using them
-in running programs) is not permitted unless you: 1. have kernel 2.2
-or better, and 2. you've complied in support for this, and 3. your
-serial hardware supports it. See
-
-
-
-Interrupt sharing and Kernels 2.2+
-
-If you only have two serial ports, ttyS0 and ttyS1, you're still OK
-since IRQ sharing conflicts don't exist for non-existent devices.
-
-
-If you add an internal modem and retain ttyS0 and ttyS1,
-then you should attempt to find an unused IRQ and set it both on your
-serial port (or modem card) and then use setserial to assign it to
-your device driver. If IRQ 5 is not being used for a sound card, this
-may be one you can use for a modem. To set the IRQ in hardware you
-may need to use isapnp, a PnP BIOS, or patch Linux to make it PnP. To
-help you determine which spare IRQ's you might have, type "man
-setserial" and search for say: "IRQ 11".
-
-
-
-
-! Laptops: PCMCIA
-
-
-If you have a Laptop, read PCMCIA-HOWTO for info on the serial
-configuration. For serial ports on the motherboard, setserial is used
-just like it is for a desktop. But for PCMCIA cards (such as a modem)
-it's a different story. The configuring of the PCMCIA system should
-automatically run setserial so you shouldn't need to run it. If you
-do run it (by a script file or by /etc/serial.conf) it might be
-different and cause trouble. The autosave feature for serial.conf
-shouldn't save anything for PCMCIA cards (but Debian did until
-2.15-7). Of course, it's always OK to use setserial to find out how
-the driver is configured for PCMCIA cards.
-
-
-
-
-
-
-
-
-
-
-!! 10.4 Stty
-
-
-!Introduction
-
-
- stty does much of the configuration of the serial port but
-since application programs (and the getty program) often handle it,
-you may not need to use it much. It's handy if you're having problems
-or want to see how the port is set up. Try typing ``stty -a'' at your
-terminal/console to see how it's now set. Also try typing it without
-the -a (all) for a short listing which shows how it's set different
-than normal. Don't try to learn all the setting unless you want to
-become a serial guru. Most of the defaults should work OK and some of
-the settings are needed only for certain obsolete dumb terminals made
-in the 1970's.
-
-
-stty is documented in the man pages with a more detailed account
-in the info pages. Type "man stty" or "info stty".
-
-
-Whereas setserial only deals with actual serial ports, stty is
-used both for serial ports and for virtual terminals such as the standard
-Linux text interface at a PC monitor. For the PC monitor, many of the
-stty settings are meaningless. Changing the baud rate, etc. doesn't
-appear to actually do anything.
-
-
-Here are some of the items stty configures: speed (bits/sec), parity,
-bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
-control, break signal, end-of-line markers, change case, padding, beep
-if buffer overrun?, echo what you type to the screen, allow background
-tasks to write to terminal?, define special (control) characters (such
-as what key to press for interrupt). See the stty man or info
-page for more details. Also see the man page: termios which
-covers the same options set by stty but (as of mid 1999) covers
-features which the stty man page fails to mention.
-
-
-
-
-
-With some implementations of getty (getty_ps package), the commands
-that one would normally give to stty are typed into a getty
-configuration file: /etc/gettydefs. Even without this configuration
-file, the getty command line may be sufficient to set things up so
-that you don't need stty.
-
-
-One may write C programs which change the stty configuration, etc.
-Looking at some of the documentation for this may help one better
-understand the use of the stty command (and its many possible
-arguments). Serial-Programming-HOWTO is useful. The manual page:
-termios contains a description of the C-language structure (of type
-termios) which stores the stty configuration in computer memory. Many
-of the flag names in this C-structure are almost the same (and do the
-same thing) as the arguments to the stty command.
-
-
-
-
-!Flow control options
-
-
- To set hardware flow control use "crtscts". For software flow
-control there are 3 settings: ixon, ixoff, and ixany.
-
-
-ixany: Mainly for terminals. Hitting any key will restarts the flow
-after a flow-control stop. If you stop scrolling with the "stop
-scroll" key (or the like) then hitting any key will resume scrolling.
-It's seldom needed since hitting the "scroll lock" key again will do
-the same thing.
-
-
-ixon: Enables the port to listen for Xoff and to stop transmitting
-when it gets an Xoff. Likewise, it will resume transmitting if it gets
-an Xon.
-
-
-ixoff: enables the port to send the Xoff signal out the transmit line
-when its buffers in main memory are nearly full. It protects the
-device where the port is located from being overrun.
-
-
-For a slow dumb terminal (or other slow device) connected to a fast
-PC, it's unlikely the the PC's port will be overrun. So you seldom
-actually need to enable ixoff. But it's often enabled "just in case".
-
-
-
-
-!Using stty at a "foreign" terminal
-
-
- Using stty to configure the terminal that you are currently
-using is easy. Doing it for a different (foreign) terminal or serial
-port may be impossible. For example, let's say you are at the PC monitor
-(tty1) and want to use stty to deal with the serial port ttyS2.
-Prior to about 2000 you needed to use the redirection operator "<".
-After 2000 (provided your version of setserial is >= 1.17 and stty >=
-2.) there is a better method using the -F option. This will work
-when the old redirection method fails. Even with the latest versions
-be warned that if there is a terminal on ttyS2 and a shell is running
-on that terminal, then what you see will likely be deceptive and
-trying to set it will not work. See
-Two interfaces at a terminal to understand it.
-
-
-The new method is ``stty -F /dev/ttyS2 ...'' (or --file instead of F).
-If ... is -a it displays all the stty settings. The old redirection
-method (which still works in later versions) is to type ``stty ... </dev/ttyS2''. If the new method works but the old one hangs, it
-implies that the port is hung due to a modem control line not being
-asserted. Thus the old method is still useful for troubleshooting.
-See the following subsection for details.
-
-
-
-
-!Old redirection method
-
-
- Here's a problem with the old redirection operator (which doesn't
-happen if you use the newer -F option instead). Sometimes when trying
-to use stty, the command hangs and nothing happens (you don't get a
-prompt for a next command even after hitting <return>). This is
-likely due to the port being stuck because it's waiting for one of the
-modem control lines to be asserted. For example, unless you've set
-"clocal" to ignore modem control lines, then if no CD signal is
-asserted the port will not open and stty will not work for it (unless
-you use the newer -F option). A similar situation seems to exist for
-hardware flow control. If the cable for the port doesn't even have a
-conductor for the pin that needs to be asserted then there is no easy
-way to stop the hang.
-
-
-One way to try to get out of the above hang is to use the newer -F
-option and set "clocal" and/or "crtscts" as needed. If you don't have
-the -F option then you may try to run some program (such a minicom) on
-the port that will force it to operate even if the control lines say
-not to. Then hopefully this program might set the port so it doesn't
-need the control signal in the future in order to open: clocal or
--crtscts. To use "minicom" to do this you likely will have to
-reconfigure minicom and then exit it and restart it. Instead of all
-this bother, it may be simpler to just reboot the PC.
-
-
-The old redirection method makes ttyS2 the standard input to stty.
-This gives the stty program a link to the "file" ttyS2 so that it may
-"read" it. But instead of reading the bytes sent to ttyS2 as one
-might expect, it uses the link to find the configuration settings of
-the port so that it may read or change them. Some people tried to use
-``stty ... > /dev/ttyS2'' to set the terminal. This will not do it.
-Instead, it takes the message normal displayed by the stty command for
-the terminal you are on (say tty1) and sends this message to ttyS2.
-But it doesn't change any settings for ttyS2.
-
-
-
-
-! Two interfaces at a terminal
-
-
- When using a shell (such as bash) with command-line-editing
-enabled there are two different terminal interfaces (what you see when
-you type stty -a). When you type in modern shells at the command line
-you have a temporary "raw" interface (or raw mode) where each
-character is read by the command-line-editor as you type it. Once you
-hit the <return> key, the command-line-editor is exited and the
-terminal interface is changed to the nominal "cooked" interface
-(cooked mode) for the terminal. This cooked mode lasts until the next
-prompt is sent to the terminal (which is only a small fraction of a
-second). Note that one never gets to type anything to this cooked
-mode but what was typed in raw mode gets executed while in cooked
-mode.
-
-
-When a prompt is sent to the terminal, the terminal goes from "cooked"
-to "raw" mode (just like it does when you start an editor since you
-are starting the command-line editor). The settings for the "raw"
-mode are based only on the basic settings taken from the "cooked"
-mode. Raw mode keeps these setting but changes several other settings
-in order to change the mode to "raw". It is not at all based on the
-settings used in the previous "raw" mode. Thus if one uses stty to
-change settings for the raw mode, such settings will be permanently
-lost as soon as one hits the <return> key at the terminal that
-has supposedly been "set".
-
-
-Now when one types stty to look at the terminal interface, one may
-either get a view of the cooked mode or the raw mode. You need to
-figure out which one you're looking at. It you use stty from another
-(foreign) terminal then you will see the raw mode settings. Any
-changes made will only be made to the raw mode and will be lost when
-someone presses <return> at the terminal you tried to "set".
-But if you type a stty command at your terminal (without the -F option
-or redirection) and then hit <return> it's a different story.
-The <return> puts the terminal in cooked mode. Your changes are
-saved and will still be there when the terminal goes back into raw
-mode (unless of course it's a setting not allowed in raw mode).
-
-
-This situation can create problems. For example, suppose you corrupt
-your terminal interface. To restore it you go to another terminal and
-"stty -F dev/ttyS1 sane" (or the like). It will not work! Of course
-you can try to type "stty sane ..." at the terminal that is corrupted
-but you can't see what you typed. All the above not only applies to
-dumb terminals but to virtual terminals used on a PC Monitor as well
-as to the terminal windows in X. In other words, it applies to almost
-everyone who uses Linux.
-
-
-Luckily, when you start up Linux, any file that runs stty at boot-time
-will likely deal with a terminal (or serial port with no terminal)
-that has no shell running on it so there's no problem for this special
-case.
-
-
-
-
-! Where to put the stty command ?
-
-
- Should you need to have stty set up the serial interface each
-time the computer starts up then you need to put the stty command
-in a file that will be executed each time the computer is started up
-(Linux boots). It should be run before the serial port is used
-(including running getty on the port). There are many possible places
-to put it. If it gets put in more than one place and you only know
-about (or remember) one of those places, then a conflict is likely.
-So make sure to document what you do.
-
-
-One place to put it would be in the same file that runs setserial when
-the system is booted. The location is distribution and version
-dependent. It would seem best to put it after the setserial command
-so that the low level stuff is done first. If you have directories in
-the /etc tree where every file in them is executed at boot-time
-(System V Init) then you could create a file named "stty" for this
-purpose.
-
-
-
-
-
-
-
-!!10.5 What is isapnp ?
-
-
-
- isapnp is a program to configure Plug-and-Play (PnP) devices
-on the ISA bus including internal modems. It comes in a package
-called "isapnptools" and includes another program, "pnpdump" which
-finds all your ISA PnP devices and shows you options for configuring
-them in a format which may be added to the PnP configuration file:
-/etc/isapnp.conf. The isapnp command may be put into a startup file
-so that it runs each time you start the computer and thus will
-configure ISA PnP devices. It is able to do this even if your BIOS
-doesn't support PnP. See Plug-and-Play-HOWTO.
-
-
-
-
-!!10.6 What is slattach?
-
-
-
- It's "serial line attach". It puts the serial line into a
-networking mode. You can thus network two computers together via a
-serial line using, for example, the slip protocol. But for the ppp
-protocol, you need to start pppd on the serial line.
-
-
-
-----
-
-!! 11. Speed (Flow Rate)
-
-
- By "speed" we really mean the "data flow rate" but almost everybody
-incorrectly calls it speed. The speed is measured in bits/sec (or
-baud). Speed is set using the "stty" command or by a program which
-uses the serial port. See
-Stty
-
-
-
-!!11.1 Can't Set a High Enough Speed
-
-
-!Speeds over 115.2k
-
-
- The top speed of 115.2k has been standard since the mid 1990's.
-But by the year 2000, most new serial ports supported higher speeds of
-230.4k and 460.8k. Some also support 921.6k. Unfortunately Linux
-seldom uses these speeds due to lack of drivers. Thus such ports
-behave just like 115.2k ports unless the higher speeds are enabled by
-special software. To get these speeds you need to compile the kernel
-with special patches but it seems that the 2.4 kernels are not yet
-supported
-
-
-The patch software is fairly simple since it only needs to enable the
-higher speeds by dialog with the hardware. But it's not quite as
-simple as just putting an enable byte in a hardware register since the
-registers weren't designed for this. But unfortunately, there is no
-standard way to enable the higher speeds so the serial driver needs to
-support a variety of hardware.
-
-
-A patch to support high-speed is called shsmod (Super High Speed
-Mode). There are both Windows and Linux versions of this patch. See
-http://www.devdrv.com/shsmod/. For Linux (as of late
-2001), most of the documentation is only in Japanese and the patch is
-for the old kernel 2.2.x. There is also a module for the VIA
-VT82C686 chip
-http://www.kati.fi/viahss/. Using it may
-result in buffer overflow.
-
-
-For internal modems, only a minority of them advertise that they
-support speeds of over 115.2k for their built-in serial ports.
-Does shsmod support these ??
-
-
-
-
-! How speed is set in hardware: the divisor and baud_base
-
-
- Speed is set by having the serial port's clock change frequency.
-But this change happens not by acutally changing the frequency of the
-oscillator driving the clock but by "dividing" the clock's frequency.
-For example, to divide by two, just ignore every other clock tick.
-This cuts the speed in half. Dividing by 3 makes the clock run at 1/3
-frequency, etc. So to slow the clock down (meaning set speed), we
-just send the clock a divisor. It's sent by the serial driver to a
-register in the port. Thus speed is set by a divisor.
-
-
-If the clock runs at a top speed of 115,000 bps (common), then here
-are the divisors for various speeds (assuming a maximum speed of
-115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24
-(4.8k), 48 (2.4k), 96 (1.2k), etc. The serial driver sets the speed
-in the hardware by sending the hardware only a "divisor" (a positive
-integer). This "divisor" divides the "maximum speed" of the hardware
-resulting in a slower speed (except a divisor of 1 obviously tells the
-hardware to run at maximum speed).
-
-
-There are exceptions to the above since for certain serial port
-hardware, speeds above 115.2k are set by using a very high divisor.
-Keep that exception in mind as you read the rest of this section.
-Normally, if you specify a speed of 115.2k (in your communication
-program or by stty) then the serial driver sets the port hardware to
-divisor 1 which sets the highest speed.
-
-
-Besides using a very high divisor to set high speed the conventional
-way to do it is as follows: If you happen to have hardware with a
-maximum speed of say 230.4k (and the 230.4k speed has been enabled),
-then specifying 115.2k will result in divisor 1. For some hardware
-this will actually give you 230.4k. This is double the speed that you
-set. In fact, for any speed you set, the actual speed will be double.
-If you had hardware that could run at 460.8k then the actual speed
-would be quadruple what you set. All the above assumes that you don't
-use "setserial" to modify things.
-
-
-
-
-!Setting the divisor, speed accounting
-
-
- To correct this accounting (but not always fix the problem) you
-may use "setserial" to change the baud_base to the actual maximal
-speed of your port such as 230.4k. Then if you set the speed (by your
-application or by stty) to 230.4k, a divisor of 1 will be used and
-you'll get the same speed as you set.
-
-
-If you have old software which will not permit such a high speed (but
-your hardware has it enabled) then you might want to look into using
-the "spd_cust" parameter for setserial with "divisor 1". Then when
-you tell the application that the speed it 38,400, it will use divisor
-1 and get the highest speed.
-
-
-There are some brands of UARTs that uses a very high divisor to set
-high speeds. There isn't any satisfactory way to use "setserial" (say
-set "divisor 32770") to get such a speed since then setserial would
-then think that the speed is very low and disable the FIFO in the
-UART.
-
-
-
-
-!Crystal frequency is higher than baud_base
-
-
- Note that the baud_base setting is usually much lower than the
-frequency of the crystal oscillator since the crystal frequency of say
-1.8432 MHz is divided by 16 in the hardware to get the actual top
-speed of 115.2k. The reason the crystal frequency needs to be higher
-is so that this high crystal speed can generate clock ticks to take a
-number of samples of each bit to determine if it's a 1 or a .
-
-
-Actually, the 1.8432 MHz "crystal frequency" is obtained from a 18.432
-MHz crystal oscillator by dividing by 10 before being fed to the UART.
-Other schemes are also possible as long as the UART performs properly.
-
-
-
-
-
-
-
-
-
-
-!! 11.2 Higher Serial Throughput
-
-
-
-If you are seeing slow throughput and serial port overruns on a
-system with (E)IDE disk drives, you can get hdparm. This
-is a utility that can modify (E)IDE parameters, including unmasking
-other IRQs during a disk IRQ. This will improve responsiveness
-and will help eliminate overruns. Be sure to read the man page very
-carefully, since some drive/controller combinations don't like this
-and may corrupt the filesystem.
-
-
-Also have a look at a utility called irqtune that will change
-the IRQ priority of a device, for example the serial port that your
-modem is on. This may improve the serial throughput on your system.
-The irqtune FAQ is at
-http://www.best.com/~cae/irqtune
-
-
-----
-
-!!12. Locking Out Others
-
-!!12.1 Introduction
-
-
-
- When you are using a serial port, you may want to prevent others
-from using it at the same time. However there may be cases where you
-do want others to use it, such as sending you an important message if
-you are using a text-terminal.
-
-
-There are various ways of preventing others (or other processes) from
-using your serial port when you are using it (locking). This should
-all happen automatically but it's important to know about this if it
-gives you trouble. If a program is abnormally exited or the PC
-is abruptly turned off (by pulling the plug, etc.) your serial port
-might wind up locked. Even if the lock remains, it's usually
-automatically removed when you want to use the serial port again.
-But in rare cases it isn't. That's when you need to understand what
-happened.
-
-
-One way to implement locking is to design the kernel to handle it but
-Linux thus far has shunned this solution (with an exception involving
-the cua device which is now obsolete). Two solutions used by Linux
-is to:
-
-
-# create lock-files
-#
-
-# modify the permissions and/or owners of devices such as /dev/ttyS2
-#
-
-
-
-
-
-!! 12.2 Lock-Files
-
-
-
- If you use the new device-filesystem (devfs) then see the next
-section.
-A lock-file is simply a file created to mean that a particular device is
-in use. They are kept in /var/lock. Formerly they were in
-/usr/spool/uucp. Linux lock-files are usually named
-LCK..''name'', where ''name'' may be a device name, a process
-id number, a device's major and minor numbers, or a UUCP site name.
-Most processes (an exception is getty) create these locks so that they
-can have exclusive access to devices. For instance if you dial out on
-your modem, some lockfiles will appear to tell other processes that
-someone else is using the modem. In older versions (in the 1990s)
-there was usually only one lockfile per process. Lock files contain
-the PID of the process that has locked the device. Note that if a
-process insists on using a device that is locked, it may ignore the
-lockfile and use the device anyway. This is useful in sending a
-message to a text-terminal, etc.
-
-
-When a program wants to use a serial port but finds it locked with
-lock-files it should check to see if the lock-file's PID is still in
-use. If it's not it means that the lock is stale and it's OK to go
-ahead and use the port anyway (after removing the stale lock-files).
-Unfortunately, there may be some programs that don't do this and give
-up by telling you that a device is already in use when it really isn't.
-
-
-When there were only lockfiles with device names, the following
-problem could arise: If the same device has two different
-names then two different processes could each use a different name for
-the same device. This results in lockfiles with different names that
-actually are the same device. Formerly each physical serial port was
-known by two different device names: ttyS0 and cua0. To solve this
-lockfile alias problem, 3 methods have been used. It may be overkill
-since any one of these methods would have fixed the problem.
-
-
-
-
-
-# The lock checking software was made aware of ttyS vs. cua.
-#
-
-# The device cua was deprecated
-#
-
-# Additional locks were created which use unique device numbers
-instead of names.
-#
-
-
-
-Using alternate names such as /dev/modem for /dev/ttyS2 may cause
-problems with older versions. For dumb terminals, lockfiles are not
-used since this would not permit someone else to send a message to
-your terminal using the write or talk program.
-
-
-
-
-!!12.3 Lock-Files and devfs Problems
-
-
-
- The new device-filesystem (devfs) has the /dev directory with
-subdirectories. As of late 2001, there were problems with lockfiles.
-For example, the lockfile mechanism considers /dev/usb/tts/0 and
-/dev/tts/0 to be the same device with name "". Ditto for all other
-devices that have the same "leaf" name.
-
-
-Also, if some applications use the old name for a device and other
-applications use the new name (devfs) for the same device, then the
-lockfiles will have different names. But the serial driver should
-know they are the same.
-
-
-
-
-!!12.4 Change Owners, Groups, and/or Permissions of Device Files
-
-
-
- In order to use a device, you (or the program you run if you have
-"set user id") needs to have permission to read and write the device
-"file" in the /dev directory. So a logical way to prevent others from
-using a device is to make yourself the temporary owner of the device
-and set permissions so that no one else can use it. A program may do
-this for you. A similar method can be used with the group of the
-device file.
-
-
-While lock files prevent other process from using the device, changing
-device file owners/permissions restricts other users (or the group)
-from using it. One case is where the group is permitted to write to
-the port, but not to read from it. Writing to the port might just
-mean a message sent to a text-terminal while reading means destructive
-reading. The original process that needs to read the data may find
-data missing if another process has already read that data. Thus a
-read can do more harm that a write since a read causes loss of data
-while a write only adds extra data. That's a reason to allow writes
-but not reads. This is exactly the opposite of the case for ordinary
-files where you allow others to read the file but not write (modify)
-it. Use of a port normally requires both read and write permissions.
-
-
-A program that changes the device file attributes should undo these
-changes when it exits. But if the exit is abnormal, then a device
-file may be left in such a condition that it gives the error
-"permission denied" when one attempts to use it again.
-
-
-
-----
-
-!! 13. Communications Programs And Utilities
-
-!!13.1 List of Software
-
-
-
-Here is a list of some communication software you can choose from,
-available via FTP, if they didn't come with your distribution.
-
-
-
-
-
-*ecu - a communications program
-*
-
-*
-C-Kermit -
-portable, scriptable, serial and TCP/IP communications including file
-transfer, character-set translation, and zmodem support
-*
-
-*gkermit Tiny GPLed kermit run only from the command line.
-Can't connect to another computer
-*
-
-*minicom - telix-like communications program
-*
-
-*seyon - X based communication program
-*
-
-*xc - xcomm communication package
-
-*
-
-*term and SLiRP offer TCP/IP functionality using a
-shell account.
-
-*
-
-*screen is another multi-session program. This one behaves
-like the virtual consoles.
-
-*
-
-*callback is where you dial out to a remote modem and then
-that modem hangs up and calls you back (to save on phone bills).
-
-*
-
-*mgetty+fax handles FAX stuff, and provides an alternate
-ps_getty.
-
-*
-
-*ZyXEL is a control program for ZyXEL U-1496 modems. It
-handles dialin, dialout, dial back security, FAXing, and voice
-mailbox functions.
-
-*
-
-*SLIP and PPP software can be found at
-
-ftp://metalab.unc.edu/pub/Linux/system/network/serial.
-*
-
-
-
-
-
-!!13.2 kermit and zmodem
-
-
-
- For use of kermit with modems see the Modem-HOWTO. One can run
-zmodem within the kermit program. To do this (for ttyS3), add the
-following to your .kermrc file:
-
-
-define rz !rz < /dev/ttyS3 > /dev/ttyS3
-define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3
-
-
-Be sure to put in the correct port your modem is on. Then, to use it,
-just type rz or sz <filename> at the kermit
-prompt.
-
-
-
-----
-
-!!14. Serial Tips And Miscellany
-
-!! 14.1 Serial Module
-
-
-
- Often the serial driver is provided as a module. Parameters may
-be supplied to certain modules in /etc/modules.conf. Since kernel 2.2
-you don't edit this file but use the program update-modules to change
-it. The info that is used to update modules.conf is put in
-/etc/modutils/. The Debian/GNU Linux has a file here named
-/etc/modutils/setserial which runs the serial script in /etc/init.d/
-every time the serial module is loaded or unloaded. When the serial
-module is unloaded this script will save the state of the module in
-/var/run/setserial.conf. Then if the module loads again this saved
-state is restored. When the serial module first loads at boot-time,
-there's nothing in /var/run/setserial.conf so the state is obtained
-from /etc/serial.conf. So there are two files that save the state.
-Other distributions may do something similar.
-
-
-One may modify the serial driver by editing the source code. Much of
-the serial driver is found in the file serial.c. For info
-regarding writing of programs for the serial port see
-Serial-Programming-HOWTO. It was revised in 1999 by Vern Hoxie but
-it's not at LDP. Get it from
-scicom.alphacdc.com/pub/linux
-
-
-
-!!14.2 Serial Console (console on the serial port)
-
-
-
- See the kernel documentation in: Documentation/serial-console.txt.
-Kernel 2.4+ has better documentation. See also "Serial Console" in
-Text-Terminal-HOWTO.
-
-
-
-
-!!14.3 Line Drivers
-
-
-
- For a text terminal, the EIA-232 speeds are fast enough but the
-usable cable length is often too short. Balanced technology could
-fix this. The common method of obtaining balanced communication with
-a text terminal is to install 2@ line drivers in the serial line to
-convert unbalanced to balanced (and conversely). They are a
-specialty item and are expensive if purchased new.
-
-
-
-
-!!14.4 Stopping the Data Flow when Printing, etc.
-
-
-
- Normally flow control and/or application programs stop the flow of
-bytes when its needed. But sometimes they don't. One example is
-printing to printer on the serial port. If you want to instantly stop
-printing you may try turning off the printer. With older versions of
-the serial driver, the printer would attempt to resume printing if you
-turned the printer back on again (before the time specified by
-closing_wait of setserial had expired). The attempt to resume would
-happen even if you used a command to stop the printing. The problem
-was that once the printer software sent bytes to the large serial
-buffer to be printed, these bytes were not removed from this buffer
-when the print job was canceled. One way to remove them (for newer
-serial drivers) is to simply turn off the printer. This will drop all
-modem control signals from the printer and empty the buffer. Modern
-printers have large buffers and often a button on the printer to empty
-the buffer.
-
-
-
-
-!!14.5 Known Defective Hardware
-
-
-! Avoiding IO Address Conflicts with Certain Video Boards
-
-
- The IO address of the IBM 8514 video board (and others) is
-allegedly 0x?2e8 where ? is 2, 4, 8, or 9. This may conflict (but
-shouldn't if the serial port is well designed) with the IO address of
-ttyS3 at 0x02e8 if the serial port ignores the leading 0 hex
-digit when it decodes the address (many do). That is bad news if you
-try to use ttyS3 at this IO address. Another story is that Linux
-will not detect your internal modem on ttyS3 but that you can use
-setserial to put ttyS3 at this address and the modem
-will work fine.
-
-
-
-
-!Problem with AMD Elan SC400 CPU (PC-on-a-chip)
-
-
- This has a race condition between an interrupt and a status register
-of the UART. An interrupt is issued when the UART transmitter
-finishes the transmission of a byte and the UART transmit buffer
-becomes empty (waiting for the next byte). But a status register of
-the UART doesn't get updated fast enough to reflect this. As a
-result, the interrupt service routine rapidly checks and determines
-(erroneously) that nothing has happened. Thus no byte is sent to the
-port to be transmitted and the UART transmitter waits in vain for a
-byte that never arrives. If the interrupt service routine had waited
-just a bit longer before checking the status register, then it would
-have been updated to reflect the true state and all would be OK.
-
-
-There is a proposal to fix this by patching the serial driver. But
-Should linux be patched to accommodate defective hardware, especially
-if this patch may impair performance of good hardware?
-
-
-
-----
-
-!! 15. Troubleshooting
-
-
- See Modem-HOWTO for troubleshooting related to modems or getty for
-modems. For a Text-Terminal much of the info here will be of value as
-well as the troubleshooting info in Text-Terminal-HOWTO.
-
-
-
-
-!! 15.1 Serial Electrical Test Equipment
-
-
-!Breakout Gadgets, etc.
-
-
- While a multimeter (used as a voltmeter) may be all that you need
-for just a few serial ports, simple special test equipment has been
-made for testing serial port lines. Some are called "breakout ... "
-where breakout means to break out conductors from a cable. These
-gadgets have a couple of connectors which connect to serial port
-connectors (either at the ends of serial cables or at the back of a
-PC). Some have test points for connecting a voltmeter. Others have
-LED lamps which light when certain modem control lines are asserted
-(turned on). The color of the light may indicate the polarity of the
-signal (positive or negative voltage). Still others have jumpers so
-that you can connect any wire to any wire. Some have switches.
-
-
-Radio Shack sells (in 2001) a "RS-232 Troubleshooter" (formerly called
-"RS-232 Line Tester") Cat. #276-1401. It checks TD, RD, CD, RTS,
-CTS, DTR, and DSR. A green light means on (+12 v) while red means
-off (-12 v). They also sell a "RS-232 Serial Jumper Box" Cat.
-#276-1403. This permits connecting the pins anyway you choose. Both
-these items are under the heading of "Peripheral hookup helpers".
-Unfortunately, they are not listed in the index to the printed
-catalog. They are on the same page as the D type connecters so
-look in the index under "Wire & Cable, Connectors. Or look
-under "Tools, D-Sub Connection Pin ...". A store chain named "Active Component
-
-
-
-
-!Measuring voltages
-
-
- Any voltmeter or multimeter, even the cheapest that sells for
-about $10, should work fine. Trying to use other methods for checking
-voltage is tricky. Don't use a LED unless it has a series resistor to
-reduce the voltage across the LED. A 470 ohm resistor is used for a
-20 ma LED (but not all LED's are 20 ma). The LED will only light for
-a certain polarity so you may test for + or - voltages. Does anyone
-make such a gadget for automotive circuit testing?? Logic probes may
-be damaged if you try to use them since the TTL voltages for which
-they are designed are only 5 volts. Trying to use a 12 V incandescent
-light bulb is not a good idea. It won't show polarity and due to
-limited output current of the UART it probably will not even light up.
-
-
-To measure voltage on a female connector you may plug in a bent paper
-clip into the desired opening. The paper clip's diameter should be no
-larger than the pins so that it doesn't damage the contact. Clip
-an alligator clip (or the like) to the paper clip to connect up. Take
-care not to touch two pins at the same time with any metal object.
-
-
-
-
-!Taste voltage
-
-
- As a last resort, if you have no test equipment and are willing to
-risk getting shocked (or even electrocuted) you can always taste the
-voltage. Before touching one of the test leads with your tongue, test
-them to make sure that there is no high voltage on them. Touch both
-leads (at the same time) to one hand to see if they shock you. Then
-if no shock, wet the skin contact points by licking and repeat. If
-this test gives you a shock, you certainly don't want to use your
-tongue.
-
-
-For the test for 12 V, Lick a finger and hold one test lead in it.
-Put the other test lead on your tongue. If the lead on your tongue is
-positive, there will be a noticeable taste. You might try this with
-flashlight batteries first so you will know what taste to expect.
-
-
-
-
-
-
-
-
-
-
-!!15.2 Serial Monitoring/Diagnostics
-
-
-
- A few Linux programs will monitor the modem control lines and
-indicate if they are positive (1) or negative (). See section
-Serial Monitoring/Diagnostics
-
-
-
-!!15.3 (The following subsections are in both the Serial and Modem HOWTOs)
-
-
-!! 15.4 My Serial Port is Physically There but Can't be Found
-
-
-
- If a physical device (such as a modem) doesn't work at all it may
-mean that the device is not at the I/O address that setserial
-thinks it's at. It could also mean (for a PnP card) that is doesn't
-yet have an address. Thus it can't be found.
-
-
-Check the BIOS menus and BIOS messages. For the PCI bus use lspci or
-scanpci. If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
-and/or see Plug-and-Play-HOWTO. Using "scanport" will scan all ISA
-bus ports and may discover an unknown port that could be a serial port
-(but it doesn't probe the port). It could hang your PC. You may try
-probing with setserial. See
-Probing. If
-nothing seems to get thru the port it may be accessible but have a bad
-interrupt. See
-Extremely Slow: Text appears on the screen slowly after long delays. Use setserial -g to
-see what the serial driver thinks and check for IRQ and I0 address
-conflicts. Even if you see no conflicts the driver may have incorrect
-information (view it by "setserial" and conflicts may still exist.
-
-
-If two ports have the same IO address then probing it will erroneously
-indicate only one port. Plug-and-play detection will find both ports
-so this should only be a problem if at least one port is not
-plug-and-play. All sorts of errors may be reported/observed for
-devices illegally "sharing" a port but the fact that there are two
-devices on the same a port doesn't seem to get detected (except
-hopefully by you). In the above case, if the IRQs are different then
-probing for IRQs with setserial might "detect" this situation by
-failing to detect any IRQ. See
-Probing.
-
-
-
-
-!! 15.5 Extremely Slow: Text appears on the screen slowly after long delays
-
-
-
- It's likely mis-set/conflicting interrupts. Here are some of the
-symptoms which will happen the first time you try to use a modem,
-terminal, or serial printer. In some cases you type something but
-nothing appears on the screen until many seconds later. Only the last
-character typed may show up. It may be just an invisible
-<return> character so all you notice is that the cursor jumps
-down one line. In other cases where a lot of data should appear on
-the screen, only a batch of about 16 characters appear. Then there is
-a long wait of many seconds for the next batch of characters. You
-might also get "input overrun" error messages (or find them in logs).
-
-
-For more details on the symptoms and why this happens see
-
-
-
-Interrupt Problem Details and/or
-Interrupt Conflicts and/or
-Mis-set Interrupts.
-If it involves Plug-and-Play devices, see also Plug-and-Play-HOWTO.
-
-
-As a quick check to see if it really is an interrupt problem, set the
-IRQ to 0 with "setserial". This will tell the driver to use
-polling instead of interrupts. If this seems to fix the "slow"
-problem then you had an interrupt problem. You should still try to
-solve the problem since polling uses excessive computer resources.
-
-
-Checking to find the interrupt conflict may not be easy since Linux
-supposedly doesn't permit any interrupt conflicts and will send you a
-/dev/ttyS?: Device or resource busy error
-message if it thinks you are attempting to create a conflict. But a
-real conflict can be created if "setserial" has told the kernel
-incorrect info. The kernel has been lied to and thus doesn't think
-there is any conflict. Thus using "setserial" will not reveal the
-conflict (nor will looking at /proc/interrupts which bases its info on
-"setserial"). You still need to know what "setserial" thinks so that
-you can pinpoint where it's wrong and change it when you determine
-what's really set in the hardware.
-
-
-What you need to do is to check how the hardware is set by checking
-jumpers or using PnP software to check how the hardware is actually
-set. For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
-"lspci" (if PCI bus). Compare this to how Linux (e.g. "setserial")
-thinks the hardware is set.
-
-
-
-
-!!15.6 Somewhat Slow: I expected it to be a few times faster
-
-
-
- An obvious reason is that the baud rate is actually set too slow.
-It's claimed that this happened by trying to set the baud rate to a speed
-higher than the hardware can support (such as 230400).
-
-
-Another reason may be that whatever is on the serial port (such as a
-modem, terminal, printer) doesn't work as fast as you thought it did.
-
-
-
-
-
-Another possible reason is that you have an obsolete serial port: UART
-8250, 16450 or early 16550 (or the serial driver thinks you do). See
-
-
-
-What Are UARTS?
-Use "setserial -g /dev/ttyS*".
-If it shows anything less than a 16550A, this may be your problem.
-If you think that "setserial" has it wrong check it out. See
-What is Setserial for more info. If you
-really do have an obsolete serial port, lying about it to setserial
-will only make things worse.
-
-
-
-
-!! 15.7 The Startup Screen Show Wrong IRQs for the Serial Ports.
-
-
-
- Linux does not do any IRQ detection on startup. When the serial
-module loads it only does serial device detection. Thus, disregard
-what it says about the IRQ, because it's just assuming the standard
-IRQs. This is done, because IRQ detection is unreliable, and can be
-fooled. But if and when setserial runs from a start-up script, it
-changes the IRQ's and displays the new (and hopefully correct) state
-on on the startup screen. If the wrong IRQ is not corrected by a
-later display on the screen, then you've got a problem.
-
-
-So, even though I have my ttyS2 set at IRQ 5, I still see
-
-
-ttyS02 at 0x03e8 (irq = 4) is a 16550A
-
-
-at first when Linux boots. (Older kernels may show "ttyS02" as
-"tty02" which is the same as ttyS2). You may need to use
-setserial to tell Linux the IRQ you are using.
-
-
-
-
-!!15.8 "Cannot open /dev/ttyS?: Permission denied"
-
-
-
- Check the file permissions on this port with "ls -l /dev/ttyS?"_
-If you own the ttyS? then you need read and write permissions: crw
-with the c (Character device) in col. 1. It you don't own it then it
-should show rw- in cols. 8 & 9 which means that everyone has read and
-write permission on it. Use "chmod" to change permissions. There are
-more complicated ways to get access like belonging to a "group" that
-has group permission.
-
-
-
-
-!!15.9 "Operation not supported by device" for ttyS?
-
-
-
- This means that an operation requested by setserial, stty, etc.
-couldn't be done because the kernel doesn't support doing it.
-Formerly this was often due to the "serial" module not being loaded.
-But with the advent of PnP, it may likely mean that there is no modem
-(or other serial device) at the address where the driver (and
-setserial) thinks it is. If there is no modem there, commands (for
-operations) sent to that address obviously don't get done. See
-What is set in my serial port hardware?
-
-If the "serial" module wasn't loaded but "lsmod" shows you it's now
-loaded it might be the case that it's loaded now but wasn't loaded
-when you got the error message. In many cases the module will
-automatically loaded when needed (if it can be found). To force
-loading of the "serial" module it may be listed in the file:
-/etc/modules.conf or /etc/modules. The actual module should reside
-in: /lib/modules/.../misc/serial.o.
-
-
-
-
-!!15.10 "Cannot create lockfile. Sorry"
-
-
-
- When a port is "opened" by a program a lockfile is created in
-/var/lock/. Wrong permissions for the lock directory will not allow a
-lockfile to be created there. Use "ls -ld /var/lock" to see if the
-permissions are OK: usually rwx for everyone (repeated 3 times). If
-it's wrong, use "chmod" to fix it. Of course, if there is no "lock"
-directory no lockfile can be created there. For more info on
-lockfiles see
-What Are Lock Files
-
-
-
-!!15.11 "Device /dev/ttyS? is locked."
-
-
-
- This means that someone else (or some other process) is supposedly
-using the serial port. There are various ways to try to find out what
-process is "using" it. One way is to look at the contents of the
-lockfile (/var/lock/LCK...). It should be the process id. If the
-process id is say 100 type "ps 100" to find out what it is. Then if
-the process is no longer needed, it may be gracefully killed by "kill
-100". If it refuses to be killed use "kill -9 100" to force it to be
-killed, but then the lockfile will not be removed and you'll need to
-delete it manually. Of course if there is no such process as 100 then
-you may just remove the lockfile but in most cases the lockfile should
-have been automatically removed if it contained a stale process id
-(such as 100).
-
-
-
-
-!! 15.12 "/dev/tty? Device or resource busy"
-
-
-
- This means that the device you are trying to access (or use) is
-supposedly busy (in use) or that a resource it needs (such as an IRQ)
-is supposedly being used by another device (the resource is "busy").
-This message is easy to understand if it only means that the device is
-busy (in use). But it often means that a resource is in use. What
-makes it even more confusing is that in some cases neither the device
-nor the resources that it needs are actually "busy".
-
-
-The ``resource busy'' part often means (example for ttyS2) ``You
-can't use ttyS2 since another device is using ttyS2's
-interrupt.'' The potential interrupt conflict is inferred from what
-"setserial" thinks. A more accurate error message would be ``Can't
-use ttyS2 since the setserial data (and kernel data) indicates
-that another device is using ttyS2's interrupt''. If two devices
-use the same IRQ and you start up only one of the devices, everything
-is OK because there is no conflict yet. But when you next try to
-start the second device (without quitting the first device) you get a
-"... busy" error message. This is because the kernel only keeps track
-of what IRQs are actually in use and actual conflicts don't happen
-unless the devices are in use (open). The situation for I/O address
-(such as 0x3f8) conflict is similar.
-
-
-This error is sometimes due to having two serial drivers: one a module
-and the other compiled into the kernel. Both drivers try to grab the
-same resources and one driver finds them "busy".
-
-
-There are two possible cases when you see this message:
-
-
-# There may be a real resource conflict that is being avoided.
-#
-
-# Setserial has it wrong and the only reason ttyS2 can't be
-used is that setserial erroneously predicts a conflict.
-#
-
-
-
-What you need to do is to find the interrupt setserial thinks
-ttyS2 is using. Look at /proc/tty/driver/serial (if you have
-it). You should also be able to see it with the "setserial" command
-for ttyS2.
-
-
-Bug in old versions: Prior to 2001 there was a bug which wouldn't let
-you see it with "setserial". Trying to see it would give the same
-"... busy" error message.
-
-
-To try to resolve this problem reboot or: exit or gracefully kill all
-likely conflicting processes. If you reboot: 1. Watch the boot-time
-messages for the serial ports. 2. Hope that the file that runs
-"setserial" at boot-time doesn't (by itself) create the same conflict
-again.
-
-
-If you think you know what IRQ say ttyS2 is using then you may
-look at /proc/interrupts to find what else (besides another serial
-port) is currently using this IRQ. You might also want to double
-check that any suspicious IRQs shown here (and by "setserial") are
-correct (the same as set in the hardware). A way to test whether or
-not it's a potential interrupt conflict is to set the IRQ to
-(polling) using "setserial". Then if the busy message goes away, it
-was likely a potential interrupt conflict. It's not a good idea to
-leave it permanently set at 0 since it will put more load on the CPU.
-
-
-
-
-!!15.13 "Input/output error" from setserial or stty
-
-
-
- You may have typed "ttys" instead of "ttyS". You will see this
-error message if you try to use the setserial command for any device
-that is not a serial port. It also may mean that the serial port is
-in use (busy or opened) and thus the attempt to get/set parameters by
-setserial or stty failed. It could also mean that there isn't any
-serial port at the IO address that setserial thinks your port is at.
-
-
-
-
-!!15.14 Overrun errors on serial port
-
-
-
- This is an overrun of the hardware FIFO buffer and you can't
-increase its size. Bug note (reported in 2002): Due to a bug in some
-kernel 2.4 versions, the port number may be missing and you will only
-see "ttyS" (no port number). But if devfs notation such as "tts/2" is
-being used, there is no bug. See
-
-
-
-
-
-
-
-
-
-
-!!15.15 Port get characters only sporadically
-
-
-
- There could be some other program running on the port. Use "top"
-(provided you've set it to display the port number) or "ps -alxw".
-Look at the results to see if the port is being used by another
-program. Be on the lookout for the gpm mouse program which often runs
-on a serial port.
-
-
-
-
-!!15.16 Troubleshooting Tools
-
-
-
- These are some of the programs you might want to use in
-troubleshooting:
-
-
-* "lsof /dev/ttyS*" will list serial ports which are open.
-*
-
-* "setserial" shows and sets the low-level hardware configuration
-of a port (what the driver thinks it is). See
-What is Setserial
-*
-
-* "stty" shows and sets the configuration of a port (except for
-that handled by "setserial").
-See the section
-Stty
-*
-
-* "modemstat" or "statserial" will show the current state of
-various modem signal lines (such as DTR, CTS, etc.)
-*
-
-* "irqtune" will give serial port interrupts higher
-priority to improve performance.
-*
-
-* "hdparm" for hard-disk tuning may help some more.
-*
-
-* "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.
-*
-
-* "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
-PnP devices on the ISA bus.
-*
-
-* Some "files" in the /proc tree (such as ioports, interrupts,
-and tty/driver/serial).
-*
-
-
-
-
-
-
-
-
-
-
-----
-
-!! 16. Interrupt Problem Details
-
-
- While the section
-Troubleshooting
-lists problems by symptom, this section explains what will happen if
-interrupts are set incorrectly. This section helps you understand what
-caused the symptom, what other symptoms might be due to the same
-problem, and what to do about it.
-
-
-
-
-!!16.1 Types of interrupt problems
-
-
-
- The "setserial" program will show you how serial driver thinks the
-interrupts are set. If the serial driver (and setserial) has it right
-then everything regarding interrupts should be OK. Of course a
-/dev/ttyS must exist for the device and Plug-and-Play (or jumpers)
-must have set an address and IRQ in the hardware. Linux will not
-knowingly permit an interrupt conflict and you will get a "Device or
-resource busy" error message if you attempt to do something that would
-create a conflict.
-
-
-Since the kernel tries to avoid interrupt conflicts and gives you the
-"resource busy" message if you try to create a conflict, how can
-interrupt conflicts happen? Easy. "setserial" may have it wrong and
-erroneously predicts no conflict when there will actually be a real
-conflict based on what is set in the hardware. When this happens
-there will be no "... busy" message but a conflict will physically
-happen. Performance is likely to be extremely slow. Both devices
-will send identical interrupt signals on the same wire and the CPU
-will erroneously think that the interrupts only come from one device.
-This will be explained in detail in the following sections.
-
-
-Linux doesn't complain when you assign two devices the same IRQ
-provided that neither device is in use. As each device starts up
-(initializes), it asks Linux for permission to use its hardware
-interrupt. Linux keeps track of which interrupt is assigned to whom,
-and if your interrupt is already in use, you'll see this "... busy"
-error message. Thus if two devices use the same IRQ and you start up
-only one of the devices, everything is OK. But when you next try to
-start the second device (without quitting the first device) you get
-"... busy" error message.
-
-
-
-
-!!16.2 Symptoms of Mis-set or Conflicting Interrupts
-
-
-
- The symptoms depend on whether or not you have a modern serial port
-with FIFO buffers or an obsolete serial port without FIFO buffers.
-It's important to understand the symptoms for the obsolete ones also
-since sometimes modern ports seem to behave that way.
-
-
-For the obsolete serial ports, only one character gets thru every
-several seconds. This is so slow that it seems almost like nothing is
-working (especially if the character that gets thru is invisible (such
-a space or newline). For the modern ports with FIFO buffers you
-will likely see bursts of up to 16 characters every several seconds.
-
-
-If you have a modem on the port and dial a number, it seemingly may
-not connect since the CONNECT message may not make it thru. But after
-a long wait it may finally connect and you may see part of a login
-message (or the like). The response from your side of the connection
-may be so delayed that the other side gives up and disconnects you,
-resulting in a NO CARRIER message.
-
-
-If you use minicom, a common test to see if things are working is to
-type the simplest "AT" command and see if the modem responds. Typing
-just at<enter> should normally (if interrupts are OK) result in
-an immediate "OK" response from the modem. With bad interrupts you
-type at<enter> and may see nothing. But then after 10 seconds
-or so you see the cursor drop down one line. What is going on is that
-the FIFO is behaving like it can only hold one byte. The "at" you
-typed caused it to overrun and both letters were lost. But the final
-<enter> eventually got thru and you "see" this invisible
-character by noticing that the cursor jumped down one line. If you were
-to type a single letter and then wait about 10 seconds, you should see
-it echo back to the screen. This is fine if your typing speed is less
-that one word per minute :-)
-
-
-
-
-!! 16.3 Mis-set Interrupts
-
-
-
- If you don't understand what an interrupt does see
-Interrupts. If a serial port has one IRQ set
-in the hardware but a different one set in the device driver, the
-device driver will not catch any interrupts sent by the serial port.
-Since the serial port uses interrupts to call its driver to service
-the port (fetching bytes from its 16-byte receive buffer or putting
-another 16-bytes in its transmit buffer) one might expect that the
-serial port would not work at all.
-
-
-But it still may work anyway --sort of. Why? Well, besides the
-interrupt method of servicing the port there's a slow polling method
-that doesn't need interrupts. The way it works is that every so often
-the device driver checks the serial port to see if it needs anything
-such as if it has some bytes that need fetching from its receive
-buffer. If interrupts don't work, the serial driver falls back to
-this polling method. But this polling method was not intended to be
-used a substitute for interrupts. It's so slow that it's not
-practical to use and may cause buffer overruns. Its purpose may have
-been to get things going again if just one interrupt is lost or fails
-to do the right thing. It's also useful in showing you that
-interrupts have failed. Don't confuse this slow polling method with
-the fast polling method that operates on ports that have their
-IRQs set to .
-
-
-For the 16-byte transmit buffer, 16 bytes will be transmitted and then
-it will wait until the next polling takes place (several seconds
-later) before the next 16 bytes are sent out. Thus transmission is
-very slow and in small chunks. Receiving is slow too since bytes that
-are received by the receive buffer are likely to remain there for
-several seconds until it is polled.
-
-
-This explains why it takes so long before you see what you typed.
-When you type say AT to a modem, the AT goes out the serial port to
-the modem. The modem then echos the AT back thru the serial port to
-the screen. Thus the AT characters have to pass twice thru the serial
-port. Normally this happens so fast that AT seems to appear on the
-screen at the same time you hit the keys on the keyboard. With slow
-polling delays at the serial port, you don't see what you typed
-until many seconds later.
-
-
-What about overruns of the 16-byte receive buffer? This will happen
-with an external modem since the modem just sends to the serial port
-at high speed which is likely to overrun the 16-byte buffer. But for
-an internal modem, the serial port is on the same card and it's likely
-to check that this receive buffer has room for more bytes before
-putting received bytes into it. In this case there will be no overrun
-of this receive buffer, but text will just appear on your screen in
-16-byte chunks spaced at intervals of several seconds.
-
-
-Even with an external modem you might not get overruns. If just a few
-characters (under 16) are sent you don't get overruns since the buffer
-likely has room for them. But attempts to send a larger number of
-bytes from your modem to your screen may result in overruns. However,
-more than 16 (with no gaps) can get thru without overruns if the
-timing is right. For example, suppose a burst of 32 bytes is sent
-into the port from the external cable. The polling might just happen
-after the first 16 bytes came in so it would pick up these 16 bytes
-OK. Then there would be space for the next 16 bytes so that entire 32
-bytes gets thru OK. While this scenario is not very likely, similar
-cases where 17 to 31 bytes make thru are more likely. But it's even
-more likely that only an occasional 16-byte chunk will get thru with
-possible loss of data.
-
-
-If you have an obsolete serial port with only a 1-byte buffer (or it's
-been incorrectly set to work like a 1-byte buffer) then the situation
-will be much worse than described above and only one character will
-occasionally make it thru the port. Every character received causes
-an overrun (and is lost) except for the last character received. This
-character is likely to be just a line-feed since this is often the
-last character to be transmitted in a burst of characters sent to your
-screen. Thus you may type AT<return> to the modem but never see
-AT on the screen. All you see several seconds later is that the
-cursor drops down one line (a line feed). This has happened to me
-with a 16-byte FIFO buffer that was behaving like a 1-byte buffer.
-
-
-When a communication program starts up, it expects interrupts to be
-working. It's not geared to using this slow polling-like mode of
-operation. Thus all sorts of mistakes may be made such as setting up
-the serial port and/or modem incorrectly. It may fail to realize when
-a connection has been made. If a script is being used for login, it
-may fail (caused by timeout) due to the polling delays.
-
-
-
-
-!! 16.4 Interrupt Conflicts
-
-
-
- When two devices have the same IRQ number it's called sharing
-interrupts. Under some conditions this sharing works out OK.
-Starting with kernel version 2.2, ISA serial ports may, if the
-hardware is designed for this, share interrupts with other serial
-ports. Devices on the PCI bus may share the same IRQ interrupt with
-other devices on the PCI bus (provided the software supports this).
-In other cases where there is potential for conflict, there should be
-no problem if no two devices with the same IRQ are ever "in use" at
-the same time. More precisely, "in use" really means "open" (in
-programmer jargon). In cases other than the exceptions mentioned
-above (unless special software and hardware permit sharing), sharing
-is not allowed and conflicts arise if sharing is attempted.
-
-
-Even if two processes with conflicting IRQs run at the same time, one
-of the devices will likely have its interrupts caught by its device
-driver and may work OK. The other device will not have its interrupts
-caught by the correct driver and will likely behave just like a
-process with mis-set interrupts. See
-Mis-set Interrupts for more details.
-
-
-
-
-!!16.5 Resolving Interrupt Problems
-
-
-
- If you are getting a very slow response as described above, then
-one test is to change the IRQ to 0 (uses fast polling instead of
-interrupts) and see if the problem goes away. Note that the polling
-due to IRQ=0 is orders of magnitude faster than the slow "polling" due
-to bad interrupts. If IRQ=0 seems to fix the problem, then there was
-likely something wrong with the interrupts. Using IRQ=0 is very
-resource intensive and is only a temporary fix. You should try to
-find the cause of the interrupt problem and not permanently use IRQ=.
-
-
-Check /proc/interrupts to see if the IRQ is currently in use by another
-process. If it's in use by another serial port you could try "top"
-(type f and then enable the TTY display) or "ps -e" to find out which
-serial ports are in use. If you suspect that setserial has a wrong
-IRQ then see
-What is the current IO address and IRQ of my Serial Port ?
-
-
-----
-
-!! 17. What Are UARTs? How Do They Affect Performance?
-
-!!17.1 Introduction to UARTS
-
-
-
- UARTs (__U__niversal __A__synchronous __R__eceiver
-__T__ransmitter) are serial chips on your PC motherboard (or on an
-internal modem card). The UART function may also be done on a chip
-that does other things as well. On older computers like many 486's,
-the chips were on the disk IO controller card. Still older computer
-have dedicated serial boards.
-
-
-The UART's purpose is to convert bytes from the PC's parallel bus to a
-serial bit-stream. The cable going out of the serial port is serial
-and has only one wire for each direction of flow. The serial port
-sends out a stream of bits, one bit at a time. Conversely, the bit
-stream that enters the serial port via the external cable is converted
-to parallel bytes that the computer can understand. UARTs deal with
-data in byte sized pieces, which is conveniently also the size of
-ASCII characters.
-
-
-Say you have a terminal hooked up to your PC. When you type a
-character, the terminal gives that character to its transmitter (also
-a UART). The transmitter sends that byte out onto the serial line,
-one bit at a time, at a specific rate. On the PC end, the receiving
-UART takes all the bits and rebuilds the (parallel) byte and puts it
-in a buffer.
-
-
-Along with converting between serial and parallel, the UART does some
-other things as a byproduct (side effect) of its primary task. The
-voltage used to represent bits is also converted (changed). Extra
-bits (called start and stop bits) are added to each byte before it is
-transmitted. See the Serial-HOWTO section, ``Voltage Waveshapes'' for
-details. Also, while the flow rate (in bytes/sec) on the parallel bus
-inside the computer is very high, the flow rate out the UART on the
-serial port side of it is much lower. The UART has a fixed set of
-rates (speeds) which it can use at its serial port interface.
-
-
-
-
-!!17.2 Two Types of UARTs
-
-
-
- There are two basic types of UARTs: dumb UARTS and FIFO UARTS.
-Dumb UARTs are the 8250, 16450, early 16550, and early 16650. They
-are obsolete but if you understand how they work it's easy to
-understand how the modern ones work with FIFO UARTS ( late 16550,
-16550A, 16c552, late 16650, 16750, and 16C950).
-
-
-There is some confusion regarding 16550. Early models had a bug and
-worked properly only as 16450's (no FIFO). Later models with the bug
-fixed were named 16550A but many manufacturers did not accept the name
-change and continued calling it a 16550. Most all 16550's in use
-today are like 16550A's. Linux will report it as being a 16550A even
-though your hardware manual (or a label note) says it's a 16550. A
-similar situation exists for the 16650 (only it's worse since the
-manufacturer allegedly didn't admit anything was wrong). Linux will
-report a late 16650 as being a 16650V2. If it reports it as 16650 it
-is bad news and only is used as if it had a one-byte buffer.
-
-
-
-
-!! 17.3 FIFOs
-
-
-
- To understand the differences between dumb and FIFO (First In,
-First Out queue discipline) first let's examine what happens when a
-UART has sent or received a byte. The UART itself can't do anything
-with the data passing thru it, it just receives and sends it. For the
-obsolete dumb UARTS, the CPU gets an interrupt from the serial device
-every time a byte has been sent or received. The CPU then moves the
-received byte out of the UART's buffer and into memory somewhere, or
-gives the UART another byte to send. The obsolete 8250 and 16450
-UARTs only have a 1 byte buffer. That means, that every time 1 byte
-is sent or received, the CPU is interrupted. At low transfer rates,
-this is OK. But, at high transfer rates, the CPU gets so busy dealing
-with the UART, that is doesn't have time to adequately tend to other
-tasks. In some cases, the CPU does not get around to servicing the
-interrupt in time, and the byte is overwritten, because they are
-coming in so fast. This is called an "overrun" or "overflow".
-
-
-FIFO UARTs help solve this problem. The 16550A (or 16550) FIFO chip
-comes with 16 byte FIFO buffers. This means that it can receive up to
-14 bytes (or send 16 bytes) before it has to interrupt the CPU. Not
-only can it wait for more bytes, but the CPU then can transfer all (14
-to 16) bytes at a time. This is a significant advantage over the
-obsolete UARTs, which only had 1 byte buffers. The CPU receives less
-interrupts, and is free to do other things. Data is rarely lost.
-Note that the interrupt threshold of FIFO buffers (trigger level) may
-be set at less than 14. 1, 4 and 8 are other possible choices. As of
-late 2000 there was no way the Linux user could set these directly
-(setserial can't do it). While many PC's only have a 16550 with
-16-byte buffers, better UARTS have even larger buffers.
-
-
-Note that the interrupt is issued slightly before the buffer gets full
-(at say a "trigger level" of 14 bytes for a 16-byte buffer). This
-allows room for a couple more bytes to be received before the
-interrupt service routine is able to actually fetch all these bytes.
-The trigger level may be set to various permitted values by kernel
-software. A trigger level of 1 will be almost like an obsolete UART
-(except that it still has room for 15 more bytes after it issues the
-interrupt).
-
-
-Now consider the case where you're on the Internet. It's just sent
-you a short webpage of text. All of this came in thru the serial
-port. If you had a 16-byte buffer on the serial port which held back
-characters until it had 14 of them, some of the last several
-characters on the screen might be missing as the FIFO buffer waited to
-get the 14th character. But the 14th character doesn't arrive since
-you've been sent the entire page (over the phone line) and there are
-no more characters to send to you. It could be that these last
-characters are part of the HTML formatting, etc. and are not
-characters to display on the screen but you don't want to lose format
-either.
-
-
-There is a "timeout" to prevent the above problem. The "timeout"
-works like this for the receive UART buffer: If characters arrive one
-after another, then an interrupt is issued only when say the 14th
-character reaches the buffer. But if a character arrives and the next
-character doesn't arrive soon thereafter, then an interrupt is issued
-anyway. This results in fetching all of the characters in the FIFO
-buffer, even if only a few (or only one) are present. There is also
-"timeout" for the transmit buffer as well.
-
-
-
-
-!!17.4 Why FIFO Buffers are Small
-
-
-
-You may wonder why the FIFO buffers are not larger. After all,
-memory is cheap and it wouldn't cost much more to use buffers in the
-kilo-byte range. The reason is flow control. Flow control stops the
-flow of data (bytes) on serial line when necessary. If a stop signal
-is sent to serial port, then the stop request is handled by software
-(even if the flow control is "hardware"). The serial port hardware
-knows nothing about flow control.
-
-
-If the serial port buffer contains 64 bytes ready to send when it
-receives a flow control signal to stop sending, it will send out the
-64 bytes anyway in violation of the stop request. There is no
-stopping it since it doesn't know about flow control. If the buffer
-was large, then many more bytes would be sent in violation of flow
-control's request to stop.
-
-
-
-
-!!17.5 UART Model Numbers
-
-
-
- Here's a list of UARTs. ''TL'' is ''T''rigger ''L''evel
-
-
-* 8250, 16450, early 16550: Obsolete with 1-byte buffers
-*
-
-* 16550, 16550A, 16c552: 16-byte buffers, TL=1,4,8,14
-*
-
-* 16650: 32-byte buffers. Speed up to 460.8 kbps
-*
-
-* 16750: 64-byte buffer for send, 56-byte for receive. Speed up
-to 921.6 kbps
-*
-
-* Hayes ESP: 1k-byte buffers.
-*
-
-
-
-The obsolete ones are only good for modems no higher than 14.4k (DTE
-speeds up to 38400 bps). For modern modems you need at least a 16550
-(and not an early 16550). For V.90 56k modems, it may be a several
-percent faster with a 16650 (especially if you are downloading large
-uncompressed files). The main advantage of the 16650 is its larger
-buffer size as the extra speed isn't needed unless the modem
-compression ratio is high. Some 56k internal modems may come with a
-16650 ??
-
-
-Non-UART, and intelligent multiport boards use DSP chips to
-do additional buffering and control, thus relieving the CPU
-even more. For example, the Cyclades Cyclom, and Stallion
-EasyIO boards use a Cirrus Logic CD1400 RISC UART, and many
-boards use 80186 CPUs or even special RISC CPUs, to handle the
-serial IO.
-
-
-Many 486 PCs (old) and all Pentiums (or the like) should have 16550As
-(usually called just 16550's) with FIFOs. Some better motherboards
-today (2000) even have 16650s. For replacing obsolete UARTs with
-newer ones in pre 1990 hardware see the Appendix: Obsolete ...
-
-
-
-----
-
-!! 18. Pinout and Signals
-
-!!18.1 Pinout
-
-
-
-
-
-
-PINOUT of the SERIAL PORT (--> direction is out of PC)
-(Note DCD is sometimes labeled CD)
-Pin # Pin # Acronym Full-Name Direction What-it-May-Do/Mean
-9-pin 25-pin
-3 2 TxD Transmit Data --> Transmits bytes out of PC
-2 3 RxD Receive Data <-- Receives bytes into PC
-7 4 RTS Request To Send --> RTS/CTS flow control
-8 5 CTS Clear To Send <-- RTS/CTS flow control
-6 6 DSR Data Set Ready <-- I'm ready to communicate
-4 20 DTR Data Terminal Ready--> I'm ready to communicate
-1 8 DCD Data Carrier Detect<-- Modem connected to another
-9 22 RI Ring Indicator <-- Telephone line ringing
-5 7 SG Signal Ground
-
-
-
-
-
-
-!!18.2 Signals May Have No Fixed Meaning
-
-
-
- Only 3 of the 9 pins have a fixed assignment: transmit, receive
-and signal ground. This is fixed by the hardware and you can't change
-it. But the other signal lines are controlled by software and may do
-(and mean) almost anything at all. However they can only be in one of
-two states: asserted (+12 volts) or negated (-12 volts). Asserted is
-"on" and negated is "off". For example, Linux software may command
-that DTR be negated and the hardware only carries out this command and
-puts -12 volts on the DTR pin. A modem (or other device) that
-receives this DTR signal may do various things. If a modem has been
-configured a certain way it will hang up the telephone line when DTR
-is negated. In other cases it may ignore this signal or do something
-else when DTR is negated (turned off).
-
-
-It's like this for all the 6 signal lines. The hardware only sends
-and receives the signals, but what action (if any) they perform is up
-to the Linux software and the configuration/design of devices that you
-connect to the serial port. However, most pins have certain functions
-which they normally perform but this may vary with the operating
-system and the device driver configuration. Under Linux, one may
-modify the source code to make these signal lines behave differently
-(some people have).
-
-
-
-
-!! 18.3 Cabling Between Serial Ports
-
-
-
- A cable from a serial port always connects to another serial port.
-An external modem or other device that connects to the serial port has
-a serial port built into it. For modems, the cable is always straight
-thru: pin 2 goes to pin 2, etc. The modem is said to be DCE (Data
-Communications Equipment) and the computer is said to be DTE (Data
-Terminal Equipment). Thus for connecting DTE-to-DCE you use
-straight-thru cable. For connecting DTE-to-DTE you must use a
-null-modem cable (also called a crossover cable). There are many ways
-to wire such cable (see examples in Text-Terminal-HOWTO subsection:
-"Direct Cable Connection")
-
-
-There are good reasons why it works this way. One reason is that the
-signals are unidirectional. If pin 2 sends a signal out of it (but is
-unable to receive any signal) then obviously you can't connect it to
-pin 2 of the same type of device. If you did, they would both send
-out signals on the same wire to each other but neither would be able
-to receive any signal. There are two ways to deal with this
-situation. One way is to have a two different types of equipment
-where pin 2 of the first type sends the signal to pin 2 of the second
-type (which receives the signal). That's the way it's done when you
-connect a PC (DTE) to a modem (DCE). There's a second way to do this
-without having two different types of equipment: Connect pin sending
-pin 2 to a receiving pin 3 on same type of equipment. That's the way
-it's done when you connect 2 PCs together or a PC to a terminal
-(DTE-to-DTE). The cable used for this is called a null-modem cable
-since it connects two PCs without use of a modem. A null-modem cable
-may also be called a cross-over cable since the wires between pins 2
-and 3 cross over each other (if you draw them on a sheet of paper).
-The above example is for a 25 pin connector but for a 9-pin connector
-the pin numbers are just the opposite.
-
-
-The serial pin designations were originally intended for connecting a
-dumb terminal to a modem. The terminal was DTE (Data Terminal
-Equipment) and the modem was DCE (Data Communication Equipment).
-Today the PC is usually used as DTE instead of a terminal (but real
-terminals may still be used this way). The names of the pins are the
-same on both DTE and DCE. The words: "receive" and "transmit" are
-from the "point of view" of the PC (DTE). The transmit pin from the
-PC transmits to the "transmit" pin of the modem (but actually the
-modem is receiving the data from this pin so from the point of view of
-the modem it would be a receive pin).
-
-
-The serial port was originally intended to be used for connecting DTE
-to DCE which makes cabling simple: just use a straight-thru cable.
-Thus when one connects a modem one seldom needs to worry about which
-pin is which. But people wanted to connect DTE to DTE (for example a
-computer to a terminal) and various ways were found to do this by
-fabricating various types of special null-modem cables. In this case
-what pin connects to what pin becomes significant.
-
-
-
-
-!! 18.4 RTS/CTS and DTR/DSR Flow Control
-
-
-
- This is "hardware" flow control. Flow control was previously
-explained in the
-Flow Control
-subsection but the pins and voltage signals were not. Linux only
-supports RTS/CTS flow control at present (but a special driver may
-exist for a specific application which supports DTR/DSR flow control).
-Only RTS/CTS flow control will be discussed since DTR/DSR flow control
-works the same way. To get RTS/CTS flow control one needs to either
-select hardware flow control in an application program or use the
-command:
-stty crtscts < /dev/ttyS2 (or the like). This enables RTS/CTS
-hardware flow control in the Linux device driver.
-
-
-Then when a DTE (such as a PC) wants to stop the flow into it, it
-negates RTS. Negated "Request To Send" (-12 volts) means "request NOT
-to send to me" (stop sending). When the PC is ready for more bytes
-it asserts RTS (+12 volts) and the flow of bytes to it resumes. Flow
-control signals are always sent in a direction opposite to the flow of
-bytes that is being controlled. DCE equipment (modems) works the same
-way but sends the stop signal out the CTS pin. Thus it's RTS/CTS flow
-control using 2 lines.
-
-
-On what pins is this stop signal received? That depends on whether we
-have a DCE-DTE connection or a DTE-DTE connection. For DCE-DTE it's a
-straight-thru connection so obviously the signal is received on a pin
-with the same name as the pin it's sent out from. It's RTS-->RTS (PC
-to modem) and CTS<--CTS (modem to PC). For DTE-to-DTE the connection
-is also easy to figure out. The RTS pin always sends and the CTS pin
-always receives. Assume that we connect two PCs (PC1 and PC2)
-together via their serial ports. Then it's RTS(PC1)-->CTS(PC2) and
-CTS(PC1)<--RTS(PC2). In other words RTS and CTS cross over. Such a
-cable (with other signals crossed over as well) is called a "null
-modem" cable. See
-Cabling Between Serial Ports
-
-What is sometimes confusing is that there is the original use of RTS
-where it means about the opposite of the previous explanation above.
-This original meaning is: I Request To Send to you. This request was
-intended to be sent from a terminal (or computer) to a modem which, if
-it decided to grant the request, would send back an asserted CTS from
-its CTS pin to the CTS pin of the computer: You are Cleared To Send to
-me. Note that in contrast to the modern RTS/CTS bi-directional flow
-control, this only protects the flow in one direction: from the
-computer (or terminal) to the modem. This original use appears to be
-little used today on modern equipment (including modems).
-
-
-
-
-!The DTR and DSR Pins
-
-
- Just like RTS and CTS, these pins are paired. For DTE-to-DTE
-connections they are likely to cross over. There are two ways to use
-these pins. One way is to use them as a substitute for RTS/CTS flow
-control. The DTR pin is just like the RTS pin while the DSR pin
-behaves like the CTS pin. Although Linux doesn't support DTR/DSR flow
-control, it can be obtained by connecting the RTS/CTS pins at the PC
-to the DSR/DTR pins at the device that uses DTR/DSR flow control. DTR
-flow control is the same as DTR/DSR flow control but it's only one-way
-and only uses the DTR pin at the device. Many text terminals and some
-printers use DTR/DSR (or just DTR) flow control. In the future, Linux
-may support DTR/DSR flow control. The software has already been
-written but it's not clear when (or if) it will incorporated into the
-serial driver.
-
-
-The normal use of DTR and DSR (not for flow control) is as follows: A
-device asserting DTR says that its powered on and ready to operate.
-For a modem, the meaning of a DTR signal from the PC depends on how
-the modem is configured. Negating DTR is sometimes called "hanging
-up" but it doesn't always do this. One way to "hang up" (negate DTR)
-is to set the baud rate to 0 using the command "stty ". Trying to do
-this from a "foreign" terminal may not work due to the two-interface
-problem. See
-Two interfaces at a terminal. For internal modem-serial_ports it worked OK with a port
-using minicom but didn't work if the port was using wvdial. Why?
-
-
-
-
-!!18.5 Preventing a Port From Opening
-
-
-
- If "stty -clocal" (or getty is used with the "local" flag negated)
-then a serial port can't open until DCD gets an assert (+12 volts)
-signal.
-
-
-
-----
-
-!! 19. Voltage Waveshapes
-
-!!19.1 Voltage for a Bit
-
-
-
- At the EIA-232 serial port, voltages are bipolar (positive or
-negative with respect to ground) and should be about 12 volts in
-magnitude (some are 5 or 10 volts). For the transmit and receive
-pins +12 volts is a -bit (sometimes called "space") and -12 volts is
-a 1-bit (sometimes called "mark"). This is known as inverted logic
-since normally a -bit is both false and negative while a one is
-normally both true and positive. Although the receive and transmit
-pins are inverted logic, other pins (modem control lines) are normal
-logic with a positive voltage being true (or "on" or "asserted") and a
-negative voltage being false (or "off" or "negated"). Zero voltage
-has no meaning (except it usually means that the unit is powered off).
-
-
-A range of voltages is allowed. The specs say the magnitude of a
-transmitted signal should be between 5 and 15 volts but must never
-exceed 25 V. Any voltage received under 3 V is undefined (but some
-devices will accept a lower voltage as valid). One sometimes sees
-erroneous claims that the voltage is commonly 5 volts (or even 3
-volts) but it's usually 11-12 volts. If you are using a EIA-422 port
-on a Mac computer as an EIA-232 (requires a special cable) or EIA-423
-then the voltage will actually be only 5 V. The discussion here
-assumes 12 V.
-
-
-Note that normal computer logic normally is just a few volts (5 volts
-was once the standard) so that if you try to use test equipment
-designed for testing 3-5 volt computer logic (TTL) on the 12 volts of a
-serial port, it may damage the test equipment.
-
-
-
-
-!! 19.2 Voltage Sequence for a Byte
-
-
-
- The transmit pin (TxD) is held at -12 V (mark) at idle when nothing
-is being sent. To start a byte it jumps to +12 V (space) for the
-start bit and remains at +12 V for the duration (period) of the start
-bit. Next comes the low-order bit of the data byte. If it's a -bit
-nothing changes and the line remains at +12 V for another bit-period.
-If it's a 1-bit the voltage jumps from +12 to -12 V. After that comes
-the next bit (-12 V if a 1 or +12 V if a ), etc., etc. After the
-last data bit a parity bit may be sent and then a -12 V (mark) stop
-bit. Then the line remains at -12 V (idle) until the next start bit.
-Note that there is no return to 0 volts and thus there is no simple
-way (except by a synchronizing signal) to tell where one bit ends and
-the next one begins for the case where 2 consecutive bits are the same
-polarity (both zero or both one).
-
-
-A 2nd stop bit would also be -12 V, just the same as the first stop
-bit. Since there is no signal to mark the boundaries between these
-bits, the only effect of the 2nd stop bit is that the line must remain
-at -12 V idle twice as long. The receiver has no way of detecting the
-difference between a 2nd stop bit and a longer idle time between
-bytes. Thus communications works OK if one end uses one stop bit and
-the other end uses 2 stop bits, but using only one stop bit is
-obviously faster. In rare cases 1 1/2 stop bits are used. This means
-that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
-50% wider than normal).
-
-
-
-
-!! 19.3 Parity Explained
-
-
-
- Characters are normally transmitted with either 7 or 8 bits of
-data. An additional parity bit may (or may not) be appended to this
-resulting in a byte length of 7, 8 or 9 bits. Some terminal emulators
-and older terminals do not allow 9 bits. Some prohibit 9 bits if 2
-stop bits are used (since this would make the total number of bits too
-large: 12 bits total after adding the start bit).
-
-
-The parity may be set to odd, even or none (mark and space parity may
-be options on some terminals or other serial devices). With odd
-parity, the parity bit is selected so that the number of 1-bits in a
-byte, including the parity bit, is odd. If a such a byte gets
-corrupted by a bit being flipped, the result is an illegal byte of
-even parity. This error will be detected and if it's an incoming byte
-to the terminal an error-character symbol will appear on the screen.
-Even parity works in a similar manner with all legal bytes (including
-the parity bit) having an even number of 1-bits. During set-up, the
-number of bits per character usually means only the number of data
-bits per byte (7 for true ASCII and 8 for various ISO character sets).
-
-
-A "mark" is a 1-bit (or logic 1) and a "space" is a -bit (or logic
-). For mark parity, the parity bit is always a one-bit. For space
-parity it's always a zero-bit. Mark or space parity (also known as
-"sticky parity") only wastes bandwidth and should be avoided if
-feasible. The stty command can't set sticky parity but it's
-supported by serial hardware and can be dealt with by programming in
-C. "No parity" means that no parity bit is added. For terminals
-that don't permit 9 bit bytes, "no parity" must be selected when using
-8 bit character sets since there is no room for a parity bit.
-
-
-
-
-!!19.4 Forming a Byte (Framing)
-
-
-
- In serial transmission of bytes via EIA-232 ports, the low-order
-bit is always sent first. Serial ports on PC's use asynchronous
-communication where there is a start bit and a stop bit to mark the
-beginning and end of a byte. This is called framing and the framed
-byte is sometimes called a frame. As a result a total of 9, 10, or 11
-bits are sent per byte with 10 being the most common. 8-N-1 means 8
-data bits, No parity, 1 stop bit. This adds up to 10 bits total when
-one counts the start bit. One stop bit is almost universally used.
-At 110 bits/sec (and sometimes at 300 bits/sec) 2 stop bits were once
-used but today the 2nd stop bit is used only in very unusual
-situations (or by mistake since it still works OK that way but wastes
-bandwidth).
-
-
-Don't confuse this type of framing with the framing used for a packet
-of bytes on a network. The serial port just frames every byte. For a
-network many bytes are framed into a packet (sometimes called a
-frame). For a network frame, instead of a start bit, there is a
-sequence of bytes called a header. On a network that uses serial
-ports (with modems), a report of a frame error usually refers to a
-multi-byte frame and not the serial port frame of a single byte.
-
-
-
-
-!!19.5 How "Asynchronous" is Synchronized
-
-
-
- The EIA-232 serial port as implemented on PC is asynchronous which
-in effect means that there is no "clock" signal sent with "ticks" to
-mark when each bit is sent.. There are only two states of the
-transmit (or receive) wire: mark (-12 V) or space (+12 V). There is
-no state of 0 V. Thus a sequence of 1-bits is transmitted by just a
-steady -12 V with no markers of any kind between bits. For the
-receiver to detect individual bits it must always have a clock signal
-which is in synchronization with the transmitter clock. Such a clock
-would generate a "tick" in synchronization with each transmitted (or
-received) bit.
-
-
-For asynchronous transmission, synchronization is achieved by framing
-each byte with a start bit and a stop bit (done by hardware). The
-receiver listens on the negative line for a positive start bit and
-when it detects one it starts its clock ticking. It uses this clock
-tick to time the reading of the next 7, 8 or 9 bits. (It actually is
-a little more complex than this since several samples of a bit are
-normally taken and this requires additional timing ticks.) Then the
-stop bit is read, the clock stops and the receiver waits for the next
-start bit. Thus async is actually synchronized during the reception
-of a single byte but there is no synchronization between one byte and
-the next byte.
-
-
-
-----
-
-!! 20. Other Serial Devices (not async EIA-232)
-
-!! 20.1 Successors to EIA-232
-
-
-
- A number of EIA standards have been established for higher speeds
-and longer distances using twisted-pair (balanced) technology.
-Balanced transmission can sometimes be a hundred times faster than
-unbalanced EIA-232. For a given speed, the distance (maximum cable
-length) may be many times longer with twisted pair. But PC-s keep
-being made with the "obsolete" EIA-232 since it works OK with modems
-and mice since the cable length is short. If this appears in the
-latest version of this HOWTO, please let me know if any of the
-non-EIA-232 listed below are supported by Linux.
-
-
-
-
-!!20.2 EIA-422-A (balanced) and EIA-423-A (unbalanced)
-
-
-
- EIA-423 is just like the unbalanced EIA-232 except that the
-voltage is only 5 volts. Since this falls within EIA-232 specs it
-can be connected to a EIA-232 port. Its specs call for somewhat
-higher speeds than the EIA-232 (but this may be of little help on a
-long run where it's the unbalance that causes interference). Since
-EIA-423 is not much of an improvement over EIA-232, it is not popular
-except on old Mac computers.
-
-
-EIA-422 is twisted pair (known as "balanced" or "differential) and is
-(per specs) exactly 100 times as fast as EIA-423 (which in turn is
-somewhat faster than EIA-232). Apple's Mac computer prior to mid-1998
-with its EIA-232/EIA-422 Port uses it. The Mac used a small round
-"mini-DIN-8" connector. It also provided conventional EIA-232 but at
-only at 5 volts (which is still legal EIA-232). To make it work like
-at EIA-232 one must use a special cable which (signal) grounds RxD+
-(one side of a balanced pair) and use RxD- as the receive pin. While
-TxD- is used as the transmit pin, for some reason TxD+ should not be
-grounded. See
-Macintosh Communications FAQ. However, due to the fact that
-Macs (and upgrades for them) cost more than PC's, they are not widely
-as host computers for Linux.
-
-
-
-
-!!20.3 EIA-485
-
-
-
- This is like EIA-422 (balanced = differential). It is
-half-duplex. It's not just point-to-point but is like ethernet or
-the USB since all devices (nodes) on it share the same "bus". It may
-be used for a multidrop LAN (up to 32 nodes or more). Since many
-nodes share the same twisted pair the need to use the electrical
-tri-state mode where besides the 0 and 1 binary states there is also
-an open circuit state to permit other nodes to uses the twisted pair
-line. Instead of a transmitter keeping a 1-state voltage on the line
-during line idle, the line is open circuited and all nodes just listen
-(receive mode).
-
-
-The most common architecture is master/slave. The master polls the
-slaves to see if they have anything to send. A slave can only
-transmit just after it's been polled.
-
-
-There is an alternative implementation where two pair of wires are used
-for sending data. One pair is only for the Master to send to the Slaves.
-Since no one transmits on this line except the master, there is no
-need for it to be tri-state. Thus the Master may just be EIA-232 but
-the slaves must still be EIA-485. See
-http://www.hw.cz/english/docs/rs485/rs485.html for more
-details.
-
-
-
-
-!!20.4 EIA-530
-
-
-
- EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
-(balanced) was intended to be a replacement for EIA-232 but few have
-been installed. It uses the same 25-pin connector as EIA-232.
-
-
-
-
-!!20.5 EIA-612/613
-
-
-
- The High Speed Serial Interface ( HSSI = EIA-612/613) uses a
-50-pin connector and goes up to about 50 Mbits/s but the distance is
-limited to only several meters. For Linux there are PCI cards
-supporting HSSI. The companies that sell the cards often provide (or
-point you to) a Linux driver. A howto or the like is needed for this
-topic.
-
-
-
-
-!!20.6 The Universal Serial Bus (USB)
-
-
-
- The Universal Serial Bus (USB) is being built into PCI chips.
-Newer PC's have them. It is 12 Mbps (with 200 Mbps planned) over a
-twisted pair with a 4-pin connector (2 wires are power supply). It
-also is limited to short distances of at most 5 meters (depends on
-configuration). Linux supports the bus, although not all devices that
-can plug into the bus are supported.
-
-
-It is synchronous and transmits in special packets like a network.
-Just like a network, it can have several devices attached to it. Each
-device on it gets a time-slice of exclusive use for a short time. A
-device can also be guaranteed the use of the bus at fixed intervals.
-One device can monopolize it if no other device wants to use it. It's
-not simple to describe in detail.
-
-
-For documentation, see the USB directory in /usr/share/doc/kernel ...
-It would be nice to have a HOWTO on the USB. See also
-http://www.linux-usb.org and/or
-http://.www.qbik.ch/usb/.
-
-
-
-
-!!20.7 Firewire
-
-
-
- Firewire (IEEE 1394) is something like the USB only faster (800
-Mbps is planned). The protocol on the bus is claimed to be more
-efficient than USB's. It uses two twisted pair for data plus two
-power conductors (6 conductors in all). A variants uses only 4
-conductors. You may compile firewire support into the Linux kernel.
-Like USB, it's also limited to short distances.
-
-
-
-
-!!20.8 MIDI
-
-
-
-Sound cards often have a 15-pin MIDI connector. There are also
-such connectors not associated with a sound card. They are for
-connecting a musical keyboard to a PC so that you can create musical
-recordings. You could also connect a MIDI sound system. The MIDI
-standard uses 31250 baud (1M/32) which is not available on an ordinary
-serial port. Some MIDI devices are designed so that they can be
-connected directly to an ordinary serial port.
-
-
-Besides the 15-pin connector, many use a 5-pin DIN connector.
-The /dev/midi00 is for MIDI.
-
-
-
-
-!! 20.9 Synchronization & Synchronous
-
-
-
- Beside the asynchronous EIA-232 (and others) there are a number of
-synchronous serial port standards. In fact EIA-232 includes
-synchronous specifications but they aren't normally implemented for
-serial ports on PC's. But first we'll explain what a synchronous
-means.
-
-
-
-
-!Defining Asynchronous vs Synchronous
-
-
- Asynchronous (async) means "not synchronous". In practice, an
-async signal is what the async serial port sends and receives which is
-a stream of bytes with each byte framed by a start and stop bit.
-Synchronous (sync) is most everything else. But this doesn't explain
-the basic concepts.
-
-
-In theory, synchronous means that bytes are sent out at a constant
-rate one after another in step with a clock signal tick. There is
-often a separate wire or channel for sending the clock signal. The
-clock signal might also be embedded in the transmitted bytes.
-Asynchronous bytes may be sent out erratically with various time
-intervals between bytes (like someone typing characters at a
-keyboard).
-
-
-When a file is being sent thru the async serial port, the flow of
-bytes will likely be at the speed of the port (say 115.2k) which is a
-constant rate. This flow may frequently start and stop due to flow
-control. Is this sync or async? Ignoring the flow control stops, it
-might seem like sync since it's a steady flow. But it's not because
-there is no clock signal and the bytes could have been sent
-erratically since they are framed by start/stop bits.
-
-
-Another case is where data bytes (without any start-stop bits) are put
-into packets with possible erratic spacing between one packet and the
-next. This is called sync since the bytes within each packet are
-transmitted synchronously.
-
-
-
-
-!Synchronous Communication
-
-
- Did you ever wonder what all the unused pins are for on a 25-pin
-connector for the serial port? Most of them are for use in
-synchronous communication which is seldom implemented in chips for
-PC's. There are pins for sync timing signals as well as for a sync
-reverse channel. The EIA-232 spec provides for both sync and async
-but PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip
-such as a 16450, 16550A, or 16650 and can't deal with sync. For sync
-one needs a USRT chip or the equivalent where the "S" stands for
-Synchronous. A USART chip supports both synchronous and asynchronous.
-Since sync is a niche market, a sync serial port is likely to be quite
-expensive.
-
-
-SCC stands for "Serial Communication Controller" or "Serial Controller
-Chip". It's likely old terminology and since it doesn't say "sync"
-or "async" it might support both.
-
-
-Besides the sync part of the EIA-232, there are various other EIA
-synchronous standards. For EIA-232, 3 pins of the connector are
-reserved for clock (or timing) signals. Sometimes it's a modem's task
-to generate some timing signals making it impossible to use
-synchronous communications without a synchronous modem (or without a
-device called a "synchronous modem eliminator" which provides the
-timing signals).
-
-
-Although few serial ports are sync, synchronous communication
-does often take place over telephone lines using modems which use
-V.42 error correction. This strips off the start/stop bits and puts
-the data bytes in packets resulting in synchronous operation over the
-phone line.
-
-
-
-----
-
-!!21. Other Sources of Information
-
-!!21.1 Books
-
-
-
-
-
-
-# Axleson, Jan: Serial Port Complete, Lakeview Research, Madison,
-WI, 1998.
-#
-
-# Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
-Computer Society Press, Los Alamitos, CA, 1996.
-#
-
-# Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.
-#
-
-# Campbell, Joe: C Programmer's Guide to Serial Communications,
-2nd ed., Unknown Publisher, 1993.
-#
-
-#
-Levine, Donald: POSIX Programmer's Guide, O'Reilly, 1991.
-#
-
-# Nelson, Mark: Serial Communications Developer's Guide, 2nd ed.,
-Hungry Minds, 2000.
-#
-
-# Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.
-#
-
-# Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall,
-1991.
-#
-
-#
-Stevens, Richard W.: Advanced Programming in the UNIX Environment,
-(ISBN -201-56317-7; Addison-Wesley)
-#
-
-# Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996.
-Chapter 7: Serial Ports
-#
-
-
-
-Notes re books:
-
-
-#"... Complete" has hardware details (including register) but the
-programming aspect is Window oriented.
-#
-
-#"Physical Layer ..." covers much more than just EIA-232.
-#
-
-
-
-
-
-!!21.2 Serial Software
-
-
-
- It's best to use the nearest mirror site, but here's the main
-sites:
-Serial Software for Linux software for the serial ports
-including getty and port monitors.
-Serial Communications for communication programs.
-
-
-
-
-
-* irqtune will give serial port interrupts higher
-priority to improve performance. Using hdparm for hard-disk tuning
-may help some more.
-
-*
-
-* modemstat and statserial show the current state of
-various modem control lines. See
-Serial Monitoring/Diagnostics
-*
-
-
-
-
-
-!!21.3 Related Linux Documents
-
-
-
-
-
-
-*man pages for: setserial and stty
-*
-
-*
-Low-Level Terminal Interface part of "GNU C Library Reference
-manual" (in libc (or glibc) docs package). It covers the detailed
-meaning of "stty" commands, etc.
-*
-
-*Modem-HOWTO: modems on the serial port
-*
-
-*PPP-HOWTO: help with PPP (using a modem on the serial port)
-*
-
-*Printing-HOWTO: for setting up a serial printer
-*
-
-*Serial-Programming-HOWTO: for some aspects of serial-port programming
-*
-
-*Text-Terminal-HOWTO: how they work and how to install and configure
-*
-
-*UPS-HOWTO: setting up UPS sensors connected to your serial port
-*
-
-*UUCP-HOWTO: for information on setting up UUCP
-*
-
-
-
-
-
-!!21.4 Usenet newsgroups:
-
-
-
-
-
-
-* comp.os.linux.answers
-*
-
-* comp.os.linux.hardware: Hardware compatibility with the Linux
-operating system.
-*
-
-* comp.os.linux.networking: Networking and communications under Linux.
-*
-
-* comp.os.linux.setup: Linux installation and system administration.
-*
-
-
-
-
-
-!!21.5 Serial Mailing List
-
-
-
-The Linux serial mailing list. To join, send email to
-majordomo@vger.rutgers.edu, with ``subscribe
-linux-serial'' in the message body. If you send ``help'' in
-the message body, you get a help message. The server also serves
-many other Linux lists. Send the ``lists'' command for a list
-of mailing lists.
-
-
-
-
-!!21.6 Internet
-
-
-
-
-
-
-*
-Linux Serial Driver home page Includes info about PCI support.
-*
-
-*
-
-Serial Suite by
-Vern Hoxie is a collection of blurbs about the care and feeding of
-the Linux serial port plus some simple programs. He also has a
-Serial-Programming-HOWTO (not yet available from the Linux
-Documentation Project). Your browser should automatically log you in
-but if you do it manually login as "anonymous" and use your full
-e-mail address as the password.
-
-*
-
-* A white paper discussing serial communications and multiport
-serial boards was available from Cyclades at
-http://www.cyclades.com.
-*
-
-
-
-
-----
-
-!!22. Appendix: Obsolete Hardware (prior to 1990) Info
-
-!!22.1 Replacing obsolete UARTS
-
-
-
- Many 486 PCs (old) and all Pentiums (or the like) should have
-modern 16550As (usually called just 16550's) with FIFOs. If you have
-something really old the chip may unplug so that you may be able to
-upgrade by buying a 16550A chip and replacing your existing 16450
-UART. If the functionality has been built into another type of chip,
-you are out of luck. If the UART is socketed, then upgrading is easy
-(if you can find a replacement). The new and old are pin-to-pin
-compatible. It may be more feasible to just buy a new serial card on
-the Internet (few retail stores stock them today) or find a used one
.
-
-
-
-
-
- END OF Serial-HOWTO
-----
+Describe [HowToSerialHOWTO]
here.
