UPGRADING REPAIRING PCs

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Chapter 4—SCSI and IDE Hard Drives and Optical Drives
The Alternative 2 Centronics latch-style connector remains
unchanged from SCSI-1.
SCSI-3 68-Pin P Cable
A new 68-conductor P cable was developed as part of the SCSI-3
specification. Shielded and unshielded high-density D-shell connectors
are specified for both the A and P cable. The shielded highdensity
connectors use a squeeze-to-release latch rather than the
wire latch used on the Centronics-style connectors. Active termination
for single-ended buses is specified, providing a high level of
signal integrity. Figure 4.9 shows the 68-pin high density SCSI connector.
34 1
68 35
Figure 4.9 High-density, 68-pin SCSI connector.
RAID Array, Hot Swappable 80-Pin Connector
Drive arrays normally use special SCSI drives with what is called an
80-pin Alternative-4 connector, which is capable of wide SCSI and
also includes power signals. Drives with the 80-pin connector are
normally hot swappable—they can be removed and installed with
the power on—in drive arrays. The 80-pin Alt-4 connector is shown
in Figure 4.10.
Pin 80
Figure 4.10 80-pin Alt-4 SCSI connector.
Pin 2 Pin 1
Apple and some other non-standard implementations from other
vendors (such as Iomega SCSI Zip drives) used a 25-pin cable and
connector for SCSI devices.
They did this by eliminating most of the grounds from the cable,
which unfortunately resulted in a noisy, error-prone connection. I
don’t recommend using 25-pin cables and connectors; you should
avoid them if possible. The connector used in these cases was a
standard female DB-25 connector, which looks exactly like a PC
parallel port (printer) connector. Unfortunately, it is possible to