SCSI (Small Computer System Interface) 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 disks and tape drives, but it can connect a wide range of other devices, including scanners, and optical drives (CD, DVD, etc.). The SCSI standard contains definitions of command sets of specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used to interface almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. The Skuzzy sternwheeler was built by Canadian Pacific Railway contractor Andrew Onderdonk at Spuzzum, British Columbia and was launched on the Fraser River on May 4, 1882. ... In computer hardware, a peripheral device is any device attached to a computer in order to expand its functionality. ... In SCSI computer storage, a command is the basic unit of communication. ... An interface defines the communication boundary between two entities, such as a piece of software, a hardware device, or a user. ... Optical Storage is made possible by data storage devices such as optical discs and holographic storage systems. ... The CD-ROM (an abbreviation for Compact Disc Read-Only Memory (ROM)) is a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. ... DVD (also known as Digital Versatile Disc or Digital Video Disc) is a popular optical disc storage media format. ... A SCSI Peripheral Device Type is a way of describing what capabilities are provided by a SCSI device. ...

• SCSI is an intelligent interface - it hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner

• SCSI is a peripheral interface - 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 is a buffered interface - it uses hand shake signals between devices, it can transfer only real data and does no error checking and correction. All those are done in a buffer.

• SCSI is a peer to peer interface - Communication can be from host to host, host to a peripheral device, peripheral device to a peripheral device

SCSI is most commonly pronounced "scuzzy".^[1][2]

History

SCSI is based on "SASI", the "Shugart Associates System Interface", introduced by that company in 1979. The Shugart SASI controller provided an interface between a hard disk's serial analog interface (called RLL) and a host computer, which needed to read sectors (blocks) of data. SASI interface boards were 5¼" x 8" in size, usually mounted on top of a hard disk drive. SASI was used in mini- and microcomputers like the Apple II. SASI defined the interface as using a 50-pin flat ribbon connector. Shugart Associates was a computer peripheral manufacturer, famous for introducing the floppy disk to the microcomputer market. ... Run Length Limited codes, or RLL codes are widely used in hard disk drives (and notably digital optical discs, such as CD, DVD and BluRay disc) to prevent long stretches of no transitions, and therefore decoding uncertainty, from creeping in. ... The 1977 Apple II, complete with integrated keyboard, color graphics, sound, a plastic BIG HAIRY DICK case, and eight expansion slots. ...

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 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 its 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. This article is about the family of home computers. ... The first Macintosh computer, introduced in 1984, upgraded to a 512K Fat Mac. The Macintosh or Mac, is a line of personal computers designed, developed, manufactured, and marketed by Apple Computer. ... Sun Microsystems, Inc. ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA), is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... The Macintosh Quadra 630 (Codenames: Crusader, Show Biz, Show & Tell; also sold with minor variations as the Macintosh LC 630 in the educational market and as the Macintosh Performa 630 in the consumer market) is a personal computer that is a part of Apple Computers (now Apple Inc. ... The Power Macintosh G3, commonly called beige G3s or platinum G3s for the color of their cases, is a series of personal computers that was designed, manufactured, and sold by Apple Computer, Inc. ... The 6-pin and 4-pin FireWire Connectors The alternative ethernet-style cabling used by 1394c FireWire is Apple Inc. ... An open Power Macintosh G3 case, showing the logic board placement on the hinged door. ... A SATA power connector. ... IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA) is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... In computing, a redundant array of inexpensive disks, also later known as redundant array of independent disks (commonly abbreviated RAID) is a system which uses multiple hard drives to share or replicate data among the drives. ... Sun SPARCstation 1+, 25 MHz RISC processor from early 1990s A workstation, such as a Unix workstation, RISC workstation or engineering workstation, is a high-end desktop or deskside microcomputer designed for technical applications. ...

Currently, 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 and FireWire connections for external devices. In information technology, a server is an application or device that performs services for connected clients as part of a client-server architecture. ... For other uses, see Raid. ... A personal computer (PC) is a computer whose price, size, and capabilities make it useful for individuals. ... For the band, see Laptop (band). ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA), is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... USB redirects here. ... The 6-pin and 4-pin FireWire Connectors The alternative ethernet-style cabling used by 1394c FireWire is Apple Inc. ...

SCSI interfaces

SCSI is available in a variety of interfaces. The first, still very common, was parallel SCSI (also called SPI). It uses a parallel electrical bus design. The traditional SPI design is making a transition to Serial Attached SCSI (SAS), which switches to a serial point-to-point design but retains other aspects of the technology. iSCSI drops physical implementation entirely, and instead uses TCP/IP as a transport mechanism. Finally, many other interfaces which do not rely on complete SCSI standards still implement the SCSI command protocol Parallel SCSI is a computer term used to describe one of the physical implementations of SCSI communications. ... In computing, a parallel port is an interface from a computer system where data is transferred in or out in parallel, that is, on more than one wire. ... An electrical bus (sometimes spelled buss) is a physical electrical interface where many devices share the same electric connection. ... 2. ... In telecommunications and computer science, serial communications is the process of sending data one bit at one time, sequentially, over a communications channel or computer bus. ... Point-to-Point telecommunications is most recently (2003) referenced regarding wireless data communications for Internet or Voice over IP via radio frequencies in the multi-gigahertz range. ... iSCSI is a protocol that allows clients (called initiators) to send SCSI commands (CDBs) to SCSI storage devices (targets) on remote servers. ... The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. ...

SCSI interfaces have traditionally been included on computers from various manufacturers for Windows, Mac and Linux environments. However, with the advent of SAS and SATA drives, motherboard manufacturers have moved SCSI connectors off of the board replacing them with the aforementioned connectivity. A handful of companies still market their SCSI interface connectivity for PCIe and PCI-X based motherboards. 2. ... SATA redirects here. ...

Connector information: See SCSI connector There are many different types of SCSI connector in use in the computer industry. ...

Parallel SCSI

It has been suggested that this article or section be merged into Low voltage differential signaling. ... Single Connector Attachment, or SCA, is a type of connection for the internal cabling of SCSI systems. ... Single Connector Attachment, or SCA, is a type of connection for the internal cabling of SCSI systems. ... Single Connector Attachment, or SCA, is a type of connection for the internal cabling of SCSI systems. ...

Fiber, serial and iSCSI

Serial Storage Architecture (SSA) is serial transport protocol used to attach disk drives to servers. ... Fibre Channel is a gigabit-speed network technology primarily used for storage networking. ... 2. ... iSCSI is a protocol that allows clients (called initiators) to send SCSI commands (CDBs) to SCSI storage devices (targets) on remote servers. ...

SCSI cabling

Internal SCSI cables are usually ribbon cables that have multiple 68 pin or 50 pin connectors. External cables are shielded and only have connectors on the ends. Left: 20-way grey ribbon cable with wire no. ...

iSCSI

iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged. iSCSI advocates project the iSCSI standard, an embedding of SCSI-3 over TCP/IP, as displacing Fibre Channel in the long run, arguing that Ethernet data rates are currently increasing faster than data rates for Fibre Channel and similar disk-attachment technologies. iSCSI could thus address both the low-end and high-end markets with a single commodity-based technology. iSCSI is a protocol that allows clients (called initiators) to send SCSI commands (CDBs) to SCSI storage devices (targets) on remote servers. ... For other uses, see Paradigm (disambiguation). ... The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. ... Fibre Channel is a gigabit-speed network technology primarily used for storage networking. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... By the mid 20th century humans had achieved a mastery of technology sufficient to leave the surface of the Earth for the first time and explore space. ... This article does not cite any references or sources. ...

Serial SCSI

Four recent versions of SCSI—SSA, FC-AL, FireWire, 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, 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^{[citation needed]}. Serial Storage Architecture (SSA) is serial transport protocol used to attach disk drives to servers. ... Fibre Channel is a gigabit-speed network technology primarily used for storage networking. ... The 6-pin and 4-pin FireWire Connectors The alternative ethernet-style cabling used by 1394c FireWire is Apple Inc. ... 2. ... Parallel SCSI is a computer term used to describe one of the physical implementations of SCSI communications. ... Parallel SCSI is a computer term used to describe one of the physical implementations of SCSI communications. ... Hot swapping or hot plugging is the ability to remove and replace components of a machine, usually a computer, while it is operating. ... // In circuit design, clock skew (sometimes timing skew) is a phenomenon in synchronous circuits in which the clock signal (sent from the clock circuit) arrives at different components at different times. ...

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. Parallel SCSI is a computer term used to describe one of the physical implementations of SCSI communications. ... ATA cables: 40 wire ribbon cable top, 80 wire ribbon cable bottom Advanced Technology Attachment (ATA), is a standard interface for connecting storage devices such as hard disks and CD-ROM drives inside personal computers. ... The USB mass storage device class is a set of computing communications protocols defined by the USB Implementers Forum that run on the Universal Serial Bus. ... Serial Bus Protocol 2 (SBP-2) [1] standard is a transport protocol within Serial Bus, IEEE Std 1394-1995 (also known as FireWire or i. ...

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. In SCSI computer storage, an initiator is the host-side endpoint of a SCSI session. ... In SCSI computer storage, a target is the storage-device side endpoint of a SCSI session. ... In SCSI computer storage, a command is the basic unit of communication. ... In SCSI computer storage, commands are sent in a Command Descriptor Block (CDB). ...

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. A SCSI Status Code is used to determine the success or failure of a SCSI command. ... In computer terminology, a Check Condition occurs when a SCSI device needs to report an error. ... The SCSI Request Sense command is used to obtain sense data (error information) from a target device. ... Key Code Qualifier is a computer term used to describe an error-code returned by a SCSI device. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

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: In SCSI computer storage, a command is the basic unit of communication. ...

• 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, also used to "ping" the device since it does not modify sense data.
• Request sense: Returns any error codes from the previous command that returned an error status.
• Send diagnostic and Receives diagnostic results: runs a simple self-test, or a specialised test defined in a diagnostic page.
• Start/Stop unit: Spins disks up and down, load/unload media.
• Read capacity: Returns storage capacity.
• Format unit: Sets all sectors to all zeroes, also allocates logical blocks avoiding defective sectors.
• Read Format Capacities:Read the capacity of the sectors
• 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 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(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 SCSI Test Unit Ready command is used to ping a target device. ... The SCSI Inquiry command is used to obtain basic information from a target device. ... The SCSI Request Sense command is used to obtain sense data (error information) from a target device. ... The SCSI Send Diagnostic command is used to instruct a target device to perform a self-test. ... The SCSI Receive Diagnostic Results command is used to interrogate the results of a self-test. ... SCSI target devices provide a number of SCSI diagnostic pages. ... The SCSI Start/Stop Unit command is used to control the motor in a rotary device such as a SCSI disk-drive. ... The SCSI Read Capacity command is used to obtain data capacity information from a target device. ... The SCSI Format Unit command is used to format the device into LBAs. ... There are four different SCSI Read commands defined. ... There are four different SCSI Write commands defined. ... The SCSI Log Sense command is used to obtain current data from log pages in a SCSI target device. ... SCSI target devices provide a number of SCSI log pages. ... The SCSI Mode Sense command is used to obtain current device information from mode pages in a SCSI target device. ... SCSI target devices provide a number of SCSI mode pages. ... The SCSI Mode Select command is used to modify device information contained in mode pages in a SCSI target device. ... In computer storage, a logical unit number or LUN is an address for an individual disk drive and by extension, the disk device itself. ... Logical block addressing (LBA) is common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks. ... There are four different SCSI Read commands defined. ... There are four different SCSI Write commands defined. ... There are four different SCSI Read commands defined. ... There are four different SCSI Read commands defined. ... There are four different SCSI Read commands defined. ... There are four different SCSI Write commands defined. ... There are four different SCSI Write commands defined. ... There are four different SCSI Write commands defined. ...

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. (Earlier devices, such as 9-track tape, tended to be fixed block, while later types, such as DAT, almost always supported variable block sizes.) IBMs System/360, released in 1964, introduced what is now generally known as 9 track tape. ... Digital audio tape can also refer to a compact cassette with digital storage. ...

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:

• BUS FREE
• ARBITRATION
• SELECTION
• MESSAGE OUT
• COMMAND OUT
• DATA OUT/IN
• STATUS IN
• MESSAGE IN
• RESELECTION

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. A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier in a Fibre Channel or Serial Attached SCSI storage network. ...

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). Fibre Channel Host Bus Adapter (64-bit PCI-X card) SCSI Host Bus Adapter (16-bit ISA card) In computer hardware, a host controller, host adapter, or host bus adapter (HBA) connects a host system (the computer) to other network and storage devices. ...

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 subystem based on the storage in those physical drives, and each virtual disk device is a logical unit). The SCSI ID, WWNN, 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.^[12]

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 specialised set of SCSI commands to access power, cooling, and other non-data characteristics. Most recent SCSI enclosure products support a protocol called SCSI Enclosure Services (SES). ...

See also

• List of device bandwidths

This is a list of device bandwidths: the channel capacity (or, more informally, bandwidth) of some computer devices employing methods of data transport is listed by bit/s, kilobit/s (kbit/s), megabit/s (Mbit/s), or gigabit/s (Gbit/s) as appropriate and also MB/s or megabytes per...

References

1. ^ "SCSI." American Heritage Dictionary.
2. ^ Field. The Book of SCSI, 1. 
3. ^ ^{a b} Clock rate in MHz for SPI, or bitrate (per second) for serial interfaces
4. ^ ^{a b} In megabytes per second, not megabits per second
5. ^ ^{a b} For daisy-chain designs, length of bus, from end to end; for point-to-point, length of a single link
6. ^ ^{a b} Including any host adapters (i.e., computers count as a device)
7. ^ ^{a b} spatial reuse
8. ^ ^{a b c d e f} full duplex
9. ^ ^{a b c d} per direction
10. ^ ^{a b c} 500 meters for multi-mode, 3 kilometers for single-mode
11. ^ 128 per expander
12. ^ Groth, David; Dan Newland (January 2001). A+ Complete Study Guide (2nd Edition). Alameda, CA, USA: Sybex, 183. ISBN 0782142443. 

This article is about the SI unit of frequency. ... In telecommunications and computing, bitrate (sometimes written bit rate, data rate or as a variable Rbit) is the number of bits that are conveyed or processed per unit of time. ... ReBoot character, see Megabyte (ReBoot). ... The Megabit is a unit of information storage, abbreviated Mbit or sometimes Mb. ... A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. ... Multi-mode optical fiber (multimode fiber or MM fiber) is a type of optical fiber mostly used for communication over shorter distances, e. ... In fiber optics, a single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest. ...

Bibliography

• (2000) in Pickett, Joseph P., et al. (ed): The American Heritage Dictionary of the English Language (AHD), Fourth Edition, Houghton Mifflin Company. ISBN 0-395-82517-2. 
• Field, Gary; Peter Ridge, John Lohmeyer, Gerhard Islinger, Stefan Groll (2000). The Book of SCSI, 2nd Edition, No Starch Press. ISBN 1-886411-10-7. 

External links

• SCSI Details, Wiring, Compaq/HP
• All About SCSI
• SCSI Help: Identifying SCSI HDs and Connectors
• T10 Technical Committee (SCSI standards)
• SCSITA terminology
• "Storage Cornucopia" SCSI links, maintained by a consultant
• SCSI/iSCSI/RAID/SAS Information Sheet
• SCSI basics
• WWW Virtual Library for SCSI
• SCSI and ATA pinouts
• Field guide to Common SCSI Connectors
• SCSI FAQ