112-534: 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 other. A SCSI device attaches to the bus but does not interrupt it. Both ends of the bus must be terminated . SCSI is a peer-to-peer peripheral interface. Every device attaches to
224-416: A parallel bus 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 , for example, uses TCP/IP as
336-416: A parallel bus 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 , for example, uses TCP/IP as
448-427: 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, such as 00h for success, 02h for an error (called a Check Condition), or 08h for busy. When
560-427: 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, such as 00h for success, 02h for an error (called a Check Condition), or 08h for busy. When
672-592: 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 – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN ( World Wide Name ). For iSCSI, because of
784-494: 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 – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN ( World Wide Name ). For iSCSI, because of
896-491: A +2.85 V linear regulator and commercially available SCSI resistor network devices (not individual resistors). Terminators must be matched to the type of SCSI bus. Using an SE (single-ended) terminator on an LVD bus causes the bus to drop back to SE speeds, even if all other devices and cables are capable of LVD operation – the same effect any other SE device has. Passive terminators may cause Ultra speed communication to be unreliable. Generally, and reflecting
1008-403: 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
1120-403: 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
1232-453: A SCSI target. SCSI-1 and SCSI-2 have the option of parity bit error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a cyclic redundancy check . The first two formal SCSI standards, SCSI-1 and SCSI-2, described parallel SCSI. The SCSI-3 standard then split the framework into separate layers which allowed the introduction of other data interfaces beyond parallel SCSI. The original SCSI-1 version of
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#17327909510081344-455: A better impedance match than passive termination because most flat ribbon cables have a characteristic impedance of approximately 110 Ω. Forced perfect termination (FPT) is similar to active termination, but with added diode clamp circuits which absorb any residual voltage overshoot or undershoot. In current practice most parallel SCSI buses are LVD and so require external, active termination. The usual termination circuit consists of
1456-833: A dozen SCSI interface names, most with ambiguous wording (like Fast SCSI, Fast Wide SCSI, Ultra SCSI, and Ultra Wide SCSI); three SCSI standards, each of which has a collection of modular, optional features; several different connector types; and three different types of voltage signaling. The leading SCSI card manufacturer, Adaptec , has manufactured over 100 varieties of SCSI cards over the years. In actual practice, many experienced technicians simply refer to SCSI devices by their bus bandwidth (i.e., SCSI 320 or SCSI 160) in Megabytes per second. As of 2003, there have only been three SCSI standards: SCSI-1, SCSI-2, and SCSI-3. All SCSI standards have been modular, defining various capabilities that manufacturers can include or not. Individual vendors and
1568-526: A jumper). Some later SCSI host adapters allow the enabling or disabling of termination through BIOS setup . Advanced SCSI devices automatically detect whether they are last on the bus and switch termination on or off accordingly. SCSI Small Computer System Interface ( SCSI , / ˈ s k ʌ z i / SKUZ -ee ) is a set of standards for physically connecting and transferring data between computers and peripheral devices , best known for its use with storage devices such as hard disk drives . SCSI
1680-536: A maximum bus cable length of 6 metres (20 ft), significantly longer than the 18 inches (0.46 m) limit of the ATA interface also popular at the time. A rarely-seen variation on the original standard featured high-voltage differential signaling and supported a maximum cable length of 25 metres (82 ft). SCSI-2 was introduced in 1994 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled
1792-459: 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 over PCIe gaining popularity as SATA can bottleneck modern solid-state drives . SCSI is available in a variety of interfaces. The first was parallel SCSI (also called SCSI Parallel Interface or SPI), which uses
1904-459: 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 over PCIe gaining popularity as SATA can bottleneck modern solid-state drives . SCSI is available in a variety of interfaces. The first was parallel SCSI (also called SCSI Parallel Interface or SPI), which uses
2016-410: 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 , etc., support variable block sizes. On a parallel SCSI bus, a device (e.g. host adapter, disk drive)
2128-410: 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 , etc., support variable block sizes. On a parallel SCSI bus, a device (e.g. host adapter, disk drive)
2240-414: A single-ended bus, or 600 mA on a differential bus. Termination can be passive or active. With passive termination each signal line is terminated by two resistors, 220 Ω to TERMPOWER and 330 Ω to ground. Active termination uses a small voltage regulator which provides a +2.85 V supply. Each signal line is then terminated by a 110 Ω resistor to this supply. Active termination provides
2352-1006: A specific sequence of events. Following a command to a target to send data to the initiator and a receipt of a command complete status, the initiator could send another command or even send a message. No version of the standard has ever specified what kind of connector should be used. Specific types of connectors for parallel SCSI devices were developed by vendors over time. Connectors for serial SCSI devices have diversified into different families for each type of serial SCSI protocol. Original parallel SCSI-1 devices typically used bulky micro ribbon connectors, and SCSI-2 devices typically used MD50 connectors. Connectors evolved to High-Density (HD) and most recently Single Connector Attachment . Connectors for wide SCSI buses have more pins and wires than those for narrow SCSI buses; typically 50 pins for narrow SCSI and 68 pins for wide SCSI. On some early devices, wide parallel SCSI buses used two or four connectors and cables while narrow SCSI buses used only one. With
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#17327909510082464-629: A terminator or includes the terminator itself. Specific capability commands allow the devices to determine whether their partners are using the whole wide bus or just the lower half and drive the bus accordingly. Alternatively, each narrow device can be attached to the wide bus through an adapter. As long as the bus is terminated with a wide – internal or external – terminator, there is no need for special termination. Single Connector Attachment (SCA) parallel SCSI devices may be connected to older controller/drive chains by using SCA adapters. Although these adapters often have auxiliary power connectors, caution
2576-453: A transport mechanism, which is most often transported over Gigabit Ethernet or faster network links. SCSI interfaces have often been included on computers from various manufacturers for use under Microsoft Windows , classic Mac OS , Unix , Amiga 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 parallel SCSI on motherboards
2688-453: A transport mechanism, which is most often transported over Gigabit Ethernet or faster network links. SCSI interfaces have often been included on computers from various manufacturers for use under Microsoft Windows , classic Mac OS , Unix , Amiga 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 parallel SCSI on motherboards
2800-522: A unified standard. In October 1981, the two companies agreed to co-develop SASI and present their standard jointly with ANSI. 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
2912-522: A unified standard. In October 1981, the two companies agreed to co-develop SASI and present their standard jointly with ANSI. 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
3024-421: 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 (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". Sometimes, redundantly, the actual LUN may be called
3136-421: 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 (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". Sometimes, redundantly, the actual LUN may be called
3248-427: Is a 5-bit field reported by a SCSI Inquiry Command ; defined SCSI Peripheral Device Types include, in addition to many varieties of storage device, printer, scanner, communications device, and a catch-all "processor" type for devices not otherwise listed. 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
3360-427: Is a 5-bit field reported by a SCSI Inquiry Command ; defined SCSI Peripheral Device Types include, in addition to many varieties of storage device, printer, scanner, communications device, and a catch-all "processor" type for devices not otherwise listed. 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
3472-496: Is a protocol that specifies how to transport SCSI commands over a reliable RDMA connection. This protocol can run over any RDMA-capable physical transport, e.g. InfiniBand or Ethernet when using RoCE or iWARP . USB Attached SCSI allows SCSI devices to use the Universal Serial Bus . The Automation/Drive Interface − Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with
Parallel SCSI - Misplaced Pages Continue
3584-425: Is a protocol that specifies how to transport SCSI commands over a reliable RDMA connection. This protocol can run over any RDMA-capable physical transport, e.g. InfiniBand or Ethernet when using RoCE or iWARP . USB Attached SCSI allows SCSI devices to use the Universal Serial Bus . The Automation/Drive Interface − Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with
3696-457: Is a special case in SCSI systems that have mixed 8-bit and 16-bit devices where high-byte termination may be required. Different SCSI transports, which are not compatible with each other, usually have unique connectors to avoid accidental mis-plugging of incompatible devices. For example, it is not possible to plug a parallel SCSI disk into an FC-AL backplane, nor to connect a cable between an SSA initiator and an FC-AL enclosure. SCSI devices in
3808-428: Is a theoretical maximum peak data rate of 640 MB/s. Due to the technical constraints of a parallel bus system, SCSI has since evolved into faster serial interfaces, mainly Serial Attached SCSI and Fibre Channel . The iSCSI protocol doesn't describe a data interface but uses any IP network , usually run over Ethernet . Parallel SCSI is not a single standard, but a suite of closely related standards. There are
3920-416: Is considered to be the "father" of SASI and ultimately SCSI due to his pioneering work first at Shugart Associates and then at Adaptec , which he founded 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
4032-416: Is considered to be the "father" of SASI and ultimately SCSI due to his pioneering work first at Shugart Associates and then at Adaptec , which he founded 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
4144-626: Is helpful to consumers because Ultra-2 SCSI device has a better-defined set of capabilities than simply identifying it as SCSI-3 . Starting with SCSI-3, the SCSI standard has been maintained as a loose collection of standards, each defining a certain piece of the SCSI architecture, and bound together by the SCSI Architectural Model . This change divorces SCSI's various interfaces from the SCSI command set , allowing devices that support SCSI commands to use any interface (including ones not otherwise specified by T10), and also allowing
4256-565: 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 Option ROM (SCSI BIOS) program that runs when the computer boots up and that program has menus that let
4368-456: 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 Option ROM (SCSI BIOS) program that runs when the computer boots up and that program has menus that let
4480-409: Is recommended when connecting them, as it is possible to damage devices by connecting external power. Each parallel SCSI device (including the computer's host adapter ) must be configured to have a unique SCSI ID on the bus. Another requirement is that any parallel SCSI bus must be terminated at both ends with the correct type of terminator . Both active and passive terminators are in common use, with
4592-585: Is revision 10 and is dated May 6, 2002. Nearly all SCSI hard drives being manufactured at the end of 2003 were Ultra-320 devices. Ultra-640 (otherwise known as Fast-320 ) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. It doubles the interface speed yet again, this time to 640 MB/s. Ultra-640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, manufacturers skipped over Ultra-640 and developed for Serial Attached SCSI instead. In addition to
Parallel SCSI - Misplaced Pages Continue
4704-404: Is typically for a CD-ROM drive. 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
4816-404: Is typically for a CD-ROM drive. 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
4928-435: 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 -based RAID subsystems as a cheaper option. Moreover, SAS offers compatibility with SATA devices, creating
5040-435: 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 -based RAID subsystems as a cheaper option. Moreover, SAS offers compatibility with SATA devices, creating
5152-742: 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) Power Mac G3 in 1999, while still offering a PCI SCSI host adapter as an option on up to the Power Macintosh G4 (AGP Graphics) models. Sun switched its lower-end range to Parallel ATA (PATA) with introduction of their Ultra 5 and 10 low end workstations using CMD640 IDE controller and continued this trend with
5264-538: 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) Power Mac G3 in 1999, while still offering a PCI SCSI host adapter as an option on up to the Power Macintosh G4 (AGP Graphics) models. Sun switched its lower-end range to Parallel ATA (PATA) with introduction of their Ultra 5 and 10 low end workstations using CMD640 IDE controller and continued this trend with
5376-584: The SCSI Trade Association have given names to specific combinations of capabilities. For example, the term Ultra SCSI is not defined anywhere in the standard, but is used to refer to SCSI implementations that signal at twice the rate of Fast SCSI . Such a signaling rate is not compliant with SCSI-2 but is one option allowed by SCSI-3. Similarly, no version of the standard requires Low-voltage differential signaling (LVD), but products called Ultra-2 SCSI include this capability. This terminology
5488-831: 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 , Atari TT and Atari Falcon 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 RAIDs became common in PC workstations for video or audio production. Recent physical versions of SCSI— Serial Attached SCSI (SAS), SCSI-over- Fibre Channel Protocol (FCP), and USB Attached SCSI (UAS)—break from
5600-682: 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 , Atari TT and Atari Falcon 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 RAIDs became common in PC workstations for video or audio production. Recent physical versions of SCSI— Serial Attached SCSI (SAS), SCSI-over- Fibre Channel Protocol (FCP), and USB Attached SCSI (UAS)—break from
5712-454: The HD connectors, a cable normally has male connectors while a SCSI device (e.g., host adapter, disk drive) has female. A female connector on a cable is meant to connect to another cable (for additional length or additional device connections). Parallel SCSI buses must always be terminated at both ends to ensure reliable operation. Without termination, data transitions reflect back from the ends of
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#17327909510085824-435: The SCSI bus in a similar manner. Depending on the version, up to 8 or 16 devices can be attached to a single bus. There can be multiple hosts and multiple peripheral devices but there should be at least one host. The SCSI protocol defines communication from host to host, host to a peripheral device, and peripheral device to a peripheral device. The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as
5936-516: The SCSI standards also include an extensive set of command 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
6048-445: The SCSI standards also include an extensive set of command 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
6160-789: The X3T9 technical committee of the American National Standards Institute (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. SCSI is derived from the Shugart Associates System Interface (SASI), developed beginning 1979 and publicly disclosed in 1981. Larry Boucher
6272-500: The X3T9 technical committee of the American National Standards Institute (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. SCSI is derived from the Shugart Associates System Interface (SASI), developed beginning 1979 and publicly disclosed in 1981. Larry Boucher
6384-408: The active type much preferred (and required on LVD buses and Ultra SCSI). Improper termination is a common problem with parallel SCSI installations. In early SCSI buses, one had to attach a physical terminator to each end, but newer devices often have terminators built in, and the user simply needs to enable termination for the devices at either end of the bus (typically by setting a DIP switch or moving
6496-495: The bus causing pulse distortion and potential data loss. A positive DC termination voltage is provided by one or more devices on the bus, typically the host adapter . This positive voltage is called TERMPOWER and is usually around +4.3 volts. TERMPOWER is normally generated by a diode connection to +5.0 volts. This is called a diode-OR circuit , designed to prevent backflow of current to the supplying device. A device that supplies TERMPOWER must be able to provide up to 900 mA on
6608-529: The controllers of the libraries (automation devices) in which they are installed. The ADI standard specifies the use of RS-422 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 . The Automation/Drive Interface − Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations. In addition to many different hardware implementations,
6720-480: The controllers of the libraries (automation devices) in which they are installed. The ADI standard specifies the use of RS-422 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 . The Automation/Drive Interface − Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations. In addition to many different hardware implementations,
6832-400: The data bus and parity signals, a parallel SCSI bus contains nine control signals: There are also three DC-level signals: There are three electrically different variants of the SCSI parallel bus: single-ended (SE), high-voltage differential (HVD), and low-voltage differential (LVD). The HVD and LVD versions use differential signaling and so they require a pair of wires for each signal. So
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#17327909510086944-456: 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. In around 1980, NCR Corporation had been developing a competing interface standard by
7056-403: 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. In around 1980, NCR Corporation had been developing a competing interface standard by
7168-441: The enclosure using a specialized set of SCSI commands to access power, cooling, and other non-data characteristics. Shugart Associates System Interface Small Computer System Interface ( SCSI , / ˈ s k ʌ z i / SKUZ -ee ) is a set of standards for physically connecting and transferring data between computers and peripheral devices , best known for its use with storage devices such as hard disk drives . SCSI
7280-673: The end of 1999, this iteration improved on the Ultra-2 standard adding the first three improvements. Devices supporting all five features were marketed as Ultra-160+ or Ultra-3 (U3). 8-bit bus width as well as HVD operation were eliminated starting with Ultra-3. Ultra-320 included the Ultra-160+ features as mandatory, doubled the clock to 80 MHz for a maximum data transfer rate of 320 MB/s, and included read/write data streaming for less overhead on queued data transfers, as well as flow control. The latest working draft for this standard
7392-538: The faster serial SCSI (SAS) host adapters. 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 , Atari , Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using the less-expensive parallel ATA (PATA, also known as IDE ) for its low-end machines with
7504-487: The faster serial SCSI (SAS) host adapters. 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 , Atari , Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using the less-expensive parallel ATA (PATA, also known as IDE ) for its low-end machines with
7616-851: The first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI or Fast-20 SCSI, were introduced in 1996. SCSI-3 itself is not as much a single document as a collection of various standards that have received updates at different points in time. The bus speed was doubled again to 20 MB/s for narrow (8-bit) systems and 40 MB/s for wide (16-bit). The maximum cable length remained 3 meters but single-ended Ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems). Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination. This standard
7728-570: The interfaces that are defined by T10 to be used in alternate manners. No version of the standard has ever specified what kind of SCSI connector should be used. See § External connectors . The original SCSI standard, SCSI-1, was derived from the Shugart Associates System Interface (SASI) and formally adopted in 1986 by ANSI . SCSI-1 features an 8-bit parallel bus (with parity ), running asynchronously at 3.5 MB/s, or 5 MB/s in synchronous mode, and
7840-429: 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
7952-429: 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
8064-475: The later Blade 100 and 150 entry level systems and did not switch to contemporary SATA interface even with the introduction of the Blade 1500 in 2003 while the higher end Blade 2500 released at the same time used Ultra320 Parallel SCSI-3. Sun moved to SATA and SAS interfaces with their last UltraSPARC-III based workstations in 2006 with the entry level Ultra 25 and mid-range Ultra 45. Commodore included SCSI on
8176-423: The later Blade 100 and 150 entry level systems and did not switch to contemporary SATA interface even with the introduction of the Blade 1500 in 2003 while the higher end Blade 2500 released at the same time used Ultra320 Parallel SCSI-3. Sun moved to SATA and SAS interfaces with their last UltraSPARC-III based workstations in 2006 with the entry level Ultra 25 and mid-range Ultra 45. Commodore included SCSI on
8288-436: The maximum transfer rate to 10 MB/s while retaining the same 50-pin cables, while Wide SCSI doubled the bus width to 16 bits on top of that to reach a maximum transfer rate of 20 MB/s, using new 68-pin cables. However, these improvements came at the cost of reducing the maximum cable length to three meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used two 16-bit cables per bus. The 32-bit implementation
8400-638: The most commonly used being: 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 (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
8512-638: The most commonly used being: 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 (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
8624-512: The name of BYSE. In the summer of 1981, NCR abandoned their in-house efforts in favor of pursuing SASI and improving on its design for their own computer systems. Fearing that their extension of the SASI standard would induce market confusion, however, NCR briefly cancelled their contract with Shugart. NCR's proposed improvements to the design of SCSI piqued the interest of Optimem, a subsidiary of Shugart, who requested that NCR and Shugart collaborate on
8736-461: The name of BYSE. In the summer of 1981, NCR abandoned their in-house efforts in favor of pursuing SASI and improving on its design for their own computer systems. Fearing that their extension of the SASI standard would induce market confusion, however, NCR briefly cancelled their contract with Shugart. NCR's proposed improvements to the design of SCSI piqued the interest of Optimem, a subsidiary of Shugart, who requested that NCR and Shugart collaborate on
8848-579: The new acronym as "scuzzy" and that stuck. The NCR facility in Wichita, Kansas developed the industry's first SCSI controller chip, the NCR 5385, released in 1983. According to its developers, the chip worked the first time it was tested. A number of companies, such as Adaptec and Optimem, were early supporters of SCSI. By late 1990 at least 45 manufactures offered 251 models of parallel SCSI host adapters Today, such host adapters have largely been displaced by
8960-476: The new acronym as "scuzzy" and that stuck. The NCR facility in Wichita, Kansas developed the industry's first SCSI controller chip, the NCR 5385, released in 1983. According to its developers, the chip worked the first time it was tested. A number of companies, such as Adaptec and Optimem, were early supporters of SCSI. By late 1990 at least 45 manufactures offered 251 models of parallel SCSI host adapters Today, such host adapters have largely been displaced by
9072-407: The number of signals required to implement a SCSI bus is a function of the bus width and voltage: All devices on a parallel SCSI bus must have a SCSI ID, which may be set by jumpers on older devices or in software. The SCSI ID field widths are: The parallel SCSI bus goes through eight possible phases as a command is processed. Not all phases will occur in all cases: The above list does not imply
9184-414: 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
9296-414: 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
9408-409: The order in which each type of terminator was introduced, unmarked terminators are passive, those marked only active are SE, and only those marked LVD (or SE/LVD) will correctly terminate an LVD bus and allow it to operate at full LVD speeds. Some early disk drives included internal terminators, but most modern disk drives do not provide termination and termination must be provided externally. There
9520-483: The parallel bus was 8 bits wide (plus a ninth parity bit ). The SCSI-2 standard allowed for faster operation (10 MHz) and wider buses (16-bit or 32-bit). The 16-bit option became the most popular. At 10 MHz with a bus width of 16 bits it is possible to achieve a data rate of 20 MB/s. Subsequent extensions to the SCSI standard allowed for faster speeds: 20 MHz, 40 MHz, 80 MHz, 160 MHz and finally 320 MHz. At 320 MHz x 16 bits there
9632-470: 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. Internal parallel SCSI cables are usually ribbons , 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
9744-470: 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. Internal parallel SCSI cables are usually ribbons , 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
9856-454: The same SCSI transport family are generally backward compatible . Within the parallel SCSI family, for example, it is possible to connect an Ultra-3 SCSI hard disk to an Ultra-2 SCSI controller. The interface operates at the lowest common supported standard, Ultra-2 in this case. Ultra-2, Ultra-160 and Ultra-320 devices may be freely mixed on the parallel LVD bus with no compromise in performance. Single-ended and LVD devices can be attached to
9968-488: The same bus, but all devices will run at a slower, single-ended speed. The SPI-5 standard (which describes up to Ultra-640) deprecates single-ended devices, so some devices may not be electrically backward compatible. Some host adapters offer compatibility using a SCSI bridge to electrically split the bus into an SE and an LVD half, so LVD devices can operate at full speed. Other adapters may provide multiple buses (channels). Both narrow and wide SCSI devices can be attached to
10080-399: The same parallel bus. All the narrow devices must be placed at one end and all the wide devices at the other end. The wider part of the bus needs to be terminated between the wide and narrow devices because the high half of the bus ends with the last wide device. This can be done with a cable designed to connect the wide part of the bus to the narrow part which either provides a place to plug in
10192-589: 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. The non-physical iSCSI preserves the basic SCSI paradigm , especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP . 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
10304-589: 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. The non-physical iSCSI preserves the basic SCSI paradigm , especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP . 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
10416-409: 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
10528-409: 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
10640-806: The specific SCSI bus width supported. The 80–pin Single Connector Attachment (SCA) is typically used for hot-pluggable devices Fibre Channel can be used to transport SCSI information units, as defined by the Fibre Channel Protocol for SCSI (FCP). These connections are hot-pluggable and are usually implemented with optical fiber. Serial attached SCSI (SAS) uses a modified Serial ATA data and power cable. iSCSI (Internet Small Computer System Interface) usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP . The SCSI RDMA Protocol (SRP)
10752-652: The specific SCSI bus width supported. The 80–pin Single Connector Attachment (SCA) is typically used for hot-pluggable devices Fibre Channel can be used to transport SCSI information units, as defined by the Fibre Channel Protocol for SCSI (FCP). These connections are hot-pluggable and are usually implemented with optical fiber. Serial attached SCSI (SAS) uses a modified Serial ATA data and power cable. iSCSI (Internet Small Computer System Interface) usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP . The SCSI RDMA Protocol (SRP)
10864-406: The standard is highly pragmatic and addressed toward commercial requirements. The initial Parallel SCSI was most commonly used for hard disk drives and tape drives , 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
10976-406: The standard is highly pragmatic and addressed toward commercial requirements. The initial Parallel SCSI was most commonly used for hard disk drives and tape drives , 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
11088-518: 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 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 commands in total, with
11200-518: 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 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 commands in total, with
11312-456: The traditional parallel SCSI bus 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 capability. The primary reason for
11424-456: The traditional parallel SCSI bus 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 capability. The primary reason for
11536-905: 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 , or the HCTL (HBA, Channel, PUN, LUN) identification mechanism, one host adapter may have more than one channels. 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
11648-805: 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 , or the HCTL (HBA, Channel, PUN, LUN) identification mechanism, one host adapter may have more than one channels. 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
11760-474: The whole bus to single-ended mode with all its limitations, including transfer speed. The standard also introduced very-high-density cable interconnect (VHDCI), a very small connector that allows placement of four wide SCSI connectors on the back of a single PCI card slot. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by Ultra-3 (Ultra-160) SCSI. Ultra-3 includes five new optional features: First introduced as Ultra-160 toward
11872-556: Was discontinued. Initially, the SCSI Parallel Interface (SPI) was the only interface using the SCSI protocol. Its standardization started as a single-ended 8-bit bus 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
11984-481: Was discontinued. Initially, the SCSI Parallel Interface (SPI) was the only interface using the SCSI protocol. Its standardization started as a single-ended 8-bit bus 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
12096-399: Was introduced c. 1997 and featured a LVD bus. For this reason, Ultra-2 is sometimes referred to as LVD SCSI. LVD's greater resistance to noise allowed a maximum bus cable length of 12 meters. At the same time, the data transfer rate was increased to 80 MB/s. Mixing earlier single-ended devices (SE) and Ultra-2 devices on the same bus is possible but connecting only a single SE device forces
12208-465: Was introduced in the 1980s and has seen widespread use on servers and high-end workstations, with new SCSI standards being published as recently as SAS-4 in 2017. The SCSI standards define commands , protocols, electrical, optical and logical interfaces . 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
12320-465: Was introduced in the 1980s and has seen widespread use on servers and high-end workstations, with new SCSI standards being published as recently as SAS-4 in 2017. The SCSI standards define commands , protocols, electrical, optical and logical interfaces . 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
12432-537: Was largely ignored because it was deemed expensive and unnecessary, and was officially retired in SCSI-3. SCSI-2 expanded the command set with the Common Command Set (CCS) for better support of devices other than disk drives, introduced command queueing (up to 256 commands per device) and tightened up the requirements on some features that were optional in SCSI-1; parity was now mandatory and the host adapter
12544-401: Was required to provide termination power in order to support active termination. SCSI-1 devices would generally remain compatible while simply ignoring the new features. A high-voltage differential (HVD) mode that was incompatible with standard single-ended (SE) was introduced to accommodate longer bus lengths. Before Adaptec and later the SCSI Trade Association codified the terminology,
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