You do not have permission to edit this page, for the following reasons:

This IP address has been blocked from editing Wikipedia.
This does not affect your ability to read Wikipedia pages.
Most people who see this message have done nothing wrong. Some kinds of blocks restrict editing from specific service providers or telecom companies in response to recent abuse or vandalism, and can sometimes affect other users who are unrelated to that abuse. Review the information below for assistance if you do not believe that you have done anything wrong.

The IP address or range 44.201.0.0/16 has been blocked (disabled) by ‪ST47‬ for the following reason(s):

The IP address that you are currently using has been blocked because it is believed to be a web host provider. To prevent abuse, web hosts may be blocked from editing Wikipedia.
You will not be able to edit Wikipedia using a web host provider because it hides your IP address, much like a proxy or VPN.
We recommend that you attempt to use another connection to edit. For example, if you use a proxy or VPN to connect to the internet, turn it off when editing Wikipedia. If you edit using a mobile connection, try using a Wi-Fi connection, and vice versa. If you are using a corporate internet connection, switch to a different Wi-Fi network. If you have a Wikipedia account, please log in.
If you do not have any other way to edit Wikipedia, you will need to request an IP block exemption.

How to appeal if you are confident that your connection does not use a web host's IP address:
If you are confident that you are not using a web host, you may appeal this block by adding the following text on your talk page: ((unblock|reason=Caught by a web host block but this host or IP is not a web host. My IP address is _______. Place any further information here. ~~~~)). You must fill in the blank with your IP address for this block to be investigated. Your IP address can be determined here. Alternatively, if you wish to keep your IP address private you can use the unblock ticket request system. There are several reasons you might be editing using the IP address of a web host (such as if you are using VPN software or a business network); please use this method of appeal only if you think your IP address is in fact not a web host.

Administrators: The IP block exemption user right should only be applied to allow users to edit using web host in exceptional circumstances, and they should usually be directed to the functionaries team via email. If you intend to give the IPBE user right, a CheckUser needs to take a look at the account. This can be requested most easily at SPI Quick Checkuser Requests. Unblocking an IP or IP range with this template is highly discouraged without at least contacting the blocking administrator.

This block will expire on 23:54, 31 July 2024.

Even when blocked, you will usually still be able to edit your user talk page, as well as email administrators and other editors.

For information on how to proceed, please read the FAQ for blocked users and the guideline on block appeals. The guide to appealing blocks may also be helpful.

Other useful links: Blocking policy · Help:I have been blocked
Your IP address is in a range that has been blocked on all Wikimedia Foundation wikis. The block was made by ‪AntiCompositeNumber‬. The reason given is Open proxy/Webhost: Visit the FAQ if you are affected .
- Start of block: 13:55, 18 August 2022
- Expiry of block: 13:55, 18 April 2025
Your current IP address is 44.201.28.123. The blocked range is 44.201.0.0/16. Please include all above details in any queries you make. If you believe you were blocked by mistake, you can find additional information and instructions in the No open proxies global policy. Otherwise, to discuss the block please post a request for review on Meta-Wiki. You could also send an email to the stewards VRT queue at stewards@wikimedia.org including all above details.

You can view and copy the source of this page:

((Short description|Original SCSI storage interface))
((Refimprove|date=January 2007))
((Use American English|date=September 2020))

[[File:Centronics 50 SCSI connector.JPG|240px|thumb|right|[[Micro ribbon|Amphenol]]-50 SCSI plug]]

'''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 [[#Termination|terminated]].

SCSI is a peer-to-peer [[peripheral]] interface. Every device attaches to 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.((efn|Most peripheral devices are exclusively [[SCSI target]]s, incapable of acting as [[SCSI initiator]]s—unable to initiate SCSI transactions themselves. Therefore, peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications.)) The [[Symbios Logic]] 53C810 chip is an example of a [[Peripheral Component Interconnect|PCI]] host interface that can act as 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]].

==History==
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 the parallel bus was 8 bits wide (plus a ninth [[parity bit]]).  The SCSI-2 standard allowed for faster operation (10&nbsp;MHz) and wider buses (16-bit or 32-bit).  The 16-bit option became the most popular.

At 10&nbsp;MHz with a bus width of 16 bits it is possible to achieve a data rate of 20&nbsp;MB/s.  Subsequent extensions to the SCSI standard allowed for faster speeds: 20&nbsp;MHz, 40&nbsp;MHz, 80&nbsp;MHz, 160&nbsp;MHz and finally 320&nbsp;MHz. At 320&nbsp;MHz x 16 bits there is a theoretical maximum peak data rate of 640&nbsp;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]].

==Standards==
Parallel SCSI is not a single standard, but a suite of closely related standards. There are 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 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 (LVD) signaling, 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 [[SCSI command|SCSI 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 be used in alternate manners.

No version of the standard has ever specified what kind of [[SCSI connector]] should be used.  See ((slink||External connectors)).

===Comparison table===
{| class="wikitable"
|- 
! rowspan=3 | Interface
! rowspan=3 | Alternative names
! rowspan=3 | Specification document((efn-ua|Specifications are maintained by the T10 subcommittee of the [[International Committee for Information Technology Standards]].))
! rowspan=3 | Connector
! rowspan=3 | Width (bits)
! rowspan=3 | Clock ([[hertz|MHz]])
! colspan=6 | Maximum
! colspan=2 | Electrical
|-
! colspan=2 | Throughput
! colspan=3 | Length ([[metre|m]])
! rowspan=2 | Devices((efn-ua|group=Comparison|Including any host adapters (i.e., computers count as a device).))
! rowspan=2 | Impedance ([[ohm|Ω]])
! rowspan=2 | Voltage ([[volt|V]])
|- 
! ([[MB/s]])
! ([[Mbit/s]])
! Single ended((efn-ua|group=Comparison|For daisy-chain designs, length of bus, from end to end; for point-to-point, length of a single link.))
! [[Low-voltage differential signaling|LVD]]((efn-ua|LVD cabling may be up to 25&nbsp;m when only a single device is attached to the host adapter, 20&nbsp;m for Ultra-640.))
! [[High-voltage differential signaling|HVD]]
|- 
| SCSI-1
| Narrow SCSI
| SCSI-1 (1986)((efn-ua|group=Comparison|The SCSI-1 specification has been withdrawn and is superseded by SCSI-2. The SCSI-3 SPI specification has been withdrawn and is superseded by SPI-2. The SCSI-3 SPI-3 and SPI-4 specifications have been withdrawn and are superseded by SPI-5.<ref>((cite web |url=http://www.t10.org/drafts.htm#OBSOLETE |title=T10 Withdrawn Standards and Technical Reports |access-date=March 18, 2010))</ref>))
| [[Insulation-displacement connector|IDC]]50; Amphenol C50
| 8
| 5 
| 5
| 40
| 6
| ((N/A|N/A))
| 25
| 8
| ((nowrap| SE 90 ± 6<ref name="random">((cite web|title=Random Problems Encountered When Mixing SE and LVD SCSI Standards |url=http://support.microsoft.com/kb/285013 |access-date=May 7, 2008))</ref>))
| SE&nbsp;5
|- 
| Fast SCSI
| 
| SCSI-2 (1994)
| IDC50; Amphenol C50
| 8
| 10
| 10
| 80
| 3
| ((N/A|N/A))
| 25
| 8
| ((nowrap| SE 90 ± 6<ref name="random"/> ))
| SE&nbsp;5 HVD&nbsp;≥5
|- 
| Fast-Wide SCSI
| Wide SCSI
| SCSI-2; <br/>SPI-5 (INCITS 367-2003)
| 68-pin
| 16
| 10
| 20
| 160
| 3
| ((N/A|N/A))
| 25
| 16
| ((nowrap| SE 90 ± 6<ref name="random"/> ))
| SE&nbsp;5 HVD&nbsp;≥5
|- 
| rowspan=2 | Ultra SCSI
| rowspan=2 | Fast-20
| rowspan=2 | SPI-5 (INCITS 367-2003)
| rowspan=2 | IDC50
| rowspan=2 | 8
| rowspan=2 | 20
| rowspan=2 | 20
| rowspan=2 | 160
| 1.5
| ((N/A|N/A))
| 25
| 8
| rowspan=2 | ((nowrap| SE 90 ± 6<ref name="random"/> ))
| rowspan=2 | SE&nbsp;5 HVD&nbsp;≥5
|- 
| 3
| ((N/A|N/A))
| ((N/A|N/A))
| 4
|- 
| rowspan=3 | Ultra Wide SCSI
| rowspan=3 |
| rowspan=3 | SPI-5 (INCITS 367-2003)
| rowspan=3 | 68-pin
| rowspan=3 | 16
| rowspan=3 | 20
| rowspan=3 | 40
| rowspan=3 | 320
| ((N/A|N/A))
| ((N/A|N/A))
| 25
| 16
| rowspan=3 | ((nowrap| SE 90 ± 6<ref name="random"/> ))
| rowspan=3 | SE&nbsp;5 HVD&nbsp;≥5
|- 
| 1.5
| ((N/A|N/A))
| ((N/A|N/A))
| 8
|- 
| 3
| ((N/A|N/A))
| ((N/A|N/A))
| 4
|- 
| Ultra2 SCSI
| Fast-40
| SPI-5 (INCITS 367-2003)
| 50-pin
| 8
| 40
| 40
| 320
| ((N/A|N/A))
| 12
| 25
| 8
| ((nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> ))
| LVD&nbsp;1.2 HVD&nbsp;≥5
|- 
| Ultra2 Wide SCSI
| 
| SPI-5 (INCITS 367-2003)
| ((nowrap|68-pin;)) ((nowrap|80-pin)) ((nowrap|([[Single Connector Attachment|SCA]]/SCA-2)))
| 16
| 40
| 80
| 640
| ((N/A|N/A))
| 12
| 25
| 16
| ((nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> ))
| LVD&nbsp;1.2 HVD&nbsp;≥5
|- 
| Ultra3 SCSI
| Ultra-160; Fast-80 wide
| SPI-5 (INCITS 367-2003)
| ((nowrap|68-pin;)) ((nowrap|80-pin)) ((nowrap|(SCA/SCA-2)))
| 16
| 40 [[double data rate|DDR]]
| 160
| 1280
| ((N/A|N/A))
| 12
| ((N/A|N/A))
| 16
| ((nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> ))
| LVD&nbsp;1.2
|- 
| Ultra-320 SCSI
| Ultra-4; Fast-160
| SPI-5 (INCITS 367-2003)
| ((nowrap|68-pin;)) ((nowrap|80-pin)) ((nowrap|(SCA/SCA-2)))
| 16
| 80 DDR
| 320
| 2560
| ((N/A|N/A))
| 12
| ((N/A|N/A))
| 16
| ((nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> ))
| LVD&nbsp;1.2
|- 
| Ultra-640 SCSI((efn-ua|group=Comparison|Ultra-640 substantially increases the requirements for cabling and backplanes, hampering a smooth transition.<ref>[http://t10.org/ftp/t10/document.01/01-224r0.pdf T10/01-224r0 Ultra640 SCSI Measured Data from Cables & Backplanes].</ref>))((efn-ua|Ultra-640 was specified but no devices were produced.<ref>[https://books.google.com/books?id=9cLFf_1PBnkC&dq=do+ultra-640+devices+exist&pg=PT724 Scott Mueller: Upgrading and Repairing Servers].</ref>))
| Ultra-5; Fast-320
| SPI-5 (INCITS 367-2003)
| ((nowrap|68-pin;)) ((nowrap|80-pin))
| 16
| 160 DDR
| 640
| 5120
| ((N/A|N/A))
| 10
| ((N/A|N/A))
| 16
| ((nowrap| LVD 125 ± 10))
| LVD&nbsp;1.2
|}

((notelist-ua))

===SCSI-1===
[[File:Ncr 53c94 scsi asic.jpeg|thumb|[[NCR 53C9x|NCR 53C94 SCSI-1 controller]] in [[plastic leaded chip carrier#Plastic leaded chip carrier|PLCC-84]] package]]
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 bit|parity]]), running asynchronously at 3.5&nbsp;MB/s, or 5&nbsp;MB/s in synchronous mode, and a maximum bus cable length of ((convert|6|m|ft)), significantly longer than the ((convert|18|in|m)) limit of the [[Advanced Technology Attachment|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 ((convert|25|m|ft)).

===SCSI-2===
[[File:53CF94 SCSI-2 ASIC.gif|thumb|[[AMD]] 53CF94 SCSI-2 controller in [[plastic leaded chip carrier#Plastic leaded chip carrier|PLCC-84]] package]]
'''SCSI-2''' was introduced in 1994 and gave rise to the '''Fast SCSI''' and '''Wide SCSI''' variants. Fast SCSI doubled the maximum transfer rate to 10&nbsp;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&nbsp;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 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 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.<ref name="MuellerSoper2006">((cite book|author1=Scott Mueller|author2=Mark Edward Soper|author3=Barrie Sosinsky|title=Upgrading and Repairing Servers|url=https://books.google.com/books?id=9cLFf_1PBnkC&pg=PT724|date=24 April 2006|publisher=Pearson Education|isbn=978-0-13-279698-9))</ref>

A [[high-voltage differential signaling|high-voltage differential]] (HVD) mode that was incompatible with standard [[Single-ended signaling|single-ended]] (SE) was introduced to accommodate longer bus lengths.

===SCSI-3===
Before Adaptec and later the [[SCSI Trade Association]] codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as '''((vanchor|Ultra SCSI))'''<ref>((cite web |url=https://www.cablewholesale.com/support/technical_articles/scsi_unscrewed.php |title=SCSI Device Unscrewed |access-date=2024-02-22))</ref> or Fast-20 SCSI,<ref>((cite web |url=https://www.globalspec.com/learnmore/industrial_computers_embedded_computer_components/bus_systems/scsi_adapters_scsi_controllers |title=SCSI Adapters and SCSI Controllers Information |access-date=2024-02-22))</ref> 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&nbsp;MB/s for ''narrow'' (8-bit) systems and 40&nbsp;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 [[SCSI terminator|terminator]]s were often to blame for instability problems).

Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination.

===Ultra-2===
This standard was introduced c. 1997 and featured a [[low voltage differential signaling|low-voltage differential]] (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&nbsp;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 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===
Ultra-3 includes five new optional features:
#Doubling the transfer rate to 160&nbsp;MB/s through the use of [[Double data rate|double-transition clocking]]
#[[cyclic redundancy check|CRC]], a robust error-correcting process more suited for high-speed operation than the parity checking used previously
#Domain validation for negotiating maximum performance for each device on the chain
#Packetization protocol with a reduced number of bus communication phases for less command and protocol overhead
#Quick arbitration and selection reduces arbitration time by eliminating bus free time

First introduced as '''Ultra-160''' toward the end of 1999, this iteration improved on the Ultra-2 standard adding the first three improvements.<ref>((cite journal|title=The Last Word on SCSI|last=Norris|first=Jim|journal=[[Maximum PC]]|date=March 2002|page=50|url=https://books.google.com/books?id=0gEAAAAAMBAJ&pg=PA50))</ref> 

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.<ref name="MuellerSoper2006">((cite book|author1=Scott Mueller|author2=Mark Edward Soper|author3=Barrie Sosinsky|title=Upgrading and Repairing Servers|url=https://books.google.com/books?id=9cLFf_1PBnkC&pg=PT724|date=24 April 2006|publisher=Pearson Education|isbn=978-0-13-279698-9))</ref>

===Ultra-320===
Ultra-320 included the Ultra-160+ features as mandatory, doubled the clock to 80&nbsp;MHz for a maximum data transfer rate of 320&nbsp;MB/s, and included read/write data streaming for less overhead on queued data transfers, as well as flow control.<ref name="MuellerSoper2006">((cite book|author1=Scott Mueller|author2=Mark Edward Soper|author3=Barrie Sosinsky|title=Upgrading and Repairing Servers|url=https://books.google.com/books?id=9cLFf_1PBnkC&pg=PT724|date=24 April 2006|publisher=Pearson Education|isbn=978-0-13-279698-9))</ref> The latest working draft for this standard is revision 10 and is dated May 6, 2002. Nearly all SCSI [[hard drive]]s being manufactured at the end of 2003 were 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. It doubles the interface speed yet again, this time to 640&nbsp;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.

==SCSI signals==
In addition to the data bus and parity signals, a parallel SCSI bus contains nine control signals:<ref name="m309x">[https://web.archive.org/web/20030506055212/http://www.fel.fujitsu.com/support/scanner/manuals/M3096gx-93gx-93dg.ome.en.pdf M3096GX/M3093GX/M3093DG Image scanner OEM Manual]</ref>

{|class="wikitable"
|-
! Signal name
! Meaning when asserted
! Meaning when deasserted
|-
|BSY Busy|| Bus in use|| Bus free
|-
|SEL Select|| Asserted by the winner of an arbitration, during selection by an initiator or reselection by a target || No device has control of the bus
|-
|RST Reset||Initiator forces all targets and any other initiators to do a warm reset||No reset requested
|-
|C/D Control/Data((efn-ua|name=Phase))|| Bus contains control information|| Bus contains data
|-
|I/O Input/Output((efn-ua|name=Phase)) || Transfer is from target to initiator. Also asserted by a target after winning arbitration to indicate reselection of an initiator|| Transfer is from initiator to target
|-
|MSG Message((efn-ua|name=Phase)) || Bus contains a message|| Bus contains data or command/status
|-
|REQ Request || Target requests initiator to transfer the next unit of information on the bus, as indicated by the 3 phase signals|| Target not requesting transfer
|-
|ACK Acknowledge|| Initiator acknowledges target request, completing the information transfer handshake|| No acknowledge
|-
|ATN Attention || Asserted by an initiator after winning arbitration to select a target || No target selection in progress
|}

((notelist-ua|refs=<ref name="Phase">These 3 signals are driven by a target during information transfer to indicate the [[#Bus operation|bus phase]].</ref>))

There are also three DC-level signals:

; TERMPOWER: See ((section link||Termination))
; DIFFSNS: Grounded in single-ended buses, otherwise floats to a positive voltage
; GROUND: Most spare pins in the connector are designated as grounds.

There are three electrically different variants of the SCSI parallel bus: [[Single-ended signalling|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 the number of signals required to implement a SCSI bus is a function of the bus width and voltage:

{|class="wikitable"
|-
! Bus width !! Voltage !! Data !! Parity !! Control !! TERMPOWER !! DIFFSNS !! GROUND !! Reserved !! Total
|-
|8-bit||SE||8||1||9||1||1||30||0||50
|-
|8-bit||LVD/HVD||16||2||18||1||1||12||0||50
|-
|16-bit||SE||16||2||9||4||1||34||2||68
|-
|16-bit||LVD/HVD||32||4||18||4||1||7||2||68
|}

==SCSI IDs==
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:

{|class="wikitable"
! Bus-width !! ID width !! IDs available
|-
|8-bit||3-bit||8
|-
|16-bit||4-bit||16
|}

==Bus operation==
The parallel SCSI bus goes through eight possible phases as a [[SCSI command|command]] is processed.  Not all phases will occur in all cases:

{|class="wikitable"
|-
! Phase !! Comments
|-
|Bus-free||This is the state in which no device communication is in process.
|-
|Arbitration||One or more devices attempt to obtain exclusive control of the bus by asserting /BSY and a single bit corresponding to the device SCSI ID.  For example, a device with a SCSI ID of 2 would generate the inverted bit pattern <code>11111011</code> on the bus.
|-
|Selection||The arbitrating device with the highest ID takes control of the bus by asserting /BSY and /SEL.  "Highest" on an eight-bit bus starts from 7 and works downward to zero.  On a 16-bit bus, the eight-bit rule applies, followed by 15 and working downward to 8, thus maintaining backward compatibility on a bus with a mix of eight and 16-bit devices.  The controlling device is now the ''initiator''.
|-
|Command||The initiator sends the [[SCSI CDB|command descriptor block]] (CDB) to a ''target'', which is another device on the bus.  The CDB tells the target what to do.
|-
|Reselection||During a transaction, the target device may be required to execute a time-consuming operation (e.g., winding or rewinding the tape in a [[tape drive]]).  In such a case, the target may temporarily disconnect from the bus, causing the bus to go to the bus-free condition and allowing other unrelated operations to take place.  Reselection is the phase where the target reconnects to the initiator to resume the previously suspended transaction.
|-
|Data||In this phase, data is transferred between initiator and target, the direction of transfer depending on the command that was issued.  For example, a command to read a [[disk sector|sector]] from a disk would result in a transfer from the disk to the host.  Or, if an error occurred, the initiator could send a ''request sense'' command to the target for details that are returned during the data phase.
|-
|Message||A [[SCSI Message Code|message code]] is exchanged between initiator and target for the purposes of interface management.
|-
|Status||A [[SCSI Status Code|status code]] is sent to the initiator to report the success or failure of the operation.
|}

The above list does not imply 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.

==External connectors==
((main article|SCSI connector))
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 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).

==Termination==
[[Image:SCSC-term.jpg|thumb|right|SCSI Terminator (Amphenol connector)]]

Parallel SCSI buses must always be [[Electrical termination|terminated]] at both ends to ensure reliable operation.  Without termination, data transitions reflect back from the ends of 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#SCSI|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|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&nbsp;mA on a single-ended bus, or 600&nbsp;mA on a differential bus.

Termination can be passive or active.  With passive termination each signal line is terminated by two resistors, 220&nbsp;Ω to TERMPOWER and 330&nbsp;Ω to ground.  Active termination uses a small voltage regulator which provides a +2.85&nbsp;V supply.  Each signal line is then terminated by a 110&nbsp;Ω resistor to this supply.  Active termination provides a better impedance match than passive termination because most flat ribbon cables have a [[characteristic impedance]] of approximately 110&nbsp;Ω.  [[Forced perfect termination]] (FPT) is similar to active termination, but with added [[Diode clamp|diode clamp circuits]] which absorb any residual voltage overshoot or undershoot.  

In current practice most parallel SCSI buses are LVD (low-voltage differential) and so require external, active termination.  The usual termination circuit consists of a +2.85&nbsp;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((snd))the same effect any other SE device has. Passive terminators may cause Ultra speed communication to be unreliable. 

Generally, and reflecting 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 is a special case in SCSI systems that have mixed 8-bit and 16-bit devices where [[SCSI high byte termination|high-byte termination]] may be required.

==Compatibility==
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.

===Mixing different speeds===
SCSI devices in the same SCSI transport family are generally [[backward compatibility|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 [[LVDS|LVD]] bus with no compromise in performance.

===Mixing single-ended and low-voltage differential===
[[Single-ended signalling|Single-ended]] and [[LVDS]] devices can be attached to 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.<ref>((cite web|url=http://download.adaptec.com/pdfs/user_guides/aha2940u2w_ur.pdf |title=Adaptec AHA-2940U2W User's Reference |date=December 1997 |access-date=2017-05-20))</ref> Other adapters may provide multiple buses (channels).

===Mixing wide and narrow===
Both narrow and wide SCSI devices can be attached to 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 a terminator or includes the terminator itself.((efn|This is sometimes referred to as a cable with high-9 termination.)) 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.

===SCA adapters===
[[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 is recommended when connecting them, as it is possible to damage devices by connecting external power.

===Device IDs and termination===
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 [[#Termination|terminator]]. Both active and passive terminators are in common use, with 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 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.

===SCAM===
''SCSI Configured Automatically'' (initially ''Automagically'') was an optional method to configure the SCSI ID without requiring user intervention.<ref>[http://t10.org/ftp/t10/document.96/96-130r0.pdf SCSI-3 Annex B 1996 – SCAM]</ref> It was dropped from later standards.

===Laptops===
SCSI interfaces had become impossible to find for laptop computers. Adaptec had years before produced [[PCMCIA]] parallel SCSI interfaces, but when PCMCIA was superseded by the [[ExpressCard]] Adaptec discontinued their PCMCIA line without supporting ExpressCard. Drivers for existing PCMCIA interfaces were not produced for newer [[operating system]]s. Ratoc produced [[USB]] and [[FireWire]] to parallel SCSI adaptors, but ceased production when the [[integrated circuit]]s required were discontinued. Since 2013, with the release of various ExpressCard and [[Thunderbolt (interface)|Thunderbolt]]-to-[[PCI Express]] adapters, it is again possible to use SCSI devices on laptops, by installing PCI Express SCSI host adapters using a laptop's ExpressCard or Thunderbolt port.((Citation needed|date=March 2015))

==Notes==
((Notelist))

==References==
((Reflist))

==External links==
* [http://www.t10.org/ T10 Technical Committee – SCSI Storage Interfaces] (SCSI standards)
* [https://web.archive.org/web/20110610024317/http://www.scsita.org/aboutscsi/termTutorial.html Termination Tutorial] (WayBack link)

((Computer-bus))

[[Category:SCSI]]

Pages transcluded onto the current version of this page (help):

Parallel SCSI (edit)
Template:Ambox (view source) (template editor protected)
Template:As of (view source) (template editor protected)
Template:Category handler (view source) (protected)
Template:Citation needed (view source) (protected)
Template:Cite book (view source) (protected)
Template:Cite journal (view source) (protected)
Template:Cite web (view source) (protected)
Template:Computer-bus (edit)
Template:Computer bus (edit)
Template:Convert (view source) (template editor protected)
Template:DMCA (view source) (template editor protected)
Template:Dated maintenance category (view source) (template editor protected)
Template:Dated maintenance category (articles) (view source) (template editor protected)
Template:Delink (view source) (protected)
Template:Efn (view source) (template editor protected)
Template:Efn-ua (view source) (extended confirmed protected)
Template:FULLROOTPAGENAME (view source) (template editor protected)
Template:Find sources mainspace (view source) (template editor protected)
Template:Fix (view source) (protected)
Template:Fix/category (view source) (protected)
Template:Hlist/styles.css (view source) (protected)
Template:Icon (view source) (template editor protected)
Template:Main (view source) (template editor protected)
Template:Main article (view source) (template editor protected)
Template:Main other (view source) (protected)
Template:More citations needed (view source) (template editor protected)
Template:N/A (view source) (template editor protected)
Template:N/a (view source) (template editor protected)
Template:Navbox (view source) (template editor protected)
Template:Notelist (view source) (template editor protected)
Template:Notelist-ua (view source) (template editor protected)
Template:Nowrap (view source) (protected)
Template:Ns has subpages (view source) (protected)
Template:Pagetype (view source) (protected)
Template:Refimprove (view source) (template editor protected)
Template:Reflist (view source) (protected)
Template:Reflist/styles.css (view source) (protected)
Template:SDcat (view source) (protected)
Template:Section link (view source) (template editor protected)
Template:Short description (view source) (protected)
Template:Short description/lowercasecheck (view source) (protected)
Template:Slink (view source) (extended confirmed protected)
Template:Snd (view source) (template editor protected)
Template:Spaced en dash (view source) (template editor protected)
Template:Use American English (view source) (template editor protected)
Template:Vanchor (view source) (template editor protected)
Template:Visible anchor (view source) (template editor protected)
Template:Visible anchor/styles.css (view source) (template editor protected)
Template:Yesno (view source) (protected)
Module:Arguments (view source) (protected)
Module:Category handler (view source) (protected)
Module:Category handler/blacklist (view source) (protected)
Module:Category handler/config (view source) (protected)
Module:Category handler/data (view source) (protected)
Module:Category handler/shared (view source) (protected)
Module:Check for unknown parameters (view source) (protected)
Module:Citation/CS1 (view source) (protected)
Module:Citation/CS1/COinS (view source) (protected)
Module:Citation/CS1/Configuration (view source) (protected)
Module:Citation/CS1/Date validation (view source) (protected)
Module:Citation/CS1/Identifiers (view source) (protected)
Module:Citation/CS1/Utilities (view source) (protected)
Module:Citation/CS1/Whitelist (view source) (protected)
Module:Citation/CS1/styles.css (view source) (protected)
Module:Convert (view source) (template editor protected)
Module:Convert/data (view source) (template editor protected)
Module:Convert/text (view source) (template editor protected)
Module:Delink (view source) (protected)
Module:Disambiguation/templates (view source) (protected)
Module:Find sources (view source) (template editor protected)
Module:Find sources/config (view source) (template editor protected)
Module:Find sources/links (view source) (template editor protected)
Module:Find sources/templates/Find sources mainspace (view source) (template editor protected)
Module:Format link (view source) (template editor protected)
Module:Hatnote (view source) (template editor protected)
Module:Hatnote/styles.css (view source) (template editor protected)
Module:Hatnote list (view source) (template editor protected)
Module:Icon (view source) (template editor protected)
Module:Icon/data (view source) (template editor protected)
Module:Labelled list hatnote (view source) (template editor protected)
Module:Message box (view source) (protected)
Module:Message box/ambox.css (view source) (protected)
Module:Message box/configuration (view source) (protected)
Module:Namespace detect/config (view source) (protected)
Module:Namespace detect/data (view source) (protected)
Module:Navbar (view source) (protected)
Module:Navbar/configuration (view source) (protected)
Module:Navbar/styles.css (view source) (protected)
Module:Navbox (view source) (template editor protected)
Module:Navbox/configuration (view source) (template editor protected)
Module:Navbox/styles.css (view source) (template editor protected)
Module:Ns has subpages (view source) (protected)
Module:Pagetype (view source) (protected)
Module:Pagetype/config (view source) (protected)
Module:Pagetype/disambiguation (view source) (protected)
Module:Pagetype/setindex (view source) (protected)
Module:Pagetype/softredirect (view source) (protected)
Module:SDcat (view source) (protected)
Module:Section link (view source) (template editor protected)
Module:String (view source) (protected)
Module:Unsubst (view source) (protected)
Module:Wikitext Parsing (view source) (protected)
Module:Yesno (view source) (protected)

Return to Parallel SCSI.

Retrieved from "https://en.wikipedia.org/wiki/Parallel_SCSI"