Definition: SCSI

(Small Computer System Interface) Pronounced "scuzzy," SCSI is an earlier hardware interface for up to 15 peripherals connected to one expansion card, called a "SCSI host adapter." Introduced in 1986 by Shugart Associates (see SASI), this parallel architecture was replaced by its serial successor (see serial attached SCSI).

SCSI hard drives were used in mainframes, servers and storage arrays in the late 1980s and 1990s because they were very robust. Initially the only kind chosen for multi-drive RAID configurations, less-costly IDE drives were eventually used (see RAID, IDE and SATA).

SCSI Was a Mini-Network
The SCSI bus connects up to 15 devices in a daisy chain topology, and any two can communicate at one time: host-to-peripheral and peripheral-to-peripheral. For more details, see SCSI Architecture Model, SCSI signaling, SCSI connectors and SCSI switch.

A "Scuzzy" Daisy Chain

SCSI enabled multiple peripherals to take up only one expansion slot in the computer.

SCSI Ports

To enable the daisy chain, all SCSI devices had one "in" and one "out" port plus a dial to set the device number. These SCSI-1 sockets are on the back of a 1990s disk drive. See SCSI connectors.