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

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Wide single-ended SCSI cables, connectors, and signals

Wide (16-bit) SCSI transfer modes use wide (68-pin) cables. Officially, a wide cable is called a SCSI P cable, but it may also be called a SCSI-2 cable or a 68-pin SCSI cable. A P cable may use any of several connectors, most commonly high-density 68-pin internal, high-density 68-pin external, and VHDCI 68-pin external. Wide SCSI uses 68 signals, each carried on one of the 68 wires in the SCSI P cable, with the 68 wires organized into 34 pairs. For SE SCSI, each pair includes a signal wire and a signal return (ground) wire. Figure 13-6 shows a SCSI P cable with an internal 68-pin high-density connector. Note the twisted pairs in the cable segment at top.

SCSI P cable with 68-pin high-density connector

Figure 13-6. SCSI P cable with 68-pin high-density connector

Tip

Early wide SCSI implementations used an awkward combination of two cables: a standard 50-pin A cable and a special 68-pin B cable. The B cable was never popular, and the combination A+B cabling was quickly replaced by the single 68-pin P cable.

Table 13-11 lists the pinouts for SCSI P cables and connectors. A “#” following a signal name indicates that signal is active-low. In a P cable, (reserved) lines are left open in SCSI devices and terminators. Although conductor numbers do not map directly to pin numbers, all P cable connectors use the same pinouts.

Tip

A 32-bit SCSI Q cable was defined, but that cable was never ...

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