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Small Computer System Interface - SCSI

The Small Computer System Interface (SCSI) is a set of parallel interface standards developed by the American National Standards Institute (ANSI) for attaching printers, disk drives, scanners and other peripherals to computers. SCSI (pronounced "skuzzy") is supported by all major operating systems.

The first version (SCSI-1), adopted by ANSI in 1986, was an 8-bit version with a 5 MBps transfer speed that allowed up to eight devices to be connected with a maximum cable length of six meters. The latest version, 16-bit Ultra-640 (Fast-320) SCSI, was introduced in 2003 and has a 640 MBps transfer speed, connecting up to 16 devices with a 12 meter cable length. Other versions include:

SCSI-2: 8-bit bus, six meter cable length, 5-10 MBps; connects 8 or 16 devices. 50-pin connector.

Wide SCSI-2: Received its name from the wider 168 line cable with 68-pin connectors to accommodate the 16-bit bus. 3 meter cable; 20 MBps transfer rate; connected 16 devices.

Fast SCSI-2: 8-bit bus, but double the clock speed of SCSI-2 allowing transfers of 10-20 MBps. 3 meter cable; connects 8 devices.

Fast Wide SCSI-2: 6-bit bus; 3 meter cable; 20 MBps; 16 devices.

Ultra SCSI-3: 8-bit and 16-bit versions, both with 1.5 meter cable length. The 8-bit version supports data rates of 20 MBps and connects 8 devices. The 16-bit version doubled the transfer rate and number of devices.

Ultra-2 SCSI: 8-bit bus; 12 meters; 40 MBps; 8 devices.

Wide Ultra-2 SCSI: 16-bit bus; 12 meters; 80 MBps; 16 devices.

Traditional SCSI has been superseded by later attachment protocols such as Serial Attached SCSI (SAS) and iSCSI, which build on the earlier SCSI command structure.

SCSI history and uses

SCSI grew out of the Shugart Associates System Interface (SASI), developed by a team led by Larry Boucher at floppy disk drive manufacturer Shugart Associates. Boucher later went on to found host bus adapter manufacturer Adaptec.

Since SCSI uses a low-voltage differential (LVD) bus, a method where data is transmitted by comparing the difference in voltage between a pair of wires, its speed and cable lengths are limited. As a result, SCSI is now largely implemented via serial communications like Serial Attached SCSI (SAS), which offers significantly higher performance. Other types of peripheral connections include the following:

Universal Serial Bus (USB)

Serial Attached SCSI (SAS)

Internet Small computer System Interface (iSCSI)

USB Attached SCSI (UAS)


Fibre Channel (FC)

Fibre Channel over IP (FC/IP)

Fibre Channel over Ethernet (FCoE)

Advanced Technology Attachment (ATA)

Serial ATA (SATA)


SCSI implementation
SCSI required the use of an adapter card, unlike ATA, where the adapter is built into the device. SCSI is also the foundation of SAS and iSCSI, two common uses of SCSI technology in enterprise storage environments.

SCSI vendors

The following vendors are part of INCITS Technical Committee 10, which sets protocols for SCSI and related technologies. These suppliers provide everything from cable connectors to complete computers and storage systems.

  • 3M Company
  • Agilent Technologies
  • Amphenol Interconnect
  • Applied Micro Circuits Corporation
  • BDT Media Automation GmbH
  • Brocade
  • Cadence Design Systems
  • Cinch Connectors
  • Crossroads Systems Inc
  • Data Direct Networks Inc
  • Dell
  • Dot Hill Systems Corporation
  • EMC Corporation
  • ENDL Texas
  • Emulex
  • FCI Electronics
  • Foxconn Electronics
  • Fujitsu America
  • Fusion-io
  • Futurewei Technologies
  • General Dynamics
  • Google
  • HCL Technologies
  • HGST
  • Ham
  • Hewlett-Packard Company
  • IBM
  • ISO/IEC JTC 1/SC25
  • Integrated Device Technology (IDT)
  • Intel
  • Intersil
  • JDS Uniphase Corporation
  • Kawasaki Microelectronics America
  • KnowledgeTekInc
  • LSI
  • LeCroy
  • Lexar Media
  • Link-A-Media Devices
  • Lorom Group East
  • Marvell Semiconductor
  • Maxim Integrated Products
  • Microsoft
  • Molex
  • NVidia
  • NXP Semiconductors
  • NetApp
  • Newisys Data Storage
  • Nexenta Systems
  • Nexsan Technologies
  • NordigitInc
  • OCZ Technology Group
  • Oracle
  • PMC-Sierra
  • Pegatron Corporation
  • Pericom Semiconductor Corporation
  • QLogic
  • QNAP Systems
  • Quantum
  • SMART Modular Technologies
  • STEC
  • Samsung Information Systems
  • SamtecInc
  • SanDisk
  • Seagate Technology
  • SerialTek
  • TE Connectivity
  • Toshiba America Information Systems
  • Unisys Corporation
  • VMware
  • Violin Memory
  • Virident Systems
  • Volex
  • Western Digital Corporation
  • Xyratex

SCSI technology

The InterNational Committee for Information Technology Standards (INCITS), which is accredited by ANSI, has three committees covering storage interfaces. T10 Technical Committee on SCSI Storage Interfaces covers the SCSI architecture, command sets, protocols and physical layers for parallel SCSI and SAS. T11 covers Fibre Channel Interfaces (Fibre Channel, HIPPI, and IPI), and T13 covers ATA storage interfaces (ATA, ATAPI and Serial ATA).

Per the T10 Committee website (www.t10.org); “is responsible for T10 is responsible for SCSI architecture standards (SAM-n), which are used by parallel SCSI, SAS, iSCSI, SCSI Express, USB Attached SCSI, Fibre Channel, SSA, ADI, and IEEE 1394. T10 is also responsible for most SCSI command set standards (e.g., SPC-4, SBC-3, SSC-4, MMC-6, SMC-3, OSD-2, SES-3, ADC-3, RBC). These standards are used by almost all modern I/O interfaces.”

A list of T10 projects is available from http://www.incits.org/tc_projects/t10.pdf. The standards ISO/IEC 14776 series) can be purchased from ANSI (http://webstore.ansi.org/). The working draft documents, including Command Standards, protocols and physical layers for SCSI, SAS, Fibre Channel, SCSI Over PCIe Architecture and others can be found at http://www.t10.org/drafts.htm#SPC_Family. A standardized set of commands used to communicate requests and responses between the initiator and target are listed at http://www.t10.org/lists/1spc-lst.htm

Next steps

Understanding SCSI ATA, SAS and SATA
Storage Protocols Debated

FCoE gets lost in Vendor Stupidity

Storage Basics: SCSI Part I

Storage Basics: SCSI Part II

Let's Bid Adieu to Block Devices and SCSI

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