TARGA 3000 Configuration Cookbook
11/25/2002
54
7.3 SCSI Bus types and potential bandwidth:
SCSI UltraWide transfers up to 40 MBps (from controller cache to drive cache), with a 16-bit wide data path.
Also referred to as "Fast Wide SCSI-2". Not recommended for data rates above DV or MPEG, as real-world
memory-to-disk transfer rates average 12 MBps for each drive.
SCSI Ultra2 transfers up to 80 MBps (from controller cache to drive cache), with a 16-bit wide data path.
Also referred to as "Fast Wide SCSI-3", "LVD or Low-Voltage Differential". Often available in dual channel
models, possibly increasing throughput capacity (pending tests will report the effectiveness of multi-channel
configurations). Recommended for most data rates, with perhaps the exception of RGB. Real-world
memory-to-disk transfer rates can average 18 MBps for each drive.
SCSI Ultra3 transfers up to 160 MBps (from controller cache to drive cache), with a 16-bit wide data path.
Also referred to as "160m", "SCSI-3 LVD or Low-Voltage Differential". These are often available in 64-bit PCI
bus slot configurations, which allows over 264 MBps transfer rates over the 64-bit portion of the PCI bus.
Like the Ultra2 controllers, these are available in dual channel models, increasing throughput capacity. The
maximum data rate over a 32-bit PCI bus is 132 MBps (this is a peak, or burst rate. Sustained rates are
lower, partly because of the traffic from other devices on the bus). Recommended for most data rates,
including the RGB codec. Real-world memory-to-disk transfer rate gains increase less dramatically over
Ultra2 LVD, and can average 25 MBps for each drive.
7.4 SCSI Cables and Terminators
Resist the temptation to use the longer SCSI cables. The best results can be achieved by using the shorter,
3-foot or shorter lengths. We always use high quality internal and external SCSI cables from trusted
vendors. This is especially true when using LVD (Ultra2 or Ultra3 SCSI), as most do are not marked with any
indication of compatibility. This is important, as LVD-compatible cables use twisted-pair wires for each pin
connection, allowing differential-voltage interference canceling. Unfortunately, they look nearly identical to
UltraWide SCSI cables, which do not use twisted-pairs. At best, the result will be inconsistent
communication over the cable.
The internal ribbon cables are easier to work with - the twisted wires are visible, usually terminate with an
active LVD terminator (LVD drives do not supply their own termination). The Ultra2 LVD ribbon cables are
usually blue and white, while Ultra3 LVD are white and pinkish-salmon. Adaptec notes that signal reflection
can occur in lengths of less than 10" between drive connectors in the Ultra2 or Ultra3 LVD ribbon cables,
degrading performance.
Sources of high quality SCSI cables and terminators:
Granite Digital
510-471-6442
www.scsipro.com
CS Electronics
714-475-9100
www.scsi-cables.com
7.5 Certified Disk Arrays
Fully integrated certified disk arrays are recommended since they provide the best guarantee of service and
performance on what can often be the weakest link in the system. A poor drive array or cables can
compromise the performance of the whole system.
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