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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 Data Transfer Modes and Feature Sets ]
Description: Ultra3 SCSI is the marketing term created by the SCSI Trade Association to refer to devices that implement some or all of the key features defined in the SCSI-3 Parallel Interface - 3 (SPI-3) standard. The units can support a maximum throughput of up to 160 MB/s using Fast-80 signaling with double transition clocking. See the SPI-3 standard discussion and the details below for more on this type of SCSI. Note that Ultra3 SCSI is wide (16-bit) only; 8-bit support was dropped for this version of the interface. This means the name is just "Ultra3 SCSI"; the "Wide" is implied and not explicitly mentioned.
Unfortunately, the decision by the SCSI TA to define the five main features of SPI-3 as optional for Ultra3 SCSI led to a revolt of sort on the part of hardware manufacturers. They made the (very valid) point that this definition of Ultra3 SCSI allows for devices with very different features to all be called "Ultra3 SCSI"--even devices that don't support Fast-80 signaling, the key feature of SPI-3! The hardware makers wanted a more concrete standard, and this led to the creation of the rather similar Ultra160(/m) SCSI and Ultra160+ SCSI feature sets.
Note: Don't confuse
Ultra3 SCSI with SCSI-3, which is a generic term that can
refer to any device corresponding to any of the myriad SCSI-3 standards (and is therefore
Special Features: Ultra3 SCSI devices include support for at least one of the following five features:
Bus Width: Wide (16-bit). Narrow mode is not supported.
Signaling Method: LVD only, if Fast-80 is being used. (Multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels.)
Signaling Speed and Bus Throughput: Depends on implementation; assuming Fast-80, 40 MHz bus speed; 160 MB/s.
Number of Devices Supported: 16 for cables up to 12m in length; 2 for cables over 12m.
Termination: LVD termination.
Cabling and Maximum Cable Length: "P" cable (68 pins). Maximum of 25m if no more than 2 devices are used, otherwise 12m.