INCITS 131-1994[S2013]: Small Computer Systems Interface

Known for its flexibility, scalability, and performance, SCSI (Small Computer System Interface) is popular in enterprise and professional settings where high-speed data transfer and reliable storage connectivity are required. INCITS 131-1994[S2013]: Information Systems – Small Computer Systems Interface-2 (SCSI-2) (Formerly ANSI X3.131-1994 (R1999)) establishes specifications for an input/output bus for interconnecting computers and peripheral devices.

What Is SCSI (Small Computer System Interface)?

Usually pronounced as “scuzzy,” SCSI (Small Computer System Interface) is an electronic interface that enables PCs and servers to communicate with peripheral hardware—such as storage devices, printers, hard drives, tape drives, CD/DVD drives, and scanners—faster and more flexibly than previous parallel data transfer interfaces. In other words, SCSI is a microprocessor-controlled smart bus that can connect up to 15 devices. It is widely used on high-performance workstations, servers, and storage appliances.

What Is INCITS 131?

Formerly known as ANSI X3.131-1994(R1999), INCITS 131-1994[S2013] defines an input/output bus for interconnecting computers and peripheral devices. This standard defines extensions to the Small Computer System Interface (ISO/IEC 9316), referred to as SCSI-1. It also provides more complete dstandardization of the previously defined command sets.

INCITS 131-1994[S2013] includes the necessary specification of the mechanical, electrical, and functional characteristics of the interface to allow interoperability of conforming devices. This standard is referred to as SCSI-2.

History of the SCSI Standard

The X3T9 technical committee published the ancestral SCSI standard, ANSI X3.131-1986 (generally referred to as SCSI-1) in 1986.  SCSI-2 was published in August 1990 as ANSI X3.T9.2/86-109 with further revisions in 1994 and subsequent adoption of a multitude of interfaces. Further refinements in the standard brought about improvements in performance as well as support for increasing data storage capacity. Eventually, the Accredited Standards Committee X3 became the National Committee for Information Technology Standards (NCITS) and then the InterNational Committee for Information Technology Standards (INCITS). With this change, ANSI X3.131 became INCITS 131.

Why was SCSI Developed?

SCSI was originally developed in the 1980s as a standard for connecting peripherals to computers, especially in high-performance and server environments. The main objective behind developing SCSI was to provide a way to seamlessly connect computers to peripherals in high-performance or server environments. It was used as both a communications protocol and a parallel physical interface, and over the years it has gone through various revisions and updates.

SCSI has been used for over 30 years, offering increasingly higher speeds, from just 5 megabytes per second at its inception to up to 640 MBps, which was considered a fairly high speed when it was introduced in 2003.

How Is SCSI Used Today?

Today, SCSI has retained and expanded the protocol, and all modern developments of SCSI have replaced the parallel physical interface with multiple different types of serial interfaces. Serial interfaces have various advantages over parallel SCSI: higher data rates, simplified cabling, longer reach, improved fault isolation, and full-duplex capability. The key reason for the shift to serial interfaces is the clock skew issue of high-speed parallel interfaces, making the faster variants of parallel SCSI susceptible to problems that cabling and termination caused.

The SCSI protocol is currently transported over a Serial Attached SCSI bus (using SAS and SPL), in Fibre Channel environments (using FCP), and over IP based networks (LAN/WAN, using iSCSI). It is used primarily for block storage devices and tape storage devices as well as by enterprise and professional environments where high-performance storage connectivity is crucial.

INCITS 131-1994[S2013]: Information Systems – Small Computer Systems Interface-2 (SCSI-2) (Formerly ANSI X3.131-1994 (R1999)) is available on the ANSI Webstore.