Scope

As with most specifications in the computer world, the original SCSI (pronounced scuzzy) specification was completed (in 1986) after work had already begun on a better version (SCSI-2). It was developed as a result of attempts by Shugart and NCR to develop a new interface for minicomputers. The basis of the interface was, and still is, the set of commands that control data transfer and communication among devices. The commands were the strength of SCSI, because they made the interface intelligent; but they were also its initial weakness, as there wasn't enough of a standard for the command set to be truly useful to device manufacturers. Consequently, in the mid-1980s, the Common Command Set (CCS) extension was developed to standardise SCSI commands.

The SCSI interface is a shared-bus configuration that supports a wide variety of peripherals, including HDDs, tape drives, optical drives, scanners, printers, and redundant arrays of independent disks (RAID) subsystems. The SCSI bus operates independently of the system CPU and the system bus through an integrated or plug-in host bus adapter (HBA).

The actual controllers are built into each SCSI device. They "chain" SCSI peripherals to the SCSI bus via the host adapter. Each device on the chain, including the host, must be identified by a unique ID number. One SCSI device must not use the same ID number as another, but they may be numbered non-sequentially. Most SCSI host adapters feature external and internal connectors, with the option for the chain to extend in either or both directions. There's no relationship between the IDs and the physical position on the bus, but both ends must be electrically "terminated" with resistors to prevent signal reflections and guarantee data integrity over long cable lengths. Termination comes in several varieties, from physical jumpers or plugs to software configurations.

The SCSI-1 standard allows for 8 devices to be connected to a single SCSI cable on a narrow bus configuration (8 data lines); SCSI-2 increases support to 16 devices on a wide bus configuration (16 data lines). New SCSI interfaces are backward compatible with most previous SCSI interfaces. SCSI devices of differing transfer speeds can coexist on the same SCSI bus. To guarantee compatibility and data reliability, transfer rates are negotiated between devices before data transfer is initiated.