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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0101:  SCSI

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VII  Connectors
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 IX  LVD/MSE
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VIII  SE/HVD

The original SCSI used positive voltage as a one and zero voltage as a zero. This became known as Single-Ended signaling (SE). The down side to SE was limits to cable length and speed limitations. The solution came by using two wires. By having zero voltage on both wires, the signal was a zero. By having two voltages of different values on the wire pair it gives a logical one (value) for signaling. This was called differential signaling. Later the term was redefined to High Voltage Differential (HVD). This allowed for longer cable lengths.

Table 13: SCSI Bus Lengths for SE and HVD

SCSI Type

Signaling Bus Speed

SE Maximum Length

HVD Maximum Length

SCSI-1

5 MHz

6 Meters

25 Meters

Fast

10 MHz

3 Meters

25 Meters

Fast-Wide

20 MHz

1.5 Meters

25 Meters

[spacer]Don’t Smoke that Drive – The Sequel

SE and HVD have different voltage levels and are incompatible at an electrical level. DO NOT mix SCSI devices. The possibility of physically smoking the hardware is very real because of the high voltage used in HVD. Unfortunately, cables and connectors for SE and differential SCSI appear the same. The Information Technology Industry Council (ITIC) has created international symbols to differentiate between the two SCSI types, as shown in Figure 65.



Figure 65: SE and HVD SCSI Symbols (Copyright ITIC, used with permission).

 

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 IX  LVD/MSE
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