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22. SCSI. SCSI HARD DRIVES
| The normal internal cable for SCSI is a 50 conductor ribbon, with all
odd numbered conductors grounded. Two conductors, numbers 25 & 26, are
often left not-connected, as they deal with Terminator power, and can be
easily shorted by cable reversals. There are no twists in this cable, and
it's length may be a maximum of 6 meters. But one is advised to use
minimum lengths to improve timing. Up to seven drives, or devices may be
attached to an SCSI cable. Each is daisy-chained on the cable, or, when a
device has two connectors, another cable may be "spliced" into the chain
starting at the second connector, and continued on. Care must be taken to
insure that cables and connectors are not reversed, as this would short
pin 26 (TERMPWR) to ground, and likely damage the drive or controller.
Also, as explained earlier, the terminating resistors should remain only
on the controller (Host Adapter) and the LAST drive on the cable,
regardless of it's address. Most SCSI Host Adapters also have a connector
for external drives in the form of a Centronics(tm) type 50 pin, or an
"alternate", DB-25F connector. Only the internal 50-pin, and the
"alternate" external connector are shown here. (see also: MORE SCSI) Also,
these diagrams refer to the single-ended SCSI connections, since this is
the most common arrangement for PCs today. The Differential SCSI requires
balanced lines, and is used mostly on high-end workstations. |
 |
Iomega ZIP SCSI:
| 1 |
req |
14,16,18,24,9,7 |
GND |
| 2 |
msg |
15 |
cmd/data |
| 3 |
i/o |
17 |
attn |
| 4 |
rst |
19 |
sel |
| 5 |
ask |
20 |
parity |
| 6 |
bsy |
21 |
db1 |
| 8 |
db0 (lsb) |
22 |
db2 |
| 10 |
db3 |
23 |
db4 |
| 11 |
db5 |
25 |
tmpwr |
| 12 |
db6 |
13 |
db7 (msb) |
SCSI Connector Pinouts
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