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Appendix B
B-5
Operating This Jukebox on a SCSI Bus
A Brief Overview of SCSI
Op
eration o
n
a
SC
SI
Bu
s
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).
Connectors
Narrow devices (50-pin) use the following connectors:
•
50-pin low-density clip Centronics-type (external)
This is a large connector that is similar to a printer cable. This type of connector
is gradually being replaced by the Micro D connector, described below.
•
50-pin high density Micro D (using thumbscrews or small clips) (external)