The single most critical item in the installation of a successful, high-reliability, maximum throughput SCSI system. Do not waste time and money on cheap SCSI cables!
The longer the cables and the faster the data throughput, the more critical cables become. Internal SCSI cables are usually non-shielded ribbon cables and external cables are usually round, shielded cables with shielded connectors. Cable impedance must be matched to the requirements of SCSI devices and cable pairs must be carefully selected for the correct SCSI signal lines.
SCSI specifications recommend the use of twisted pair cable for either ribbon cables or external shielded cables. With the higher data throughput of today's SCSI systems, twisted pairs are strongly recommended. For a correctly made round, shielded cable assembly, SCSI signal pairs must be placed on wire pairs in the cable. Also, sensitive signals such as REQ and ACK must be placed on pairs in the core of the cable with data lines in the second layer of pairs and data lines in the outer layer. Ribbon cables are generally not a problem as the wire location in the ribbon is fixed.
* Alt 1 Cable An A-cable having 50-pin male high-density micro-D connectors with spring-latch fasteners. May be nonshielded or shielded.
* Alt 2 Cable An A-cable having 50-pin male Centronics-type connectors for round, shielded cabling or two rows of 25 pins on 0.1 inch centers for internal nonshielded ribbon cabling.
* A-Cable A 50 pin narrow SCSI cable. See Alt 1 and Alt 2 above for the 4 types of A-Cable.
* B-Cable Identical in construction to the A-cable introduced in SCSI-2 for use with WIDE SCSI. Never popular because of requirement for two cables. The B-cable was rendered obsolete by the introduction of the P-cable in SCSI-3. Use of WIDE SCSI immediately began to increase.
* L-Cable A 110-pin high-density male micro-D connector considered for use in 32-bit WIDE SCSI systems. No longer viable as 32-bit SCSI was obsoleted in the SPI-3 document of SCSI-3.
* P-Cable In SCSI-3, the P-cable is defined for 16-bit WIDE SCSI systems to eliminate the necessity to use two cables (an A-cable and a B-cable) for 16-bit WIDE SCSI. Uses 34 twisted pair cable (68 wires) and 68-pin high-density male micro-D connectors with 2-56 thumbscrew fasteners.
* Q-Cable Physically and electrically identical to the P-cable. Both a P-cable and a Q-cable were specified for 32-bit WIDE SCSI buses. This cable is no long viable as 32-bit wide SCSI was rendered obsolete in the SPI-3 document of SCSI-3.
See also Connectors.

This definition is in context to SCSI. See more contextual defintions for Cables.