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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 ]

Ultra SCSI

Description: Ultra SCSI is the term usually applied to devices that use 20 MHz SCSI bus speed on a narrow, 8-bit bus. This is sometimes called Fast-20 SCSI in reference to the Fast-20 signaling specification that defines Ultra SCSI speed. Note that "Ultra SCSI" is sometimes used vaguely; for example, some people call both Wide Ultra SCSI and (non-wide) Ultra SCSI just "Ultra SCSI" collectively.

Defining Standard: SCSI-3 / SPI (including Fast-20 addendum).

Special Features: None.

Bus Width: Narrow (8-bit).

Signaling Method: SE or HVD.

Signaling Speed and Bus Throughput: 20 MHz bus speed; 20 MB/s.

Number of Devices Supported: 8 if HVD signaling is used or SE signaling is used with a maximum cable length of 1.5m; 4 if SE signaling is used with a cable length of over 1.5m.

Termination: For SE, either active or forced perfect termination. For HVD, HVD termination.

Cabling and Maximum Cable Length: "A" cable (50 pins). Maximum of 3m for SE if no more than 4 devices are used, otherwise 1.5m; 25m for HVD.

Next: Wide Ultra SCSI

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