Small Computer System Interface

Published on Nov 03, 2015


SCSI is actually an acronym for Small Computer System Interface and it is pronounced as "skuzzy". It is the second-most popular hard disk interface used in PCs today. It's a high-speed, intelligent peripheral I/O bus with a device independent protocol for transferring data between different types of peripheral devices. The SCSI bus connects all parts of a computer system so that they can communicate with each other.

The bus frees the host processor from the responsibility of I/O internal tasks. A SCSI bus can be either internal, external, or cross the boundary from internal to external. The SCSI protocol is a peer-to-peer relationship: one device does not have to be subordinated to another device in order to perform I/0 activities.

Only two of these devices can communicate on the bus at any given time. Each SCSI bus can connect up to 8 or up to 16 peripherals; one of those devices will always be the computer or the SCSI card, because they too are devices on the SCSI. SCSI devices are designated as either initiators (drivers) or targets (receivers) and the interface to the host computer is called the host adapter. Every device connected to the bus will have a different SCSI ID, ranging from 0 to 7. The host adapter takes up one ID leaving 7 ID's for other hardware. SCSI hardware typically consists of hard drives, tape drives, CD-ROMs, printers and scanners.

The reason for the slow taking of SCSI is the lack of standard. Each company seems to have its own idea of how SCSI should work. While the connections themselves have been standardized, the actual driver specs used for communication have not been. The end result is that each piece of SCSI hardware has its own host adapter. So, due to the lack of an adapter standard, a standardized software interface, and a standard BIOS for hard drives attached to the SCSI. Adapter.

History and Evolution

In the beginning, one couldn't even use a hard drive on the bus. This was mainly because the BIOS in those systems were designed to use the ST506/412 controller. With the IDE, the BIOS was easily changed because of the similarity to ST506/412 on the WD1003 controller. At the register level, though, SCSI was very different, and would have required an entirely new set of BIOS in the PC.

What we currently know of as the SCSI interface had its beginnings back in 1979. Shugart Associates, led by storage industry pioneer Alan Shugart (who was a leader in the development of the floppy disk, and later founded Seagate Technology) created the Shugart Associates Systems Interface (SASI).

This very early predecessor of SCSI was very rudimentary in terms of its capabilities, supporting only a limited set of commands compared to even fairly early "true" SCSI, and rather slow signaling speeds of 1.5 Mbytes/second. For its time, SASI was a great idea, since it was the first attempt to define an intelligent storage interface for small computers. The limitations must be considered in light of the era: we are talking about a time when 8" floppy drives were still being commonly used.