Parallel SCSI: Difference between revisions

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==Standards==

[[File:SCSI symbols.svg|thumb|upright=0.9|Diagrams of different Parallel سجلات البروتوكول (IANA)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.

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{{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}}.

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! rowspan=3 | Connector

! rowspan=3 | Width (bits)

! rowspan=3 | Clock ([[hertz|MHz]])

! colspan=6 | Maximum

! colspan=2 | Electrical

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! rowspan=2 | Voltage ([[volt|V]])

|-

! {{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&nbsp;m when only a single device is attached to the host adapter, 20&nbsp;m for Ultra-640.}}

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| 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

|-

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|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.

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|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.

|}

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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==

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===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 LVD bus with no compromise in performance.

===Mixing single-ended and low-voltage differential===