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{{Short description|Original SCSI storage interface}}
{{Refimprove|date=January 2007}}
{{Use American English|date=September 2020}}
[[File:Centronics 50 SCSI connector.JPG|240px|thumb
[[File:Scsi intern hd68 lvd term.jpg|thumb|upright|68-pin [[twisted pair|twisted]] [[ribbon cable]] used for [[Low-voltage differential signaling|LVD]] SCSI connections]]
'''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.
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.
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==History==
The first two formal SCSI standards, SCSI-1 and SCSI-2, described parallel SCSI.
At 10 MHz with a bus width of 16 bits it is possible to achieve a data rate of 20 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==
[[File:SCSI symbols.svg|thumb|upright=0.9|Diagrams of different Parallel SCSI symbols<ref>{{cite book |last=Jernigan |first=Elton |date=2002 |url=https://books.google.com/books?id=GkEBejuiTt4C&pg=PA171 |title=Server+ Certification: Training Guide |publisher=Que |page=171 |isbn=9780735710870 |via=Google Books}}</ref>|alt=single-ended, low-voltage differential, low-voltage differential/single-ended multi-mode, high-voltage differential]]
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 signaling]] (LVD), 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
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! rowspan=3 | Connector
! rowspan=3 | Width (bits)
! rowspan=3 |
! 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 ([[
! rowspan=2 | Voltage ([[
|-
! {{nowrap|([[MB/s]])}}
! {{nowrap|([[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 m when only a single device is attached to the host adapter, 20 m for Ultra-640.}}
! [[High-voltage differential signaling|HVD]]
|-
| SCSI-1
| Narrow SCSI
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| [[Insulation-displacement connector|IDC]]50; Amphenol C50
| 8
| 5
| 5
| 40
| 6
| {{N/A|N/A}}
| 25
| 8
| {{nowrap|
| SE 5
|-
| Fast SCSI
|
| SCSI-2 (1994)
| IDC50; Amphenol C50
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| 80
| 3
| {{N/A|N/A}}
| 25
| 8
| {{nowrap| SE 90 ± 6<ref name="random"/> }}
| SE 5 HVD ≥5
|-
| Fast-Wide SCSI
| Wide SCSI
| SCSI-2; <br/>
| 68-pin
| 16
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| 160
| 3
| {{N/A|N/A}}
| 25
| 16
| {{nowrap| SE 90 ± 6<ref name="random"/> }}
| SE 5 HVD ≥5
|-
| rowspan=2 | Ultra SCSI
| rowspan=2 | Fast-20
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| rowspan=2 | 160
| 1.5
| {{N/A|N/A}}
| 25
| 8
| rowspan=2 | {{nowrap| SE 90 ± 6<ref name="random"/> }}
| rowspan=2 | SE 5 HVD ≥5
|-
| 3
| {{N/A|N/A}}
| {{N/A|N/A}}
| 4
|-
| rowspan=3 | Ultra Wide SCSI
| rowspan=3 |
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| 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 5 HVD ≥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
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| 40
| 320
| {{N/A|N/A}}
| 12
| 25
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| {{nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> }}
| LVD 1.2 HVD ≥5
|-
| Ultra2 Wide SCSI
|
| SPI-5 (INCITS 367-2003)
| {{nowrap|68-pin;}} {{nowrap|80-pin}} {{nowrap|([[Single Connector Attachment|SCA]]/SCA-2)}}
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| 80
| 640
| {{N/A|N/A}}
| 12
| 25
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| {{nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> }}
| LVD 1.2 HVD ≥5
|-
| Ultra3 SCSI
| Ultra-160; Fast-80 wide
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| {{nowrap|68-pin;}} {{nowrap|80-pin}} {{nowrap|(SCA/SCA-2)}}
| 16
| 40 [[
| 160
| 1280
| {{N/A|N/A}}
| 12
| {{N/A|N/A}}
| 16
| {{nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> }}
| LVD 1.2
|-
| Ultra-320 SCSI
| Ultra-4; Fast-160
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| 320
| 2560
| {{N/A|N/A}}
| 12
| {{N/A|N/A}}
| 16
| {{nowrap| [[Low-voltage differential signaling|LVD]] 125 ± 10<ref name="random"/> }}
| LVD 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
| Ultra-5; Fast-320
| SPI-5 (INCITS 367-2003)
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| 640
| 5120
| {{N/A|N/A}}
| 10
| {{N/A|N/A}}
| 16
| {{nowrap| LVD 125 ± 10}}
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===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 MB/s, or 5 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}}.{{cn|date=May 2024}}
===SCSI-2===
[[File:53CF94 SCSI-2 ASIC.gif|thumb|[[AMD]] 53CF94 SCSI-2 controller in
'''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 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 was largely ignored because it was deemed expensive and unnecessary, and was officially retired in SCSI-3.
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===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>{{
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 [[SCSI terminator|terminator]]s were often to blame for instability problems).
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===Ultra-2===
This standard was introduced c. 1997 and featured a
===Ultra-3===
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#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>
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{|class="wikitable"
|-
! Signal name
! Meaning when asserted ! Meaning when deasserted |-
|BSY Busy|| Bus in use|| Bus free
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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).
{|class="wikitable"
|-
! Bus
|-
|8-bit||SE||8||1||9||1||1||30||0||50
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==SCSI IDs==
[[File:Iomega Jaz V2000S-9377.jpg|thumb|SCSI ID selector]]
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"
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==Bus operation==
The parallel SCSI bus goes through eight possible phases as a [[SCSI command|command]] is processed.
{|class="wikitable"
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|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.
|-
|Selection||The arbitrating device with the highest ID takes control of the bus by asserting /BSY and /SEL.
|-
|Command||The initiator sends the
|-
|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]]).
|-
|Data||In this phase, data is transferred between initiator and target, the direction of transfer depending on the command that was issued.
|-
|Message||A [[SCSI Message Code|message code]] is exchanged between initiator and target for the purposes of interface management.
|-
|Status||A
|}
The above list does not imply a specific sequence of events.
==External connectors==
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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.
==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.
A positive DC termination voltage is provided by one or more devices on the bus, typically the
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 a better impedance match than passive termination because most flat [[ribbon cable]]s have a [[characteristic impedance]] of approximately 110 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 and so require external, active termination. The usual termination circuit consists of 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{{snd}}the same effect any other SE device has. Passive terminators may cause Ultra speed communication to be unreliable.
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 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
===Mixing single-ended and low-voltage differential===
[[Single-ended signalling|Single-ended]] and LVD 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
===SCA adapters===
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===Device IDs and termination===
Each parallel SCSI device (including the computer's [[host adapter]]) must be configured to have a unique SCSI
===SCAM===
''SCSI Configured Automatically'' (initially ''Automagically'') was an optional method to configure the SCSI ID without requiring user intervention
===Laptops===
[[File:Sony-pcmcia-scsi-ii hg.jpg|thumb|upright|Sony SCSI-2 PCMCIA card from [[Adaptec]]]]
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==
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==External links==
* [http://www.t10.org/ T10 Technical Committee
* [https://web.archive.org/web/20110610024317/http://www.scsita.org/aboutscsi/termTutorial.html Termination Tutorial] (WayBack link)
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