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The icon/logo used for SCSI.

Small Computer System Interface, or SCSI (pronounced scuzzy), is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drive. 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 an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. SCSI uses hand shake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a CRC32 checksum. The SCSI protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device. However most peripheral devices are exclusively SCSI targets, incapable of acting as SCSI initiators—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.


SCSI was derived from "SASI", the "Shugart Associates System Interface", developed c. 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 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 compatibleANSI SCSI Standard, X3.131-1986, June 23, 1986, 2nd, foreword..

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, Kansasmarker may 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, Apple Macintosh and Sun Microsystems 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 G3 in 1999. Sun has switched its lower end range to Serial ATA (SATA). 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. SCSI drives and even SCSI RAIDs became common in PC workstations for video or audio production, but the appearance of large cheap SATA drives means that SATA is rapidly taking over this market.

Three recent versions of SCSI—Serial Storage Architecture (SSA), SCSI-over-Fibre Channel Protocol (FCP), and Serial Attached SCSI (SAS)—break from the traditional parallel SCSI standards and perform data transfer via serial communications. Although much of the documentation of SCSI talks about the parallel interface, most contemporary development effort is on serial SCSI. Serial SCSI has a number of advantages over parallel SCSI: faster data rates, hot swapping (some but not all parallel SCSI interfaces support it), 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. Serial SCSI devices are more expensive than the equivalent parallel SCSI devices, but this is likely to change soon .

iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP.

SCSI is popular on high-performance workstations and servers. RAIDs on servers almost always use SCSI hard disks, though a number of manufacturers offer SATA-based RAID systems as a cheaper option. Desktop computers and notebooks more typically use the ATA/IDE or the newer SATA interfaces for hard disks, and USB, eSATA, and FireWire connections for external devices.

SCSI interfaces

Two SCSI connectors.

SCSI is available in a variety of interfaces. The first, still very common, was parallel SCSI (now also called SPI), which uses a parallel electrical bus design. As of 2008, SPI is being replaced by Serial Attached SCSI (SAS), which uses a serial design but retains other aspects of the technology. iSCSI drops physical implementation entirely, and instead uses TCP/IP as a transport mechanism. Many other interfaces which do not rely on complete SCSI standards still implement the SCSI command protocol.

SCSI interfaces have often been included on computers from various manufacturers for use under Microsoft Windows, Mac OS, Unix 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 SCSI on motherboards is being discontinued. A few companies still market SCSI interfaces for motherboards supporting PCIe and PCI-X.

Parallel SCSI

Interface Alternative


Connector Width

Clock Maximum Electrical
Bandwidth (MB/s) Bandwidth (Mbit/s) Length

(single ended)
Length LVD Length HVD Devices Impedance [Ω] Voltage [V]
SCSI-1 Narrow SCSI SCSI-1 (1986) IDC50; Centronics C50 8 5 MHz 5 MB/s 40 Mbit/s 6 m NA 25m 8 5
Fast SCSI SCSI-2 (1994) IDC50; Centronics C50 8 10 MHz 10 MB/s 80 Mbit/s 1.5–3 m NA 25m 8 5
Fast-Wide SCSI SCSI-2;

SCSI-3 SPI (1996)
2 x 50-pin (SCSI-2);

1 x 68-pin (SCSI-3)
16 10 MHz 20 MB/s 160 Mbit/s 1.5–3 m NA 25m 16 5
Ultra SCSI Fast-20 SCSI-3 SPI IDC50 8 20 MHz 20 MB/s 160 Mbit/s 1.5–3 m NA 25m 8-4 (HVD:8) 5
Ultra Wide SCSI SCSI-3 SPI 68-pin 16 20 MHz 40 MB/s 320 Mbit/s 1.5–3 m NA 25m 8-4 (HVD:16) 5
Ultra2 SCSI Fast-40 SCSI-3 SPI-2 (1997) 50-pin 8 40 MHz 40 MB/s 320 Mbit/s NA 12m 25m 8
Ultra2 Wide SCSI SCSI-3 SPI-2 68-pin; 80-pin (SCA/SCA-2) 16 40 MHz 80 MB/s 640 Mbit/s NA 12m 25m 16
Ultra3 SCSI Ultra-160; Fast-80 wide SCSI-3 SPI-3 (1999) 68-pin; 80-pin (SCA/SCA-2) 16 40 MHz DDR 160 MB/s 1280 Mbit/s NA 12m NA 16
Ultra-320 SCSI Ultra-4 SCSI or Fast-160 SCSI SCSI-3(2002) 68-pin; 80-pin (SCA/SCA-2) 16 80 MHz DDR 320 MB/s 2560 Mbit/s NA 12m NA 16
Ultra-640 SCSI Ultra-5; SCSI-3(2003) 68-pin; 80-pin 16 160 MHz DDR 640 MB/s 5120 Mbit/s ?? 16

Other SCSI interfaces

Interface Alternative


Connector Width

Clock Maximum
Throughput (MB/s) Throughput (Mbit/s) Length Devices
SSA 1 200 MHz 40 MB/s 320 Mbit/s 25 m 96
SSA 40 1 400 MHz 80 MB/s 640 Mbit/s 25 m 96
FC-AL 1Gb 1 1 GHz 100 MB/s 800 Mbit/s 500m/3 km 127
FC-AL 2Gb 1 2 GHz 200 MB/s 1600 Mbit/s 500m/3 km 127
FC-AL 4Gb 1 4 GHz 400 MB/s 3200 Mbit/s 500m/3 km 127
SAS 1 3 GHz 300 MB/s 2400 Mbit/s 6 m 16,256
iSCSI Implementation- and network-dependent

SCSI cabling

Traditional (Parallel) SCSI

Internal SCSI cables are usually ribbon cables that have two or more 68 pin or 50 pin connectors. External cables are typically shielded (but can be unshielded) and only have connectors on the ends with either 50 or 68 pins depending upon the specific SCSI type .

Serial attached SCSI

Serial attached SCSI uses the same connectors and cables as Serial ATA.


iSCSI usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP.

SCSI command protocol

In addition to many different hardware implementations, the SCSI standards also include a complex set of command protocol definitions. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with iSCSI and serial SCSI. Other technologies which use the SCSI command set include the ATA Packet Interface, USB Mass Storage class and FireWire SBP-2.

In SCSI terminology, communication takes place between an initiator and a target. The initiator sends a command to the target which then responds. SCSI commands are sent in a Command Descriptor Block (CDB). The CDB consists of a one byte operation code followed by five or more bytes containing command-specific parameters.

At the end of the command sequence the target returns a Status Code byte which is usually 00h for success, 02h for an error (called a Check Condition), or 08h for busy. When the target returns a Check Condition in response to a command, the initiator usually then issues a SCSI Request Sense command in order to obtain a Key Code Qualifier (KCQ) from the target. The Check Condition and Request Sense sequence involves a special SCSI protocol called a Contingent Allegiance Condition.

There are 4 categories of SCSI commands: N (non-data), W (writing data from initiator to target), R (reading data), and B (bidirectional). There are about 60 different SCSI commands in total, with the most common being:

Each device on the SCSI bus is assigned at least one Logical Unit Number (LUN). Simple devices have just one LUN, more complex devices may have multiple LUNs. A "direct access" (i.e. disk type) storage device consists of a number of logical blocks, usually referred to by the term Logical Block Address (LBA). A typical LBA equates to 512 bytes of storage. The usage of LBAs has evolved over time and so four different command variants are provided for reading and writing data. The Read and Write commands contain a 21-bit LBA address. The Read, Read, Read Long, Write, Write, and Write Long commands all contain a 32-bit LBA address plus various other parameter options.

A "sequential access" (i.e. tape-type) device does not have a specific capacity because it typically depends on the length of the tape, which is not known exactly. Reads and writes on a sequential access device happen at the current position, not at a specific LBA. The block size on sequential access devices can either be fixed or variable, depending on the specific device. Tape devices such as half-inch 9-track tape, DDS (4 mm tapes physically similar to DAT), Exabyte, etc.., support variable block sizes.

How SCSI works

SCSI uses a protocol method to transfer data between devices on the bus. It is a circular process which starts and ends up in the same layer. From the first layer, all additional layers of protocol must be executed before any data is transferred to or from another device and the layers of protocol must be completed after the data has been transferred to the end of the process. The protocol layers are referred to as "SCSI bus phases". These phases are:


The SCSI bus can be in only one phase at a given time.

SCSI device identification

In the modern SCSI transport protocols, there is an automated process of "discovery" of the IDs. SSA initiators "walk the loop" to determine what devices are there and then assign each one a 7-bit "hop-count" value. FC-AL initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN (World Wide Name). For iSCSI, because of the unlimited scope of the (IP) network, the process is quite complicated. These discovery processes occur at power-on/initialization time and also if the bus topology changes later, for example if an extra device is added.

On a parallel SCSI bus, a device (e.g. host adapter, disk drive) is identified by a "SCSI ID", which is a number in the range 0-7 on a narrow bus and in the range 0–15 on a wide bus. On earlier models a physical jumper or switch controls the SCSI ID of the initiator (host adapter). On modern host adapters (since about 1997), doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains a BIOS program that runs when the computer boots up and that program has menus that let the operator choose the SCSI ID of the host adapter. Alternatively, the host adapter may come with software that must be installed on the host computer to configure the SCSI ID. The traditional SCSI ID for a host adapter is 7, as that ID has the highest priority during bus arbitration (even on a 16 bit bus).

The SCSI ID of a device in a drive enclosure that has a backplane is set either by jumpers or by the slot in the enclosure the device is installed into, depending on the model of the enclosure. In the latter case, each slot on the enclosure's back plane delivers control signals to the drive to select a unique SCSI ID. A SCSI enclosure without a backplane often has a switch for each drive to choose the drive's SCSI ID. The enclosure is packaged with connectors that must be plugged into the drive where the jumpers are typically located; the switch emulates the necessary jumpers. While there is no standard that makes this work, drive designers typically set up their jumper headers in a consistent format that matches the way that these switches implement.

Note that a SCSI target device (which can be called a "physical unit") is often divided into smaller "logical units." For example, a high-end disk subsystem may be a single SCSI device but contain dozens of individual disk drives, each of which is a logical unit (more commonly, it is not that simple—virtual disk devices are generated by the subsystem based on the storage in those physical drives, and each virtual disk device is a logical unit). The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the logical unit number (LUN) identifies a disk device within the subsystem.

It is quite common, though incorrect, to refer to the logical unit itself as a "LUN." Accordingly, the actual LUN may be called a "LUN number" or "LUN id".

Setting the bootable (or first) hard disk to SCSI ID 0 is an accepted IT community recommendation. SCSI ID 2 is usually set aside for the floppy disk drive while SCSI ID 3 is typically for a CD-ROM drive.

SCSI enclosure services

In larger SCSI servers, the disk-drive devices are housed in an intelligent enclosure that supports SCSI Enclosure Services . The initiator can communicate with the enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics.

See also



External links

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