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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 ]

Ultra2 SCSI

Description: Ultra2 SCSI is the marketing term for devices corresponding to the SCSI-3 Parallel Interface - 2 (SPI-2) standard that run on a narrow (8-bit) bus. These units support a maximum throughput of 40 MB/s using Fast-40 signaling; they are occasionally (rarely) called Fast-40 SCSI devices. See the SPI-2 standard discussion and the details below for more on this type of SCSI.

Note: While narrow Ultra2 SCSI has been formally defined, it was never very popular; most devices using Ultra2 SCSI are in fact Wide Ultra2 SCSI devices, because "giving away" half the maximum throughput by using narrow cabling has become quite unpopular. The sparsity of narrow Ultra2 implementations means that many people use just "Ultra2 SCSI" to refer to what are really wide devices. Watch out for this.

Note: Don't confuse Ultra2 SCSI with SCSI-2, which is a totally different (and much older) standard.

Defining Standard: SCSI-3 / SPI-2.

Special Features: LVD signaling; multimode (LVD/SE) optional.

Bus Width: Narrow (8-bit).

Signaling Method: LVD or HVD. (HVD is officially supported for Ultra2 SCSI, though it is not generally used; LVD offers significant advantages over HVD and has become the standard for modern high-speed SCSI buses.) Note that multimode drives may optionally run in SE mode, but throughput will drop to Fast-20 (Ultra) levels if this is done.

Signaling Speed and Bus Throughput: 40 MHz bus speed; 40 MB/s.

Number of Devices Supported: 8 for HVD or LVD cables up to 12m in length; 2 for LVD cables over 12m.

Termination: For LVD, LVD termination; for HVD, HVD termination.

Cabling and Maximum Cable Length: "A" cable (50 pins). Maximum of 25m for LVD if no more than 2 devices are used, otherwise 12m; 25m for HVD.

Next: Wide Ultra2 SCSI

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