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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 | SCSI-3 ]

SCSI(-3) Parallel Interface - 3 (SPI-3)

The third generation of the SCSI parallel interface is, unsurprisingly, called the SCSI(-3) Parallel Interface - 3 or SPI-3. This document builds upon the physical and protocol definitions of the SPI-2 document. It is in the process of approval at the time of this writing, so it has no ANSI standard number yet, but should be published by early 2001 (it's T10 project 1302-D).

Five main features were added to parallel SCSI in the SPI-3 standard:

  • Fast-80(DT) Data Transfer: Reflecting the continuing appetite for speed on the SCSI bus, data transfer rates were again doubled, this time to 160 MB/s on a wide bus. This was accomplished not by increasing the speed of the bus from 40 MHz to 80 MHz, but rather through the use of double transition clocking; thus the "DT" sometimes found in the name for this signaling speed. See here for more.
  • Cyclic Redundancy Check (CRC): This is a common error checking protocol used to ensure data integrity. It was added as a safety measure since transfer speeds were being increased, leading to the possibility of data corruption.
  • Domain Validation: This feature improves the robustness of the process by which different SCSI devices determine an optimal data transfer rate; read more about it here.
  • Quick Arbitration and Selection (QAS): This feature represents a change in the way devices determine which has control of the SCSI bus, providing a small improvement in performance.
  • Packetization: Another small change to improve performance, packetization reduces the overhead associated with each data transfer; it is described here.

Other, smaller changes were also made. SPI-3 also does some "cleanup" of the parallel SCSI standard, by making obsolete several older features that either never caught on in the industry, or were replaced with superior ways of accomplishing the same tasks:

  • High Voltage Differential: With the widespread adoption of low voltage differential, the older "high voltage" differential became unnecessary. Since it was never very popular, it was removed from the standard.
  • 32-Bit Bus Width: Introduced in SCSI-2, the 32-bit parallel SCSI option never caught on in the industry and was finally removed from the specification in SPI-3.
  • SCAM: SPI-3 removed the "SCSI Configured AutoMatically" (SCAM) feature, which was a good idea but never was universally adopted and sometimes led to configuration problems. In doing so, the SCSI world was mercifully rid of one of the worst acronyms in the history of the computer industry. :^)
  • Narrow High-Speed Transfers: Narrow (8-bit) SCSI hasn't been technically "made obsolete", but 8-bit transfers are not defined for Fast-80 transfers. (Considering that faster transfer modes are used to get more throughput, increasing data transfer speeds while staying on an 8-bit bus never really made much sense.)

Unfortunately, despite the lessons that should have been learned in the past regarding what happens when standards aren't kept universal, the SCSI industry managed to create another mess out of the SPI-3 standard. The SCSI Trade Association defined the marketing term "Ultra3 SCSI" to correspond to the features introduced in SPI-3. However, they allowed a device that implemented any sub-set of the five main new features to be called "Ultra3 SCSI"; this "optionality" meant that there was no guarantee that any two devices labeled "Ultra3 SCSI" had the same features! Hardware manufacturers didn't like this, so they decided to market alternative names for more concrete subsets of the Ultra3 features, and we were off to the competing standards races yet again. The results were Ultra160 and Ultra160/m SCSI, and Ultra160+ SCSI.

(Someday, all these companies will get their act together and these things won't happen any more--and I'll probably die of shock. ;^) )

Next: SCSI(-3) Parallel Interface - 4 (SPI-4)

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