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
