IBM 150 Manual De Usuario

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.

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.

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.