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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 ]

Wide Ultra2 SCSI

Description: Wide Ultra2 SCSI is the marketing term for devices corresponding to the SCSI-3 Parallel Interface - 2 (SPI-2) standard that run on a wide (16-bit) bus. These units support a maximum throughput of 80 MB/s using Fast-40 signaling; they are occasionally (rarely) called Wide Fast-40 SCSI devices. See the SPI-2 standard discussion and the details below for more on this type of SCSI.

Note: Most SCSI implementations that use Ultra2 signaling are in fact wide implementations. Narrow Ultra2 SCSI is supposed to be called just "Ultra2 SCSI" while the wide version is "Wide Ultra2"; however, since narrow never really caught on for Ultra2, some people say "Ultra2 SCSI" when they really mean "Wide Ultra2 SCSI".

Note: Don't confuse Wide Ultra2 SCSI with Wide SCSI-2; the latter is a less common name for Wide SCSI, a much older and slower transfer mode.

Defining Standard: SCSI-3 / SPI-2.

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

Bus Width: Wide (16-bit).

Signaling Method: LVD or HVD. (HVD is officially supported for Wide 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; 80 MB/s.

Number of Devices Supported: 16 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: "P" cable (68 pins). Maximum of 25m for LVD if no more than 2 devices are used, otherwise 12m; 25m for HVD.

Next: Ultra3 SCSI

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