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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) ]

SCSI Standards

There was a time that SCSI standards were relatively few, and not that difficult to understand. That time is now long past. :^) In some ways, the best way I could describe the current situation regarding SCSI standards, feature sets and marketing terms is that it makes the standards and terms associated with IDE/ATA seem simple by comparison. That would really be a rather strong indictment, however, so I won't say that. ;^) Still, understanding all of the documents and labels associated with SCSI can be very baffling at times.

It's not that the standards are poorly written, or that the technology is all that hard to understand. The main issue with SCSI today is that it has become so broad, and includes so many different protocols and methods, that it's hard to get a handle on all of it. The confusion surrounding SCSI standards has increased since the creation of SCSI-3, which is really a collection of different standards, some of them rather different from each other. The situation is made worse by manufacturers that like to create funky new "unofficial names" for transfer modes or feature sets, or apply overly-broad labels to specific hardware.

As described in the page describing the history of SCSI, the first organization that was charged with developing the first SCSI standard was ANSI technical committee X3T9.2. Today, SCSI standards are developed, maintained and approved by a number of related organizations, each playing a particular role. Here's how they all fit together:

  • American National Standards Institute: ANSI is usually thought of as an organization that develops and maintains standards, but in fact they do neither. They are an oversight and accrediting organization that facilitates and manages the standards development process. As such, they are the "high level management" of the standards world. They qualify other organizations as Standards Developing Organizations or SDOs. ANSI also publishes standards once they have been developed and approved.
  • Information Technology Industry Council: ITIC is a group of several dozen companies in the information technology (computer) industry. ITIC is the SDO approved by ANSI to develop and process standards related to many computer-related topics.
  • National Committee for Information Technology: NCITS is a committee established by ITIC to develop and maintain standards related to the information technology world. NCITS was formerly known under the name "Accredited Standards Committee X3, Information Technology", or more commonly, just "X3". It maintains several sub-committees that develop and maintain standards for various technical subjects.
  • T10 Technical Committee: T10 is the actual technical standards committee responsible for the SCSI interface.

Note: If this description looks similar to the one on the page where I defined the structure of the organizations supporting the T13 technical committee that develops ATA standards, that's because it is. T10 and T13 are sibling committees.

If you boil all of this down, T10 is the group that actually does the work of developing new SCSI standards. ;^) The other organizations support their activities. The T10 group is comprised primarily of technical people from various hard disk and other technology companies, but the group (and the development process itself) is open to all interested parties. Comments and opinions on standards under development are welcomed from anyone, not just T10 members. The standards development process is intended to create consensus, to ensure that everyone who will be developing hardware and software agrees on how to implement new technology.

Once the T10 group is done with a particular version of a standard, they submit it to NCITS and ANSI for approval. This approval process can take some time; which is why the official standards are usually published several years after the technology they describe is actually implemented. While approval of the standard is underway, companies develop products using technology described in the standard, confident that agreement has already been reached. Meanwhile, the T10 group starts work on the next version of the standard. With SCSI-3 now including a number of different "sub-standards" (hmm, bad name :^) ), it is in some ways constantly "under development".

There are also other organizations that are involved in the creation and maintenance of SCSI-related standards. Since SCSI-3 has a broad scope, it defines and structures certain standards that are in fact "owned" by other groups. In particular, the documents describing the physical layer for Fibre Channel are developed by the T11 technical committee, and the IEEE-1394 interface is of course an IEEE standard.

In this section I describe the three main standards that define SCSI. They are listed in chronological order, and SCSI-3 is expanded into its own full section, reflecting its new status as an "umbrella" standard containing several others.

Note: Standards that have been approved and published by ANSI are available for purchase in either print form or electronic format from ANSI's web site. Draft standards that are under development (as well as older drafts of approved standards) can be found at the T10 Technical Committee web site.

Tip: If there's a SCSI term or "standard" that you are looking for information on but can't find in this section, it might in fact be a transfer mode or feature set.

Warning: You may occasionally see a hardware device being sold based on the name of a standard; for example, a "SCSI-3 drive". Be aware that this is a meaningless label, because it is very vague. With the possible exception of SCSI-1, the standards define several different transfer speeds and signaling methods, so just giving the name of a standard is insufficient information to properly describe a SCSI device. With SCSI-3 especially, the label could mean just about anything--always ask for specifics.

Next: SCSI-1

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