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
