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
