HP C7401A - SureStore Ultrium 230 Tape Drive Technical Reference Manual page 37

With low voltage differential (LVD) signaling, signals travel along two wires
and the difference in voltage between the wire pairs determines the signal
value. This enables faster data rates and longer cabling with less susceptibility
to noise than SE signaling and reduced power consumption.
Narrow and Wide, Fast, Ultra and Ultra2 SCSI
Narrow SCSI devices can transfer data one byte at-a-time (and are
sometimes called "8-bit SCSI" devices). They can conform to either the SCSI-2
or SCSI-3 protocols. They have a 50-pin connection to the SCSI bus.
Wide SCSI devices can transfer two bytes of data simultaneously ("16-bit
SCSI"). They usually have a single, 68-pin connection to the SCSI bus. (This
physical arrangement is part of the SCSI-3 specification.) They may support
either SCSI-2 or SCSI-3 protocols. Wide and narrow devices can
simultaneously be connected to the same bus without problem, provided
certain rules are followed.
Fast SCSI can transfer data at up to 10 MB/sec, using a cable of up to 6
meters total length.>/dd>
Ultra SCSI can transfer data at up to 20 MB/sec, but the cable length cannot
exceed 3 meters (it is also known as "Fast20").
Ultra2 SCSI can transfer data at up to 40 MB/sec, using a cable of up to 25
meters total length for a single device, or up to 12 meters for two or more
devices (it is also known as "Fast40").
Ultra SCSI supports both SE and LVD interfaces. Ultra2 SCSI supports LVD
interfaces only. In normal situations, slower devices can coexist with faster
devices, and narrow devices can be used on the same SCSI bus as wide
devices using a suitable adapter.
HP's Ultrium drives are Ultra2, wide SCSI-3 compatible devices. They can be
used with both LVD and SE host bus adapters.
Sequential access devices store data sequentially in the order in which it is
sequential access
received. Tape devices are the most common sequential access devices.
Devices such as disk drives are direct access devices, where data is stored in
blocks, not necessarily sequentially. Direct access allows for speed of
retrieval, but is significantly more costly.
Glossary 37