Small Computer System Interface (SCSI, /ˈskʌzi/ SKUZ-ee) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interfaces. SCSI is most commonly used for hard disk drives and tape drives, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.
The ancestral SCSI standard, X3.131-1986, generally referred to as SCSI-1, was published by the X3T9 technical committee of the American National Standards Institute (ANSI) in 1986. SCSI-2 was published in August 1990 as X3.T9.2/86-109, with further revisions in 1994 and subsequent adoption of a multitude of interfaces. Further refinements have resulted in improvements in performance and support for ever-increasing storage data capacity.
SCSI is derived from "SASI", the "Shugart Associates System Interface", developed circa 1978 and publicly disclosed in 1981. Larry Boucher is considered to be the "father" of SASI and ultimately SCSI due to his pioneering work first at Shugart Associates and then at Adaptec.
A SASI controller provided a bridge between a hard disk drive's low-level interface and a host computer, which needed to read blocks of data. SASI controller boards were typically the size of a hard disk drive and were usually physically mounted to the drive's chassis. SASI, which was used in mini- and early microcomputers, defined the interface as using a 50-pin flat ribbon connector which was adopted as the SCSI-1 connector. SASI is a fully compliant subset of SCSI-1 so that many, if not all, of the then-existing SASI controllers, were SCSI-1 compatible.
Until at least February 1982, ANSI developed the specification as "SASI" and "Shugart Associates System Interface" however, the committee documenting the standard would not allow it to be named after a company. Almost a full day was devoted to agreeing to name the standard "Small Computer System Interface", which Boucher intended to be pronounced "sexy", but ENDL's Dal Allan pronounced the new acronym as "scuzzy" and that stuck.
A number of companies such as NCR Corporation, Adaptec and Optimem were early supporters of SCSI. The NCR facility in Wichita, Kansas is widely thought to have developed the industry's first SCSI controller chip; it worked the first time.
The "small" reference in "small computer system interface" is historical; since the mid-1990s, SCSI has been available on even the largest of computer systems.
Since its standardization in 1986, SCSI has been commonly used in the Amiga, Atari, Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using the less-expensive parallel ATA (PATA, also known as IDE) for its low-end machines with the Macintosh Quadra 630 in 1994 and added it to its high-end desktops starting with the Power Macintosh G3 in 1997. Apple dropped on-board SCSI completely in favor of IDE and FireWire with the (Blue & White) Power Mac G3 in 1999, while still offering a PCI SCSI host adapter as an option on up to the Power Macintosh G4 (AGP Graphics) models. Sun switched its lower-end range to Serial ATA (SATA). Commodore included SCSI on the Amiga 3000/3000T systems and it was an add-on to previous Amiga 500/2000 models. Starting with the Amiga 600/1200/4000 systems Commodore switched to the IDE interface. Atari included SCSI as standard in its Atari MEGA STE, Atari TT and Atari Falcon computer models. SCSI has never been popular in the low-priced IBM PC world, owing to the lower cost and adequate performance of ATA hard disk standard. However, SCSI drives and even SCSI RAIDs became common in PC workstations for video or audio production.
Recent physical versions of SCSI—Serial Attached SCSI (SAS), SCSI-over-Fibre Channel Protocol (FCP), and USB Attached SCSI (UAS)—break from the traditional parallel SCSI bus and perform data transfer via serial communications using point-to-point links. Although much of the SCSI documentation talks about the parallel interface, all modern development efforts use serial interfaces. Serial interfaces have a number of advantages over parallel SCSI, including higher data rates, simplified cabling, longer reach, improved fault isolation, and full-duplex capability. The primary reason for the shift to serial interfaces is the clock skew issue of high-speed parallel interfaces, which makes the faster variants of parallel SCSI susceptible to problems caused by cabling and termination.
The non-physical iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged, through the embedding of SCSI-3 over TCP/IP. Therefore, iSCSI uses logical connections instead of physical links and can run on top of any network supporting IP. The actual physical links are realized on lower network layers, independently from iSCSI. Predominantly, Ethernet is used which is also of serial nature.
SCSI is popular on high-performance workstations, servers, and storage appliances. Almost all RAID subsystems on servers have used some kind of SCSI hard disk drives for decades (initially Parallel SCSI, recently SAS and Fibre Channel), though a number of manufacturers offer SATA-based RAID subsystems as a cheaper option. Moreover, SAS offers compatibility with SATA devices, creating a much broader range of options for RAID subsystems together with the existence of nearline SAS (NL-SAS) drives. Instead of SCSI, modern desktop computers and notebooks typically use SATA interfaces for internal hard disk drives, with NVME over PCIe gaining popularity as SATA can bottleneck modern solid-state drives.