HP Ultrium tape drives Q1538-90925 - Manual (Page 31)

HP Ultrium drives technical reference manual, volume 5: UNIX configuration guide 31
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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.
sequential access Sequential access devices store data sequentially in the order in which it is 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 speedy retrieval, but is significantly more costly.