SCSI - Small Computers System Interface
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SCSI3 is the natural evolution of SCSI2, and has been designed to provide
specification for the following elements:
SCSI3 is too big a standard to be published in one unique specification
(as the ANSI did for the SCSI2 specification).
- fast, wide and ultra host adapter cards (see
- new device types not listed in the SCSI2 standard (like CD-Rs, CD
changers, hardware enclosures, etc.)
SCSI3 is, better, a collection of specifications that is continuously
growing. So it is not a "closed" standard like SCSI2, and this way it fixes
some limits of its predecessor.
T10 is the Technical Committee in charge for drafting
specification inside the SCSI3 family of standards. Drafts approved
by the ANSI become the actual SCSI3 standard specifications.
T10's site is a mine of
information, and you can also download all the drafts you are interested
Please note that T10 draft specifications *might* be different from the
final ANSI approved specifications. Sometimes some minor differences
happen to be found.
But the actual difference between T10 drafts and ANSI specifications is
the following: the former are freely downloadable from T10's site, and
the latter must be bought at some fee from ANSI, and cannot be
Home users and hobbysts can use T10 drafts, but professional users should
buy from ANSI. In fact professional users can't afford the risk of
producing out of standard software or devices, so it is correct for them
to buy from ANSI.
SCSI3 defines the following device types in addition to those already
defined in the SCSI2 specs:
More than one specification you can find for each device type, because
generations of devices have been introduced. This eliminate the need
for a future SCSI4 to come.
Some SCSI3 general purpose commands have been enclosed into one big
specification that has been called SCSI3 Primary Commands (
SPC for short) and that has already been released in the 3rd
- CD-R, CD-RW, DVD and DVD-R
- RAID controller
- enclosure (fan, power supply, etc.)
- CD disc changer
Commands for CD-R, CD-RW, DVD and DVD-R have been enclosed into the
Multi Media Commands specification (MMC for short) that
has already been released in the 4th generation set.
If you are willing to write a program for a CD-R you should download
the proper MMC and SPC documents from T10's site. You should also get
the SCSI3 command set reference from the manufacturer of the device.
But in my little experience in the world of SCSI3 I noticed that command
set references from the manufacturers of SCSI3 devices are closely
matching the original SCSI3 specifications from T10, providing for a
much more guaranteed device interchangeability, and relieving the
programmer from the heavy effort to support devices from different
manufacturers. Very helpful.
Sending SCSI3 commands to SCSI3 devices is very similar to sending SCSI2
commands to SCSI2 devices, and you can still use use the ASPI manager invented and provided by
Adaptec for the programmer to use.
Error reporting in SCSI3 is very close to error reporting explained in
Sense key, additional sense code (ASC) and additional sense code
qualifier (ASCQ) are defined in SCSI3 specifications and into the
SCSI command set references provided by the manufacturers of devices.
SCSI3 has been properly planned to be a robust standard, flexible
and expandable without affecting the standard itself.
SCSI3 has been designed to let a soft transition from SCSI2. This is
of great importance for the programmer that can make use of her/his previous
experience in SCSI2 programming.
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