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Glossary
Index of all terms
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Terminating the SCSI Bus |
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Terminating
the SCSI bus
What is Termination?
Cables make up the
physical connections of the SCSI bus. Since the SCSI bus is a chain of
devices on a cable rather than a loop, the two ends of the bus must be
terminated. Every wire in the cable has a specific impedance, or
resistance to the passing of electrical signals. When signals reach the
end of the cable that makes up the SCSI bus, they encounter the air,
which has very high impedance and acts as a wall of infinite resistance.
The problem with high impedance is that any signal coming down the bus
is reflected back in the other direction once it hits this barrier.
Terminating both ends of the cable prevents the signal from being
reflected.
You terminate the bus
by attaching a circuit, the terminator, to the physical ends of the SCSI
bus. The terminator provides an impedance that matches the cable's,
thereby preventing the signal from bouncing back. The terminators use
power, and the power to operate them comes from the SCSI interface
card through the termination power wire on the bus.
What is Passive
Termination?
Passive Termination
is the oldest method of termination, defined in the specs for SCSI-1.
Basically, a passive terminator sits on the bus to minimize reflections
at the end of the cable. The terminator doesn't really do any work to
regulate power for termination; it relies on the interface card to
provide steady power. A passive terminator simply provides impedance
that's close to the impedance of the cable.
What is Active
Termination?
Active termination
works to control the impedance at the end of the SCSI bus by using a
voltage regulator, not just the power supplied by the interface card.
Because it is active, regulating the power that it gets from the
interface card, active termination is more stable than passive
termination.
What is Forced
Perfect Termination (FPT)?
Forced perfect
termination is the most complex of the terminators, going beyond merely
stabilizing the power applied to the terminator. It can actually alter
its impedance to compensate for variations in impedance among many
different cables, devices, and terminators. It is usually used in
high-speed SCSI systems that have many different devices, cables, and
terminator types. The complexity of such a system can cause impedance
mismatches that degrade the signals sent through the bus. FPT actively
compensates for these impedance variations by means of diode switching
and biasing to force the impedance of the cable to match each device.
How do I terminate
my Seagate SCSI disc drive?
If you are installing
a Seagate drive in a system that has other SCSI devices installed,
terminate only the end devices on the SCSI chain. A SCSI
"device" is any disc drive, scanner, tape backup unit, or
other piece of hardware connected to your system using the SCSI bus.
The example above
shows an internal hard disc at one end of the SCSI bus with the SCSI controller
at the other end (both are terminated). The bottom example shows two
additional SCSI devices connected externally-this means the SCSI
controller is no longer on the end of the SCSI chain and should not be
terminated.
Note: Some
controllers prefer to remain terminated even if they are in the middle
of the chain. Also, some controllers treat the internal and external
chains as separate logical buses. This means you may need to terminate
both the first and last devices on both logical buses to achieve proper
termination. If necessary, refer to your system or controller
documentation to see how this is handled in your particular system. For
information about how to terminate your specific drive, refer to our drive encyclopedia.
Last
edited 23-Feb-2001 |
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