HP Ultrium Drive, Техническое справочное руководство, Страница: 37 / 40

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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 10 MB/sec, using a cable of up to 6 meters
total length.
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 80 MB/sec, using a cable of up to 12
meters.
Ultra160
SCSI can transfer data at up to 160 MB/sec, using a cable of up to 12
meters.
Ultra320 SCSI can transfer data at up to 320 MB/sec, using a cable of up to 12
meters.
Ultra SCSI supports both SE and LVD interfaces. Although Ultra2 SCSI and above
can support SE devices, this is not recommended as the whole bus is slowed to
Ultra speeds; instead, use LVD devices only.
HP’s Ultrium drives are Ultra-320 compatible devices. They should be used only on
LVD host bus adapters for maximum performance.