SCSI stands for "Small Computer System Interface", and is a standard interface and command set for transferring data between devices on a computer bus. SCSI is pronounced "scuzzy" when spoken aloud, while occasional attempts to promulgate the more flattering pronunciation "sexy" have never succeeded. SCSI is most commonly used for hard disk and Tape storage devices, but also connects a wide range of other devices, including image scanners, CD-ROM drives, CD recorders, and DVD drives. In fact, the entire SCSI standard promotes device independence, which means that theoretically anything can be made SCSI (SCSI printers actually exist). In the past, SCSI was very popular on all kinds of computers. SCSI remains popular on high-performance workstations, servers, and high-end peripherals. Personal computer and laptop more typically use the Advanced Technology Attachment/Integrated Drive Electronics interfaces for hard disks and Universal Serial Bus (which uses a subset of the SCSI command set for hard disks and floppy drives) for other devices. ==History== In 1979, Shugart Associates introduced an interface called SASI (Shugart Associates System Interface). At the same time, NCR Corporation's Peripherals division (now Engenio), had developed a more sophisticated product called BYSE, and was developing an Application-specific integrated circuit to implement it. In late 1981, NCR and Shugart agreed to merge the best features of the two solutions, and to jointly promote the concept as an ANSI standard. After several committee meetings and after a number of other companies decided to adopt the combined standard, it received the new name "SCSI." In 1986, with SCSI already in widespread use, ANSI approved the SCSI specification (as X3.131-1986). Since then, SCSI has developed as an industry-wide standard, capable of being applied to virtually any computer system (there were even SCSI implementations for the venerable Commodore 64 home computer). The first working SCSI ASIC was donated by NCR to the Smithsonian Institution. ==Standards== SCSI has evolved over the years. Before summarizing the evolution, a distinction should be made between the terminology used in the SCSI standard itself, as promulgated by the T10 committee of INCITS, and common parlance, as codified by the SCSI trade association, SCSITA. 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 which manufacturers can include or not. Individual vendors and SCSITA 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 signalling 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 (LVD) signalling, but products called Ultra-2 SCSI include this capability. This terminology 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 command set, allowing devices that support SCSI commands to use any interface (including ones not otherwise specified by T10), and also allowing the interfaces that are defined by T10 to develop on their own terms. This change is also why there is no "SCSI-4". No version of the standard has ever specified what kind of connector should be used. The connectors used by vendors have tended to evolve over time. Although SCSI-1 devices ''typically'' used bulky Blue Ribbon ("Centronics") connectors, and SCSI-2 devices ''typically'' "Mini-D" connectors, it is not correct to refer to these as "SCSI-1" and "SCSI-2" connectors. The mainstream implementations of SCSI (in chronological order) are as follows, using common parlance:
===SCSI-1===
The original standard that was derived from SASI and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit computer bus (with parity), running asynchronously at 3.5 MB/s or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 meters (just under 20 feet -- compare that to the 18 inch (0.45 meter) limit of the Advanced Technology Attachment interface). A variation on the original standard included a high-voltage differential signalling (HVD) implementation whose maximum cable length was many times that of the single-ended versions.
===SCSI-2===
This standard was introduced in 1989 and gave rise to the Fast SCSI and Wide SCSI variants. Fast SCSI doubled the maximum transfer rate to 10 MB/s and Wide SCSI doubled the bus width to 16 bits on top of that (to reach 20 MB/s). However, these improvements came at the minor cost of a reduced maximum cable length to 3 meters. SCSI-2 also specified a 32-bit version of Wide SCSI, which used 2 16-bit cables per bus; this was largely ignored by SCSI device makers, and was officially retired in SCSI-3.
===SCSI-3===
Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI and fast-20 SCSI, were introduced in 1992. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide. The maximum cable length stayed at 3 meters but 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).
===Ultra-2===
This standard was introduced c. 1997 and featured a low voltage differential (LVD) bus. For this reason ultra-2 is sometimes referred to as LVD SCSI. Using LVD technology, it became possible to allow a maximum bus cable length of 12 meters (almost 40 feet!), with much greater noise immunity. At the same time, the data transfer rate was increased to 80 MB/s. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by ultra-3 (ultra-160) SCSI.
===Ultra-3===
Also known as Ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double pumped. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation.
===Ultra-320===
This is the ultra-160 standard with the data transfer rate doubled to 320 MB/s. Nearly all new SCSI hard drives being manufactured at the time of this writing (October 2003) are actually ultra-320 devices.
===Ultra-640===
Ultra-640 (otherwise known as Fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. Ultra-640 doubles the interface speed yet again, this time to 640 MB/s. Ultra640 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, most manufacturers have skipped over Ultra640 and are developing for Serial Attached SCSI instead.
===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 technology. iSCSI could thus address both the low-end and high-end markets with a single commodity-based technology.
===Serial SCSI===
Three recent versions of SCSI Serial_Storage_Architecture, Fibre_Channel and Serial Attached SCSI break from the traditional parallel SCSI standards and perform data transfer via serial communications.
==Compatibility==
Note: Ultra-2, ultra-160 and ultra-320 devices may be freely mixed on the LVD bus with no compromise in performance, as the host adapter will negotiate the operating speed and bus management requirements for each device. ''Single-ended devices should not be attached to the LVD bus, as doing so will force all devices to run at the slower single-ended speed.'' Support for single-ended interfaces has been deprecated in the SPI-5 standard (which describes Ultra-640), so future devices may not be electrically backward compatible.
Caution: Modern Single_Connector_Attachment (SCA) devices may be connected to older controller/drive chains by using SCA adapters. Although these adapters often have auxillary power connectors, use caution: it is possible to quickly destroy the drive by connecting external power. Always try the drive without auxillary power first.
SCSI devices are generally backward compatibility , i.e., it is possible to connect an ultra-3 SCSI hard disk to an ultra-2 SCSI controller and use it (though with reduced speed and feature set).
Each SCSI device (including the computer's host adapter) must be configured to have a unique SCSI identifier on the bus. Also, the SCSI bus must be terminated with a terminator. Both active and passive terminators are in common use, with the active type much preferred (and required on LVD buses). Improper termination is a common problem with SCSI installations.
It is possible to convert a wide bus to a narrow one, with widedevices closer to the adapter. To do this properly requires a cable which terminates the wide part of the bus. This is sometimes referred to as a cable with high-9 termination. Specific commands allow the host to determine the active width of the bus. This arrangement is discouraged.
==Termination==
SCSI buses must always be terminated at both ends.
There are four ways to terminate:
# Internal
# External
# Passive
# Active - the standard resistance for active SCSI terminators is 100 ohms.
See [http://www.scsita.org/aboutscsi/termTutorial.html Termination Tutorial].
==External links==
* [http://www.t10.org/ SCSI standards]
* [http://www.scsita.org/terms/scsiterms.html SCSITA terminology]
* [http://www.bswd.com/cornucop.htm "Storage Cornucopia" SCSI links, maintained by a consultant]
* [http://home.nc.rr.com/woodsmall/SCSI.htm SCSI/iSCSI/RAID/SAS Information Sheet]
SCSI
Computer storage
Computing
Macintosh internals
Computer buses
Fiber Channel is not an alternative spelling of Fibre Channel. == Pleonasm? == "SCSI is pronounced "scuzzy" ''when spoken aloud''"... Isn't this a bit pleonastic? --User:Edcolins 11:51, Oct 28, 2004 (UTC) "while occasional attempts to promulgate the more flattering pronunciation "sexy" have never succeeded." Nowadays we're trying to get it to be pronounced "sucksy" ;) --User:148.84.19.92 15:25, 9 Mar 2005 (UTC) == IDE? == The article says that PATAPI is SCSI over IDE. Isn't this a misnomer? As I recall, parallel ATA is not the only form of IDE, though PATAPI is, of course, over parallel ata. :Not quite; something the article doesn't make clear is that SCSI is a command set as much as (and, now even more than) it is an interface. If you look at the official specs for ATAPI, it is indeed sending SCSI commands over the ATA bus; in fact, most ATAPI devices these days are actually SCSI devices that speak enough ATA to get by. User:Lee Cremeans 07:11, 13 Jun 2005 (UTC) == spice up SCSI == good article, Mac users in particular grew up on SCSI and SCSI peripherals How about some pics of the typical fat cables and giant terminators that people remember: http://images.google.com/images?q=scsi&hl=en&lr=&sa=N&tab=wi Also what about the SCSI logo ? http://images.google.com/images?hl=en&lr=&q=scsi+logo&btnG=Search == SCSI protocol == This article only seems to talk about devices...as far as I know, in the Linux kernel (for 2.6 at least), firewire IEEE1394 / USB storage devices / Ipod's, etc. they also use the SCSI protocol to communicate to the kernel. Any elaboration on this? -- User:Natalinasmpf 02:27, 19 Apr 2005 (UTC) :You are correct; most of this article focuses on what is now officially referred to as the SCSI Parallel Interface, which is just one part of SCSI-3 and has little to do now with the SCSI command set (which is, as you and others have noted, is implemented on things that are about as far from SPI as you can get). At some point (not tonight, since it's late) I'll go through and sort things out. User:Lee Cremeans 07:05, 13 Jun 2005 (UTC) ---- June 22, 2005: The article should mention "packetized" SCSI that was introduced with Ultra-320. IBM preferred the term "information units" over "packets". I think this was around 2002 or early 2003. Also, the "Ultra" designations were promoted by the SCSI Trade Association but not used in the ANSI standards. After SCSI-2, the physical interface standard was separated from the logical layers, and became SPI (for SCSI Parallel Interface) and went through generations SPI-1, SPI-2, SPI-3, SPI-4 and SPI-5. STA wanted marketing-oriented nomenclature that reflected the actual maximum data transfer rate of the bus. Note that the SPI acronym can easily cause confusion with System_Packet_Interface Serial_Peripheral_Interface. I don't think "All SCSI standards have been modular, defining various capabilities which manufacturers can include or not" is true of SCSI-1 or SCSI-2, which were monolithic specifications, unless "modular" means something different from what I take it to mean. :I didn't write that part. But certainly SCSI-2 has wide scope for variation in device capabilities, with all sorts of contexts in which initiator and target are supposed to negotiate for a set of common capabilities. :I think this is too opinionated for the article, but to my mind one of the ''weaknesses'' of SCSI is that it is a humongous specification--SCSI-2 is about an inch thick--and it is very easy for devices to fail to implement some portions of it correctly. Obviously, if an Adaptec card doesn't work properly with Seagate disks they'll catch it and fix it, but obscure corners of the specification can be ignored, resulting in SCSI cards that work with most but not all devices, and vice versa. :It is similar to problems encountered with TIFF in, say, the early nineties, when it was quite possible to have a TIFF file that completely conformed to the TIFF spec but which could not be opened by PageMaker. :Another problem, to my way of thinking, is that the safety margins on things like cable length appear not to be conservative enough. I notice that Adaptec consistently specifies maximum cable lengths that are exactly half those allowed by the spec! And, worse yet, in the real world, if you have a cable configuration that theoretically should work, but the terminations are slightly bollixed, or there are slight impedance mismatches where cables join at connectors, the result is subtle and sometimes intermittent problems, rather than obvious failures. :Vendors try to solve these problems by building in automagic self-configurating features, which help when they work and make things worse when they don't. For a while we had an issue with Adaptec cards, when they first introduced a feature they called "domain validation." This meant that instead of trusting what the device said its capabilities were, the card would ''dynamically test'' for certain device characteristics and capabilities. Our device would sit there and say "I'm synchronous, I'm synchronous, please please run synchronous so we can get the data transfer rates we need" and for some reason the card would say "Nyaah, nyaah, I don't BELEEEEVE you, say what you like, I'm going to run asynchronous anyway" making our stuff basically not work unless the end-user turned off domain validation. This is no longer true, but we never found out exactly what had changed about their "domain validation" process. User:Dpbsmith User_talk:dpbsmith 19:21, 22 Jun 2005 (UTC)
See the main article on SCSI. Computer buses
#redirect SCSI
See other meanings of words starting from letter:
Words begining with SCSI:
SCSI
SCSI
SCSI
Scsi
SCSI-1
SCSI-2
SCSI-3
SCSI1
SCSI2
SCSI3
SCSI_host_adapter
SCSI_initiator
SCSI_Parallel_Interface
SCSI
SCSI stands for "Small Computer System Interface", and is a standard interface and command set for transferring data between devices on a computer bus. SCSI is pronounced "scuzzy" when spoken aloud, while occasional attempts to promulgate the more flattering pronunciation "sexy" have never succeeded. SCSI is most commonly used for hard disk and Tape storage devices, but also connects a wide range of other devices, including image scanners, CD-ROM drives, CD recorders, and DVD drives. In fact, the entire SCSI standard promotes device independence, which means that theoretically anything can be made SCSI (SCSI printers actually exist). In the past, SCSI was very popular on all kinds of computers. SCSI remains popular on high-performance workstations, servers, and high-end peripherals. Personal computer and laptop more typically use the Advanced Technology Attachment/Integrated Drive Electronics interfaces for hard disks and Universal Serial Bus (which uses a subset of the SCSI command set for hard disks and floppy drives) for other devices. ==History== In 1979, Shugart Associates introduced an interface called SASI (Shugart Associates System Interface). At the same time, NCR Corporation's Peripherals division (now Engenio), had developed a more sophisticated product called BYSE, and was developing an Application-specific integrated circuit to implement it. In late 1981, NCR and Shugart agreed to merge the best features of the two solutions, and to jointly promote the concept as an ANSI standard. After several committee meetings and after a number of other companies decided to adopt the combined standard, it received the new name "SCSI." In 1986, with SCSI already in widespread use, ANSI approved the SCSI specification (as X3.131-1986). Since then, SCSI has developed as an industry-wide standard, capable of being applied to virtually any computer system (there were even SCSI implementations for the venerable Commodore 64 home computer). The first working SCSI ASIC was donated by NCR to the Smithsonian Institution. ==Standards== SCSI has evolved over the years. Before summarizing the evolution, a distinction should be made between the terminology used in the SCSI standard itself, as promulgated by the T10 committee of INCITS, and common parlance, as codified by the SCSI trade association, SCSITA. 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 which manufacturers can include or not. Individual vendors and SCSITA 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 signalling 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 (LVD) signalling, but products called Ultra-2 SCSI include this capability. This terminology 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 command set, allowing devices that support SCSI commands to use any interface (including ones not otherwise specified by T10), and also allowing the interfaces that are defined by T10 to develop on their own terms. This change is also why there is no "SCSI-4". No version of the standard has ever specified what kind of connector should be used. The connectors used by vendors have tended to evolve over time. Although SCSI-1 devices ''typically'' used bulky Blue Ribbon ("Centronics") connectors, and SCSI-2 devices ''typically'' "Mini-D" connectors, it is not correct to refer to these as "SCSI-1" and "SCSI-2" connectors. The mainstream implementations of SCSI (in chronological order) are as follows, using common parlance:
| Interface | Bus width | Clock speed | Bus bandwidth | Max. cable length | Max. number of devices |
|---|---|---|---|---|---|
| SCSI | 8 bits | 5 MHz | 5 MB/s | 6m | 8 |
| Fast SCSI | 8 bits | 10 MHz | 10 MB/s | 1.5-3m | 8 |
| Wide SCSI | 16 bits | 10 MHz | 20 MB/s | 1.5-3m | 16 |
| Ultra SCSI | 8 bits | 20 MHz | 20 MB/s | 1.5-3m | 5-8 |
| Ultra Wide SCSI | 16 bits | 20 MHz | 40 MB/s | 1.5-3m | 5-8 |
| Ultra2 SCSI | 8 bits | 40 MHz | 40 MB/s | 12m | 8 |
| Ultra2 Wide SCSI | 16 bits | 40 MHz | 80 MB/s | 12m | 16 |
| Ultra3 SCSI | 16 bits | 40 MHz DDR | 160 MB/s | 12m | 16 |
| Ultra-320 SCSI | 16 bits | 80 MHz DDR | 320 MB/s | 12m | 16 |
| Serial_Storage_Architecture | 1 bit | 400 MBit | 80 MB/s | 25m | 96 |
| Fibre_Channel | 1 bit | 2GBit | 2000 MB/s | ? | 127 |
| iSCSI | Dependent upon IP network | ?? | |||
| Serial Attached SCSI 3Gbit | 1 bit | N/A | 375 MB/s per direction; full duplex | 10m | 16,256 (128 per expander) |
SCSI
Fiber Channel is not an alternative spelling of Fibre Channel. == Pleonasm? == "SCSI is pronounced "scuzzy" ''when spoken aloud''"... Isn't this a bit pleonastic? --User:Edcolins 11:51, Oct 28, 2004 (UTC) "while occasional attempts to promulgate the more flattering pronunciation "sexy" have never succeeded." Nowadays we're trying to get it to be pronounced "sucksy" ;) --User:148.84.19.92 15:25, 9 Mar 2005 (UTC) == IDE? == The article says that PATAPI is SCSI over IDE. Isn't this a misnomer? As I recall, parallel ATA is not the only form of IDE, though PATAPI is, of course, over parallel ata. :Not quite; something the article doesn't make clear is that SCSI is a command set as much as (and, now even more than) it is an interface. If you look at the official specs for ATAPI, it is indeed sending SCSI commands over the ATA bus; in fact, most ATAPI devices these days are actually SCSI devices that speak enough ATA to get by. User:Lee Cremeans 07:11, 13 Jun 2005 (UTC) == spice up SCSI == good article, Mac users in particular grew up on SCSI and SCSI peripherals How about some pics of the typical fat cables and giant terminators that people remember: http://images.google.com/images?q=scsi&hl=en&lr=&sa=N&tab=wi Also what about the SCSI logo ? http://images.google.com/images?hl=en&lr=&q=scsi+logo&btnG=Search == SCSI protocol == This article only seems to talk about devices...as far as I know, in the Linux kernel (for 2.6 at least), firewire IEEE1394 / USB storage devices / Ipod's, etc. they also use the SCSI protocol to communicate to the kernel. Any elaboration on this? -- User:Natalinasmpf 02:27, 19 Apr 2005 (UTC) :You are correct; most of this article focuses on what is now officially referred to as the SCSI Parallel Interface, which is just one part of SCSI-3 and has little to do now with the SCSI command set (which is, as you and others have noted, is implemented on things that are about as far from SPI as you can get). At some point (not tonight, since it's late) I'll go through and sort things out. User:Lee Cremeans 07:05, 13 Jun 2005 (UTC) ---- June 22, 2005: The article should mention "packetized" SCSI that was introduced with Ultra-320. IBM preferred the term "information units" over "packets". I think this was around 2002 or early 2003. Also, the "Ultra" designations were promoted by the SCSI Trade Association but not used in the ANSI standards. After SCSI-2, the physical interface standard was separated from the logical layers, and became SPI (for SCSI Parallel Interface) and went through generations SPI-1, SPI-2, SPI-3, SPI-4 and SPI-5. STA wanted marketing-oriented nomenclature that reflected the actual maximum data transfer rate of the bus. Note that the SPI acronym can easily cause confusion with System_Packet_Interface Serial_Peripheral_Interface. I don't think "All SCSI standards have been modular, defining various capabilities which manufacturers can include or not" is true of SCSI-1 or SCSI-2, which were monolithic specifications, unless "modular" means something different from what I take it to mean. :I didn't write that part. But certainly SCSI-2 has wide scope for variation in device capabilities, with all sorts of contexts in which initiator and target are supposed to negotiate for a set of common capabilities. :I think this is too opinionated for the article, but to my mind one of the ''weaknesses'' of SCSI is that it is a humongous specification--SCSI-2 is about an inch thick--and it is very easy for devices to fail to implement some portions of it correctly. Obviously, if an Adaptec card doesn't work properly with Seagate disks they'll catch it and fix it, but obscure corners of the specification can be ignored, resulting in SCSI cards that work with most but not all devices, and vice versa. :It is similar to problems encountered with TIFF in, say, the early nineties, when it was quite possible to have a TIFF file that completely conformed to the TIFF spec but which could not be opened by PageMaker. :Another problem, to my way of thinking, is that the safety margins on things like cable length appear not to be conservative enough. I notice that Adaptec consistently specifies maximum cable lengths that are exactly half those allowed by the spec! And, worse yet, in the real world, if you have a cable configuration that theoretically should work, but the terminations are slightly bollixed, or there are slight impedance mismatches where cables join at connectors, the result is subtle and sometimes intermittent problems, rather than obvious failures. :Vendors try to solve these problems by building in automagic self-configurating features, which help when they work and make things worse when they don't. For a while we had an issue with Adaptec cards, when they first introduced a feature they called "domain validation." This meant that instead of trusting what the device said its capabilities were, the card would ''dynamically test'' for certain device characteristics and capabilities. Our device would sit there and say "I'm synchronous, I'm synchronous, please please run synchronous so we can get the data transfer rates we need" and for some reason the card would say "Nyaah, nyaah, I don't BELEEEEVE you, say what you like, I'm going to run asynchronous anyway" making our stuff basically not work unless the end-user turned off domain validation. This is no longer true, but we never found out exactly what had changed about their "domain validation" process. User:Dpbsmith User_talk:dpbsmith 19:21, 22 Jun 2005 (UTC)
SCSI
See the main article on SCSI. Computer buses
Scsi
#redirect SCSI
See other meanings of words starting from letter:
S
SB | SC | SD | SE | SF | SG | SH | SI | SJ | SK | SL | SM | SN | SO | SP | SR | SS | ST | SU | SW | SX | SY | SZ |Words begining with SCSI:
SCSI
SCSI
SCSI
Scsi
SCSI-1
SCSI-2
SCSI-3
SCSI1
SCSI2
SCSI3
SCSI_host_adapter
SCSI_initiator
SCSI_Parallel_Interface
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