SCSI has it's roots in the mainframe world, but it's first implementation in the PC world came soon after the first PC. Shugart Associates devised an interface that they designated the SASI, or "Shugart Associates Standard Interface." They proposed that SASI be adopted by ANSI for small computers, and began the work required for ratification. During this process, ideas were exchanged with the IPI group, who were already well into their effort. The SASI group decided to take features of both interfaces, and put forth a specification for a new, improved interface, to be called SCSI, or Small Computer Systems Interface. SCSI was ratified in 1986.
The original specifications for SCSI-1 called for a few, core commands to be required, and all others to be optional. This led to many problems of compatibility between Manufacturers, where a peripheral from one Company would not play with a peripheral from another Company. Much of this was corrected with the advent of SCSI-2. The SCSI-2 specification also detailed a faster SCSI bus speed, labeled Fast SCSI, and a wider bus, labeled Wide SCSI. The SCSI Committee is currently working on SCSI-3, which adds many diverse peripherals to the SCSI spec. and increases the number of required SCSI commands.
The normal internal cable for SCSI is a 50 conductor ribbon, with all
odd numbered conductors grounded. 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.
The normal internal SCSI connector is a 50-pin, dual row "Berg" type (male gender). A diagram of this type connector is illustrated by figure TT12. (The internal connector for Wide SCSI is not illustrated.)
Most SCSI-1 Host Adapters also have a connector for external drives in the form of a "D" shaped, 50-pin dual row "Centronics"(tm) type connector. A diagram of this connector is illustrated by figure TT13.
fig. tt12 fig. tt13
An "alternate" SCSI-1 external connector was used by some Manufacturers as a space/cost saving feature. This called for using a standard DB-25 connector (female gender) with staggered pins. Two different thoughts emerged on how this alternate SCSI-1 connector should be wired. Apple Computer went with one implementation, illustrated by figure TT14, while Future Domain Corp. went with another, illustrated by figure TT15.
fig. tt14 fig. tt15