ULTRA-SCSI

8/16/97

Ultra SCSI -- Ultra SCSI (SCSI-3) is an improved Version of the Small Computer System Interface intended to offer significantly greater bandwidth than the 10MB/Sec supported by SCSI-2. Ultra-SCSI is an extension to the SCSI-2 standard defined by the ANSI XT310 -Fast-20 Committee. Ultra-SCSI supports 8, 16 and 32 bit data paths vs SCSI-2's 8 and 16. It uses active termination; an expanded command set; and lengthy (256 entry) command queues. It is capable of 20MB/Sec. Both SCSI-2 and SCSI-3 come in a fast-wide flavor capable of 20MB/Sec and 40MB/Sec bandwidths respectively. 80MB/Sec is anticipated in 1998-1999.

"Fibre channel" a 100 MB/Sec plus serial SCSI is being designed in parallel with Ultra-SCSI but is not part of the SCSI-3 effort per se.

SCSI-3 doubles the data transfer rate. The changes primarily affect the I/O subsystem, not disk drive internals and mechanics. One limitation of Ultra-SCSI is that cable length is limited to 1.5 meters -- a bit less than 5 feet. Where longer cables must be used, there is an option to use differential signals which supports 25 meter cables at substantially higher cost. The SCSI-3 standard also includes provisions for Bus Extenders which allow a 6 meter extension to the cable.