What is a SCSI terminator? Why do I need them?
A SCSI bus is a transmission line. To prevent reflections from the ends of the bus, you need a device which makes the transmission line appear to be of infinite length. This is done by attaching resistors, which have the same resistance as the characteristic impedance of the transmission line, to the ends of the bus. Also, since SCSI line drivers are open-collector (which can only pull a signal low), a pull-up resistor is needed to pull the signal high when it’s not asserted. If the ends of the bus are not terminated, the signal pulses will reflect off these open ends and travel back along the bus in the other direction. The resultant adding and cancelling of signal amplitudes distorts and corrupts the SCSI signals. There are two basic types of terminators, active and passive: • Passive terminators consist of pairs of resistors. A 220 Ohm pulling each signal up to TERMPWR and a 330 Ohm pulling each signal down to GROUND. Passive terminators were considered adequate in SCSI-1 when the b
