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146 Glossary
as a fabric. The SAN fabric enables any-server-to-any-storage device
connectivity through the use of Fibre Channel switching technology.
SCSI Small Computer System Interface—a standard command specification and
command set that enables computers and peripherals to communicate with
each other. HP’s Ultrium drives adhere to the SCSI specifications (see
Chapter 1, “Interface Implementation” in Volume 3, the SCSI Interface, of the
HP Ultrium Technical Reference Manual) and support all features required by
those standard.
Single-Ended and Low Voltage Differential SCSI
These terms define how the signals are transmitted along the cable.
With single-ended (SE) SCSI, each signal travels over a single wire and each
signal’s value is determined by comparing the signal to a paired ground wire.
Signal quality tends to decrease over longer cable lengths or at increased
signal speed.
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 20 MB/s wide, using a cable of up to
6 meters total length.
Ultra SCSI can transfer data at up to 40 MB/s wide, but the cable length
cannot exceed 3 meters (it is also known as “Fast20”).
Ultra2 SCSI can transfer data at up to 80 MB/s wide, 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”).
Ultra3 or Ultra160 can transfer data at up to 160 MB/s wide. Cable lengths
are as for Ultra2.