104 RS/6000 43P 7043 Models 150 and 260 Handbook
4.2.4.3 SCSI Differential
Normally, there is one wire in a SCSI cable for each signal. However, over
long distances and with high clocking rates, the signals can degrade and
errors can occur. To solve this, SCSI Differential was developed, which uses
two wires for each signal.
Consider Figure 30 on page 104 as an example. Signal A is transmitted along
two wires, A+ and A-. A- is the mirror image of A+. When some line noise
appears such as a signal spike, the spike will be of the same polarity on both
A+ and A-. On the receiving end, the signals are subtracted, and the result is
A+/A- as shown in the diagram. The spike is canceled out by the subtraction.
Figure 30. SCSI Differential Example
With SCSI Differential, longer cable lengths, up to 25 meters, can be
maintained. The trade-off is the additional cost of the cables, connectors and
the circuitry needed to perform the calculations. SCSI Differential is not
commonly used for long distances. However, SCSI repeaters, as described in
the below section, are more common.
4.2.5 SCSI Repeaters
A SCSI repeater is a device that enhances the signal quality of a SCSI bus
and allows the bus to be physically longer. The repeater can be in the form of
either an adapter or an external "black box."
As noted in Table 19 on page 105, the maximum length of the SCSI bus for a
SCSI-II F/W configuration is 3.0 meters. With a repeater, it is possible to
extend this distance.
A+
A -
1
0
1
0
1
0
A+