Small Computer System Interface (SCSI, is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical and optical 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.
SCSI is derived from "SASI", the "Shugart Associates System Interface", developed circa 1978 and publicly disclosed in 1981. 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 first-generation SCSI (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.
Larry Boucher is considered to be the "father" of SASI and SCSI due to his pioneering work first at Shugart Associates and then at Adaptec.
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 the SCSI standard. The NCR facility in Wichita, Kansas is widely thought to have developed the industry's first SCSI chip; it worked the first time.
The "small" part in SCSI 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 (now part of Oracle Corporation) computer lines and PC server systems. Apple started using Parallel ATA (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 controller card 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 a SCSI interface 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 interface 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 versions of SCSI—?Serial Attached SCSI (SAS), SCSI-over-Fibre Channel Protocol (FCP), and USB Attached SCSI (UAS)—?break from the traditional parallel SCSI standards and perform data transfer via serial communications. 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, and improved fault isolation. 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. iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP, predominantly on Ethernet which is also of serial nature.
SCSI is popular on high-performance workstations, servers, and storage appliances. RAID subsystems on servers had almost always used some kind of SCSI hard disk drives (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, what together with the existence of nearline SAS (NL-SAS) drives creates a much broader range of options for RAID subsystems. Instead of SCSI, modern desktop computers and notebooks typically use SATA interface for internal hard disk drives, and USB, eSATA, and FireWire connections for external devices.
Two SCSI connectors.
Main article: SCSI connector
SCSI is available in a variety of interfaces. The first was parallel SCSI (also called SCSI Parallel Interface or SPI), which uses a parallel bus design. Since 2005, SPI was gradually replaced by Serial Attached SCSI (SAS), which uses a serial design but retains other aspects of the technology. Many other interfaces which do not rely on complete SCSI standards still implement the SCSI command protocol; others drop physical implementation entirely while retaining the SCSI architectural model. iSCSI, for example, uses TCP/IP as a transport mechanism, which is most often transported over Gigabit Ethernet or faster network links.
SCSI interfaces have often been included on computers from various manufacturers for use under Microsoft Windows, Mac OS, Unix, Commodore Amiga and Linux operating systems, either implemented on the motherboard or by the means of plug-in adaptors. With the advent of SAS and SATA drives, provision for parallel SCSI on motherboards was discontinued.
Main article: Parallel SCSI
Initially, the SCSI Parallel Interface (SPI) was the only interface using the SCSI protocol. Its standardization started as a single-ended 8-bit bus in 1986, transferring up to 5 MB/s, and evolved into a low-voltage differential 16-bit bus capable of up to 320 MB/s. The last SPI-5 standard from 2003 also defined a 640 MB/s speed which failed to be realized.
Parallel SCSI specifications include several synchronous transfer modes for the parallel cable, and an asynchronous mode. The asynchronous mode is a classic request/acknowledge protocol, which allows systems with a slow bus or simple systems to also use SCSI devices. Faster synchronous modes are used more frequently.