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Termination
Basics
Why terminate?
A basic principle of SCSI
says that a SCSI bus requires correct electrical termination
at both ends in order to function properly. Unfortunately, termination
is implemented differently from SCSI device to SCSI device (some
use jumpers, some use removable resistor packs, others use combinations
of the two), and this can cause some headaches when configuring
multiple devices on a SCSI bus.
Where do you terminate the bus?
Termination must be present at two and only two positions on
the SCSI bus, at the beginning of the bus, and at the end, and
must occur within four inches of the physical ends of the SCSI
bus. Often, the host will be installed at one end of the bus
and will provide one of the two terminations required.
When do you terminate the bus?
Whenever you want the SCSI bus to operate reliably! Proper termination
ensures that the signal travelling down the SCSI bus doesn't
"reflect back", a situation that can cause a variety of problems
including "ghosted" SCSI devices, data errors, and other anomalies.
How are terminators powered?
Terminators are powered from the "Term Power" line on the SCSI
bus. Term Power can come from any device on the bus, and is
provided by either the host, a drive on the bus, the drive enclosure
backplane, or any combination thereof. Term Power is provided
through a diode and fuse. The drop across the diode and cable
allows for a Term Power range of 4.0 to 5.25 volts.
What kinds of terminators are available?
- Internal:
- Pre-LVD SCSI drives had passive terminators installed
in the drive, and could be enabled/disabled by setting a
jumper. Drives with LVD interface do not have this on-board
termination.
- External:
- In-line terminators connect in series with the SCSI device,
either by plugging into a device connector, or by crimping
directly onto the SCSI cable.
- Passive:
- Primarily associated with SCSI 1, passive terminators
are called "Passive" since they don't do any active work
to regulate power for termination. They simply use resistors
to provide impedance close to that of the cable and rely
on the interface card to provide steady power.
- Active:
- Providing a more advanced form of termination, usually
associated with SCSI 2 and SCSI 3, "Active" terminators
control the impedance at the end of the SCSI bus by incorporating
voltage regulators or diodes to maintain proper voltage,
in addition to resistors and capacitors to eliminate reflections
at the end of the bus.
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