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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 - 2 (SPI-2)

The second generation of the SCSI-3 parallel interface standard is called the SCSI(-3) Parallel Interface - 2, or SPI-2. (The "-3" was dropped from "SCSI-3" to uh... reduce confusion. Sure. :^) ) This ANSI standard, document X3.302-1999, replaced the older SPI standard, and also incorporated the SCSI-3 Interlocked Protocol (SIP) document. Thus, SPI-2 included everything from the earlier SCSI-2, SPI, SIP and Fast-20 documents--as well as adding some new features of course. SPI-2 formally replaced the earlier collection of SPI documents in 1999, and in doing so simplified matters significantly, since at least now everything associated with parallel SCSI was back in one document.

Several important new technologies and features were defined as part of SPI-2; the most important changes are the following:

  • Fast-40 Data Transfer: SPI-2 defines another doubling of the maximum speed of the SCSI bus, from 20 MHz to 40 MHz, allowing maximum throughput of 40 MB/s on a narrow (8-bit) channel or 80 MB/s on a wide (16-bit) channel. The document also defines several restrictions associated with these faster signaling speeds, such as the use of differential signaling.
  • Low Voltage Differential Signaling: A new type of signaling for the SCSI bus, called low voltage differential or LVD signaling, was specified as part of SPI-2. LVD is an attempt to blend the best attributes of conventional single-ended (SE) signaling and the older type of differential signaling that is now called high voltage differential (HVD). LVD (or the older HVD) is required to run the SCSI bus at Fast-40 speeds; you can read more about it here.
  • Multimode Operation: Specification is provided for a way to create devices that will automatically work on both LVD and regular single-ended buses; such units are called multimode devices. They are also discussed in the section on LVD.
  • SCA-2 Single Connector Attachment Connectors: An improvement to the original SCA connectors, called SCA-2, was defined.
  • Very High Density Connectors: SPI-2 defined a smaller version of the older high-density 68-pin connectors. This new standard is called Very High Density Cable Interconnect, abbreviated VHDCI. Read more about them here.

Note: The features defined as part of SPI-2 are sometimes referred to by the informal (marketing) terms Ultra2 SCSI.and Wide Ultra2 SCSI.

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

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