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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0101:  SCSI

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VI  Speeds and Feeds

At the time of this writing, there are seven generations of SCSI. As SCSI has evolved, so have the connectors interfacing the devices. Therefore, to some degree, the issue of SCSI speeds is tied to connectors. In this work, you will find the discussion of the generations of SCSI first, then the connectors that are available in SCSI.

The first official SCSI is now known as SCSI-1. It supported 5MB/second transfers, and used an 8-bit bus. SCSI-2 the next generation was Fast SCSI, and supported 10MB/second transfer. The release of CPU's such as the 80286, made the option for sixteen-bit available. This is known as Wide SCSI. Combining Fast and Wide yields Fast Wide SCSI. Some references optionally will denote not wide SCSI as Narrow SCSI. If wide is not specified, narrow is assumed. The modifier Ultra was used after Fast/Wide. The Ultra2 came into being after Ultra. The latest method for using Ultra is to use the maximum throughput in megabytes as a number after Ultra. An example of this would be Ultra 160, which would yield 160MB/second transfer rates.

Adding to the confusion is the fact that ANSI did not specify all the versions of SCSI. SCSI-1 and SCSI-2 are well defined by ANSI standards. When SCSI-3 came along, ANSI broke up the specification into a series of smaller specifications. For example, the electrical connector, pin assignment, etc. are specified separately, by the document referred to as SCSI Parallel Interface (SPI). Other issues such as protocols were covered in separate documents.

As SCSI continued forward, some vendors chose to call certain specifications, such as Ultra SCSI 320, SCSI-4. Well folks, sorry but ANSI did not ratify Ultra SCSI 320 as SCSI-4. And as of this writing, calling Ultra SCSI 640 SCSI-5 doesn't work either. At least it doesn't according to ANSI. A vendors’ marketing department is of course, a different story.

[spacer]QAS

TCQ was not the Holy Grail for SCSI. With Ultra320 (SPI-4) comes the draft proposal for Quick Arbitrate and Select (QAS), which has appeared in drives and controllers in the second half of 2001



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V  Termination
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VII  Connectors
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