Any driver on a circuit that has a non-zero OutputImpedance? requires ImpedanceMatching to deliver the maximum amount of power to the load.

If the load InputImpedance (which can be modelled as a Resistance? at a single frequency is equal to the drivers OutputImpedance?, the power at the load is at a maximum. If there is a TransmissionLine connecting the driver to the load, it must also have a CharacteristicImpedance close to the OutputImpedance?, so as to instantaneously load the driver to the right load, to prevent ringing in the circuit. Imagine you take a TransmissionLine and make an incision, both sides of the TransmissionLine will be the same CharacteristicImpedance as the original line. This can be modeled as a amplifier with unity gain (0dB) and an InputImpedance / OutputImpedance? equal to the CharacteristicImpedance of each respective side, ensuring maximum power is transferred.

The reason why SCSI busses need to be terminated is that all the drive inputs have a high InputImpedance which means they don't draw power over the bus (which is a TransmissionLine). The terminators at the end of a SCSI Chain ensure that there is enough load on the drivers to maintain proper signal levels. The result is also that when the rising edge of the signal gets to the end of the cable, it has an (ImpedanceMatched?) path to ground, thus signals do not reflect back down the line, degrading signal integrity and lowering the maximum speed we can drive the bus.

CategoryElectronics