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SCSI Data Transfer Modes and Feature Sets

The technologies used in SCSI are defined by the formal standards created and maintained by industry groups and standards associations. However, SCSI hardware is usually not sold labeled with the name of the standard whose features it implements. This is probably a good thing; considering the number of different protocols and transfer modes that are covered by both the SCSI-2 and SCSI-3 names, being told a drive is a "SCSI-2 drive" or "SCSI-3 device" doesn't really tell you much of anything at all!

Instead, SCSI devices are usually sold using specific names that define particular "flavors" of SCSI. Each of these is a particular intersection of various important SCSI characteristics, such as bus speed, bus width and signaling type. In most cases new feature sets are created when new transfer modes are created by the adoption of new standards; they are then given cute names by various hardware makers or manufacturers' associations and used to promote the new products. The various feature set names can be hard to understand since some of them are similar to each other despite different-sounding names.

In this section I provide a description of each of the common feature sets used for regular parallel SCSI. For each one I provide the following information in a standardized format:

• Description: A brief description of the particular SCSI "flavor".
• Defining Standard: The specific SCSI standard that introduced the technology used in this type of SCSI.
• Special Features: Specific features or requirements of a hardware device of this particular SCSI type.
• Bus Width: The width of the bus for this SCSI type, either narrow (8-bit) or wide (16-bit).
• Signaling Method: What combination of single-ended (SE), differential (HVD) and low voltage differential (LVD) signaling is used by this type of SCSI. Note that HVD has been withdrawn from the SCSI standard, but some HVD hardware is still "out there" and will of course still work with HVD host adapters.
• Signaling Speed and Bus Throughput: The speed that this SCSI bus runs at, and the data throughput in MB/s.
• Number of Devices Supported: The number of devices that can be put on a single SCSI chain. This is the total number of devices; remember that the host adapter counts as one device.
• Termination: The type of SCSI bus termination required or normally used.
• Cabling and Maximum Cable Length: The type of cables supported, and the maximum length of the SCSI chain supported, in meters. Some types of SCSI support different cable lengths depending on the signaling method. Oh, for you folks still stuck in the land of Imperial measure, 1 meter is about 3.28 feet. :^)

Next: "Regular" SCSI (SCSI-1)

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