HP Q153090901 User Manual
Page 189
Glossary
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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 SCSI and above
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.
Ultra4 or Ultra320 transfer data at up to 320 MB/s. Cable lengths are as for
Ultra2.