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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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