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Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and
peripheral devices A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...
. The SCSI standards define commands, protocols, electrical, optical and logical
interfaces Interface or interfacing may refer to: Academic journals * Interface (journal), ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Lin ...
. 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 initial
Parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...
was most commonly used for
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
s and
tape drive A tape drive is a data storage device that reads and writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and a long archival stability. ...
s, but it can connect a wide range of other devices, including scanners and CD drives, although not all controllers can handle all devices. 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 The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
(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 data storage capacity.


History


Parallel interface

SCSI is derived from "SASI", the "
Shugart Associates Shugart Associates (later Shugart Corporation) was a computer peripheral manufacturer that dominated the floppy disk drive market in the late 1970s and is famous for introducing the -inch "Minifloppy" floppy disk drive. In 1979 it was one of the f ...
System Interface", developed beginning 1979 and publicly disclosed in 1981.
Larry Boucher Larry is a masculine given name in English, derived from Lawrence or Laurence. It can be a shortened form of those names. Larry may refer to the following: People Arts and entertainment *Larry D. Alexander, American artist/writer *Larry Boone, ...
is considered to be the "father" of SASI and ultimately SCSI due to his pioneering work first at Shugart Associates and then at
Adaptec Adaptec was a computer storage company and remains a brand for computer storage products. The company was an independent firm from 1981 to 2010, at which point it was acquired by PMC-Sierra, which itself was later acquired by Microsemi, which itse ...
. 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"Working document for ANSI meeting on March 3, 1982, "SASI SHUGART ASSOCIATES SYSTEM INTERFACE, Revision D, February 17, 1982" 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 NCR Corporation, previously known as National Cash Register, is an American software, consulting and technology company providing several professional services and electronic products. It manufactures self-service kiosks, point-of-sale termin ...
, Adaptec and Optimem were early supporters of SCSI. The NCR facility in
Wichita, Kansas Wichita ( ) is the largest city in the U.S. state of Kansas and the county seat of Sedgwick County, Kansas, Sedgwick County. As of the 2020 United States census, 2020 census, the population of the city was 397,532. The Wichita metro area had ...
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 Amiga is a family of personal computers introduced by Commodore in 1985. The original model is one of a number of mid-1980s computers with 16- or 32-bit processors, 256 KB or more of RAM, mouse-based GUIs, and significantly improved graphi ...
,
Atari Atari () is a brand name that has been owned by several entities since its inception in 1972. It is currently owned by French publisher Atari SA through a subsidiary named Atari Interactive. The original Atari, Inc. (1972–1992), Atari, Inc., ...
,
Apple Macintosh The Mac (known as Macintosh until 1999) is a family of personal computers designed and marketed by Apple Inc. Macs are known for their ease of use and minimalist designs, and are popular among students, creative professionals, and software en ...
and
Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, the ...
computer lines and PC server systems. Apple started using the less-expensive
parallel ATA Parallel ATA (PATA), originally , also known as IDE, is a standard interface designed for IBM PC-compatible computers. It was first developed by Western Digital and Compaq in 1986 for compatible hard drives and CD or DVD drives. The connection ...
(PATA, also known as ''IDE'') for its low-end machines with the
Macintosh Quadra The Macintosh Quadra is a family of personal computers designed, manufactured and sold by Apple Computer, Inc. from October 1991 to October 1995. The Quadra, named for the Motorola 68040 central processing unit, replaced the Macintosh II family as ...
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 IEEE 1394 is an interface standard for a serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple in cooperation with a number of companies, primarily Sony an ...
with the (Blue & White) Power Mac G3 in 1999, while still offering a
PCI PCI may refer to: Business and economics * Payment card industry, businesses associated with debit, credit, and other payment cards ** Payment Card Industry Data Security Standard, a set of security requirements for credit card processors * Pro ...
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 (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard t ...
(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 The Atari Mega STE is Atari Corporation's final personal computer in the Atari ST series. Released in 1991, the MEGA STE is a late-model Motorola 68000-based STE mounted in the case of an Atari TT computer. Description The MEGA STE is based on ST ...
,
Atari TT The Atari TT030 is a member of the Atari ST family, released in 1990. It was originally intended to be a high-end Unix workstation, but Atari took two years to release a port of Unix SVR4 for the TT, which prevented the TT from ever being serious ...
and
Atari Falcon The Atari Falcon030 (usually shortened to Atari Falcon), released in 1992, was the final personal computer product from Atari Corporation. A high-end model of the Atari ST line, the machine is based on a Motorola 68030 CPU and a Motorola 56001 dig ...
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
RAID Raid, RAID or Raids may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college ...
s became common in PC workstations for video or audio production.


Modern SCSI

Recent physical versions of SCSI Serial Attached SCSI (SAS), SCSI-over-
Fibre Channel Protocol Fibre Channel Protocol (FCP) is the SCSI interface protocol utilising an underlying Fibre Channel connection. The Fibre Channel standards define a high-speed data transfer mechanism that can be used to connect workstations, mainframes, supercomput ...
(FCP), and
USB Attached SCSI USB Attached SCSI (UAS) or USB Attached SCSI Protocol (UASP) is a computer protocol used to move data to and from USB storage devices such as hard drives (HDDs), solid-state drives (SSDs), and thumb drives. UAS depends on the USB protocol, and u ...
(UAS)break from the traditional
parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
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 A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
capability. The primary reason for the shift to serial interfaces is the
clock skew Clock skew (sometimes called timing skew) is a phenomenon in synchronous digital circuit systems (such as computer systems) in which the same sourced clock signal arrives at different components at different times due to gate or, in more advanced s ...
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 Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...
preserves the basic SCSI
paradigm In science and philosophy, a paradigm () is a distinct set of concepts or thought patterns, including theories, research methods, postulates, and standards for what constitute legitimate contributions to a field. Etymology ''Paradigm'' comes f ...
, especially the command set, almost unchanged, through embedding of SCSI-3 over
TCP/IP The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
. 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 Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
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, interim Fibre Channel, recently SAS), though a number of manufacturers offer
SATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard to ...
-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 NVM Express (NVMe) or Non-Volatile Memory Host Controller Interface Specification (NVMHCIS) is an open, logical-device interface specification for accessing a computer's non-volatile storage media usually attached via PCI Express (PCIe) bus. The ...
over PCIe gaining popularity as SATA can bottleneck modern
solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is ...
s.


Interfaces

SCSI is available in a variety of interfaces. The first was
parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...
(also called SCSI Parallel Interface or SPI), which uses a
parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster of IBM ...
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
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 Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...
, for example, uses
TCP/IP The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
as a transport mechanism, which is most often transported over
Gigabit Ethernet In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use i ...
or faster
network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...
links. SCSI interfaces have often been included on computers from various manufacturers for use under
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
,
classic Mac OS Mac OS (originally System Software; retronym: Classic Mac OS) is the series of operating systems developed for the Macintosh family of personal computers by Apple Computer from 1984 to 2001, starting with System 1 and ending with Mac OS 9. The ...
,
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
,
Commodore Amiga Amiga is a family of personal computers introduced by Commodore in 1985. The original model is one of a number of mid-1980s computers with 16- or 32-bit processors, 256 KB or more of RAM, mouse-based GUIs, and significantly improved graphi ...
and
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...
operating systems, either implemented on the
motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...
or by the means of plug-in adaptors. With the advent of SAS and
SATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard to ...
drives, provision for parallel SCSI on motherboards was discontinued.


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 A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
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.


SCSI interfaces


Cabling


SCSI Parallel Interface

Internal parallel SCSI cables are usually
ribbons A ribbon or riband is a thin band of material, typically cloth but also plastic or sometimes metal, used primarily as decorative binding and tying. Cloth ribbons are made of natural materials such as silk, cotton, and jute and of synthetic mater ...
, with two or more 50–, 68–, or 80–pin connectors attached. External cables are typically shielded (but may not be), with 50– or 68–pin connectors at each end, depending upon the specific SCSI bus width supported. The 80–pin
Single Connector Attachment A SCSI connector ( ) is used to connect computer parts that use a system called SCSI to communicate with each other. Generally, two connectors, designated male and female, plug together to form a connection which allows two components, such as a c ...
(SCA) is typically used for hot-pluggable devices


Fibre Channel

Fibre Channel Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data cen ...
can be used to transport SCSI information units, as defined by the
Fibre Channel Protocol Fibre Channel Protocol (FCP) is the SCSI interface protocol utilising an underlying Fibre Channel connection. The Fibre Channel standards define a high-speed data transfer mechanism that can be used to connect workstations, mainframes, supercomput ...
for SCSI (FCP). These connections are hot-pluggable and are usually implemented with optical fiber.


Serial attached SCSI

Serial attached SCSI (SAS) uses a modified
Serial ATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard t ...
data and power cable.


iSCSI

iSCSI Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...
(Internet Small Computer System Interface) usually uses
Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP.


SRP

The
SCSI RDMA Protocol In computing the SCSI RDMA Protocol (SRP) is a protocol that allows one computer to access SCSI devices attached to another computer via remote direct memory access (RDMA).InfiniBand InfiniBand (IB) is a computer networking communications standard used in high-performance computing that features very high throughput and very low latency. It is used for data interconnect both among and within computers. InfiniBand is also used ...
or
Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
when using RoCE or
iWARP iWARP is a computer networking protocol that implements remote direct memory access (RDMA) for efficient data transfer over Internet Protocol networks. Contrary to some accounts, iWARP is not an acronym. Because iWARP is layered on Internet Eng ...
.


USB Attached SCSI

USB Attached SCSI USB Attached SCSI (UAS) or USB Attached SCSI Protocol (UASP) is a computer protocol used to move data to and from USB storage devices such as hard drives (HDDs), solid-state drives (SSDs), and thumb drives. UAS depends on the USB protocol, and u ...
allows SCSI devices to use the
Universal Serial Bus Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
.


Automation/Drive Interface

The Automation/Drive Interface − Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with the controllers of the libraries (automation devices) in which they are installed. The ADI standard specifies the use of
RS-422 RS-422, also known as TIA/EIA-422, is a technical standard originated by the Electronic Industries Alliance that specifies electrical characteristics of a digital signaling circuit. It was meant to be the foundation of a suite of standards that ...
for the physical connections. The second-generation ADT-2 standard defines iADT, use of the ADT protocol over IP (Internet Protocol) connections, such as over
Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
. The Automation/Drive Interface − Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations.


SCSI command protocol

In addition to many different hardware implementations, the SCSI standards also include an extensive set of command definitions. The SCSI command architecture was originally defined for
parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...
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 ATA Packet Interface (ATAPI) is a protocol that has been added to Parallel ATA and Serial ATA so that a greater variety of devices can be connected to a computer than with the ATA command set alone. It carries SCSI commands and responses through ...
, USB Mass Storage class and FireWire SBP-2. In SCSI terminology, communication takes place between an
initiator An initiator can refer to: * A person who instigates something. * Modulated neutron initiator, a neutron source used in some nuclear weapons ** Initiator, an Explosive booster ** Initiator, the first Nuclear chain reaction * Pyrotechnic initiator, ...
and a
target Target may refer to: Physical items * Shooting target, used in marksmanship training and various shooting sports ** Bullseye (target), the goal one for which one aims in many of these sports ** Aiming point, in field artillery, fi ...
. The initiator sends a
command Command may refer to: Computing * Command (computing), a statement in a computer language * COMMAND.COM, the default operating system shell and command-line interpreter for DOS * Command key, a modifier key on Apple Macintosh computer keyboards * ...
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, such as 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 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, optical and logical interface ...
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 four 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 command In SCSI computer storage, computers and storage devices use a client-server model of communication. The computer is a client which requests the storage device to perform a service, e.g., to read or write data. The SCSI command architecture was or ...
s in total, with the most commonly used being: *Test unit ready: Queries device to see if it is ready for data transfers (disk spun up, media loaded, etc.). *Inquiry: Returns basic device information. *Request sense: Returns any error codes from the previous command that returned an error status. *Send diagnostic and Receive diagnostic results: runs a simple self-test, or a specialised test defined in a diagnostic page. *Start/Stop unit: Spins disks up and down, or loads/unloads media (CD, tape, etc.). *Read capacity: Returns storage capacity. *Format unit: Prepares a storage medium for use. In a disk, a low level format will occur. Some tape drives will erase the tape in response to this command. *Read: (four variants): Reads data from a device. *Write: (four variants): Writes data to a device. *Log sense: Returns current information from log pages. *Mode sense: Returns current device parameters from mode pages. *Mode select: Sets device parameters in a mode page. Each device on the SCSI bus is assigned a unique SCSI identification number or ID. Devices may encompass multiple logical units, which are addressed by
logical unit number In computer storage, a logical unit number, or LUN, is a number used to identify a logical unit, which is a device addressed by the SCSI protocol or by Storage Area Network protocols that encapsulate SCSI, such as Fibre Channel or iSCSI. A LUN m ...
(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, addressed by 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(6) and Write(6) commands contain a 21-bit LBA address. The Read(10), Read(12), Read Long, Write(10), Write(12), and Write Long commands all contain a 32-bit LBA address plus various other parameter options. The capacity of a "sequential access" (i.e. tape-type) device is not specified because it depends, amongst other things, on the length of the tape, which is not identified in a machine-readable way. Read and write operations on a sequential access device begin at the current tape 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 The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
, etc., support variable block sizes.


Device identification


Parallel interface

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 In computer hardware, a host controller, host adapter, or host bus adapter (HBA), connects a computer system bus, which acts as the host system, to other network and storage devices. The terms are primarily used to refer to devices for conne ...
). On modern host adapters (since about 1997), doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains a
Option ROM An Option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device and ...
(SCSI 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 back plane 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 back plane 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. 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.


General

Note that a SCSI target device (which can be called a "physical unit") is sometimes 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. Further, a RAID array may be a single SCSI device, but may contain many logical units, each of which is a "virtual" disk—a stripe set or mirror set constructed from portions of real disk drives. The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the
logical unit number In computer storage, a logical unit number, or LUN, is a number used to identify a logical unit, which is a device addressed by the SCSI protocol or by Storage Area Network protocols that encapsulate SCSI, such as Fibre Channel or iSCSI. A LUN m ...
(LUN) identifies a disk device (real or virtual) 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". In modern SCSI transport protocols, there is an automated process for the "discovery" of the IDs. The SSA initiator (normally the host computer through the 'host adaptor') "walk the loop" to determine what devices are connected and then assigns each one a 7-bit "hop-count" value.
Fibre Channel Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data cen ...
– Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN (
World Wide Name A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier used in storage technologies including Fibre Channel, Parallel ATA, Serial ATA, SCSI and Serial Attached SCSI (SAS). A WWN may be employed in a variety of roles, such ...
). 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. SCSI has the CTL (Channel, Target or Physical Unit Number, Logical Unit Number) identification mechanism per
host bus adapter In computer hardware, a host controller, host adapter, or host bus adapter (HBA), connects a computer system bus, which acts as the host system, to other computer network, network and computer storage, storage devices. The terms are primarily ...
, or the HCTL (HBA, Channel, PUN, LUN) identification mechanism, one host adapter may have more than one channels.


Device Type

While all SCSI controllers can work with read/write storage devices, i.e. disk and tape, some will not work with some other device types; older controllers are likely to be more limited, sometimes by their driver software, and more Device Types were added as SCSI evolved. Even CD-ROMs are not handled by all controllers. Device Type is a 5-bit field reported by a
SCSI Inquiry Command 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, optical and logical interface ...
; defined
SCSI Peripheral Device Type A SCSI Peripheral Device Type describes the capabilities provided by a SCSI device. It is a five-bit field in the first byte returned in response to an INQUIRY SCSI command In SCSI computer storage, computers and storage devices use a client-serv ...
s include, in addition to many varieties of storage device, printer, scanner, communications device, and a catch-all "processor" type for devices not otherwise listed.


SCSI enclosure services

In larger SCSI servers, the disk-drive devices are housed in an intelligent enclosure that supports SCSI Enclosure Services (SES). 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

*
Fibre Channel Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data cen ...
*
List of device bandwidths This is a list of interface bit rates, is a measure of information transfer rates, or digital bandwidth capacity, at which digital interfaces in a computer or network can communicate over various kinds of buses and channels. The distinction can ...
*
Parallel SCSI Parallel SCSI (formally, SCSI Parallel Interface, or SPI) is the earliest of the interface implementations in the SCSI family. SPI is a parallel bus; there is one set of electrical connections stretching from one end of the SCSI bus to the othe ...
* Serial Attached SCSI


Notes


References


Bibliography

*


External links


InterNational Committee for Information Technology Standards: T10 Technical Committee on SCSI Storage Interfaces
(SCSI standards committee) {{DEFAULTSORT:Scsi Macintosh internals Logical communication interfaces Electrical communication interfaces Computer storage buses