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PC Hardware in a Nutshell, 3rd Edition by Barbara Fritchman Thompson, Robert Bruce Thompson

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LVD SCSI cables, connectors, and signals

LVD SCSI transfer modes use a wide (68-pin) cable of special design and construction, which is labeled and referred to as a SCSI LVD cable. An LVD cable uses the same high-density 68-pin external and VHDCI 68-pin external connectors as a P cable. However, all LVD connectors, internal or external, must be shielded, so the high-density 68-pin internal connector is not supported for LVD.

Tip

Although a narrow (50-pin) LVD cable is defined by the SCSI standard, all actual LVD implementations are wide, so you will never encounter a narrow LVD cable.

Table 13-12 lists the pinouts for SCSI LVD cables and connectors. Because LVD uses differential signaling rather than the signal/ground method used by SE implementations, each LVD signal is actually a plus and minus signal pair, carried on a twisted pair within the cable. So, for example, whereas in SE SCSI conductors 2 and 1 carry the DB(12)# (active-low) signal and its “signal return” (ground), in LVD SCSI those same conductors carry the DB(12)- (negative) and DB(12)+ (positive) signal pair, respectively. LVD adds one signal not used by earlier variants. The DIFFSENS signal (conductor 31 in LVD Wide, and conductor 21 on LVD Narrow) is used to control differential signaling.

Table 13-12. SCSI LVD cable pinouts

Signal

Pin #

Cable conductor #

Pin #

Signal

DB(12)+

1

1

2

35

DB(12)-

DB(13)+

2

3

4

36

DB(13)-

DB(14)+

3

5

6

37

DB(14)-

DB(15)+

4

7

8

38

DB(15)-

DB(Parity1)+

5

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