HP Surestore 80ex - Optical Jukebox, User Manual

Page: 76 / 96

B-4 Appendix B
Operating This Jukebox on a SCSI Bus
A Brief Overview of SCSI
Termination
To keep signals on the SCSI bus from being reflected, “terminators” must be placed
at each end of the physical bus. These terminators may be either active or passive.
Passive termination is a resistor network. Active termination uses a voltage
regulator (the active component) to regulate the power of the resistor network to
provide more stable termination. Active termination is always preferred over
passive termination.
Depending on your SCSI device, termination is supplied by a using a physical
connector, by flipping a dipswitch, or by selecting the termination setting in
software.
Termination is always at both physical ends of the bus.
Single-ended and Differential Interfaces
The SCSI bus is electrically implemented in two ways: single-ended and
differential. These terms come from the way the signals are asserted on the bus.
Single-ended buses use a 5-volt signal that is referenced to ground. This method
makes the bus somewhat susceptible to noise and loss of signal quality over
distance. The maximum allowable length of a single-ended SCSI bus was initially 6
meters (19.7 feet). Fast SCSI required the maximum length of the bus to be reduced
to 3 meters (9.8 feet).
The differential bus uses two lines for each signal and measures the voltage
difference between the two signals. Differential retains more signal quality than
single-ended and is not as sensitive to noise, allowing for a longer cable length. The
maximum allowable length of a differential bus is 25 meters (82 feet) for all types
buses (narrow, wide, fast, Fast-20).