RAID depends on several technologies to make it work, including SCSI
controllers and wiring schemes that RAID and SCSI controllers use. Here are
definitions of these technologies. For more information about SCSI technology,
see Sean Daily, "SCSI and IDE," June 1997.
SCSI Standards
SCSI-1
The original SCSI standard, offering synchronous (5MBps) and
asynchronous (3MBps) modes. Supports single-ended configurations, with passive
and active termination.
SCSI-2
The second standard, and the most common today. You can mix
SCSI-1 and -2 devices on one SCSI-2 bus because it's backward-compatible to
SCSI-1. SCSI-2 offers much higher burst and sustained transfer rates (although
the standard is only 5MBps), with a clock speed of 20MHz.
Fast SCSI-2
Runs at 10MBps.
Wide SCSI-2
Runs at 10MBps but extends the data path to 16 or 32
bits.
Fast and Wide SCSI-2
Runs at 20MBps over a 16-bit data path.
SCSI-3
Also known as Ultra SCSI (or Fast 20). It is fully
backward-compatible to SCSI-2 and uses 4-bit (deep) addressing. This new
standard extends performance to 40MBps.
Fast SCSI-3
Runs at 20MBps; cable length limits are the same as
SCSI-2, unless you have differential wiring.
Wide SCSI-3
Runs at 40MBps. SCSI-3 is by nature "fast";
doublewide uses 32 bits instead of 16.
SCSI-4
Also known as Ultra 2. It is a 64-bit standard designed for
64-bit PCI and runs at 80MBps with a clock speed of 40MHz.
Deep SCSI
Uses 4 bits instead of 3 to identify devices on the bus,
allowing for up to 15 total devices on a channel.
Single-ended wiring
Cable wiring scheme that is more prone to noise
because all signal lines use a common ground. It has a 6-meter cable
length limit.
Differential wiring
Wiring scheme that uses signal/ground wire pairs,
which reduces noise and allows for up to 25-meter cables. The two
versions of differential are low-voltage (LVDS) and high-voltage (HVDS). The one
you choose depends on the cable length you want, data speeds, and other factorsHVDS
is for SCSI-4. You cannot mix single-ended and differential devices on the same
bus.
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