SCSI terminators

What is a SCSI terminator? Why do I need them?

A SCSI bus is a transmission line. To prevent reflections from the ends of
the bus, you need a device which makes the transmission line appear to be
of infinite length. This is done by attaching resistors, which have the
same resistance as the characteristic impedance of the transmission line,
to the ends of the bus. Also, since SCSI line drivers are open-collector
(which can only pull a signal low), a pull-up resistor is needed to pull
the signal high when it’s not asserted.

If the ends of the bus are not terminated, the signal pulses will reflect
off these open ends and travel back along the bus in the other direction.
The resultant adding and canceling of signal amplitudes distorts and
corrupts the SCSI signals.

There are two basic types of terminators, active and passive:

Passive terminators consist of pairs of resistors. A 220 Ohm pulling each
signal up to TERMPWR and a 330 Ohm pulling each signal down to GROUND.
Passive terminators were considered adequate in SCSI-1 when the bus only
ran at 5 MHz. In SCSI-2, passive terminators were given the name
"Alternative 1". Active terminators consist of 110 Ohm resistors connected
from each signal line to a common 2.85 Volt regulated power supply. Active
terminators both terminate the bus better (less reflection), and supply
cleaner pull-up current (due to their Voltage regulation). They were first
defined in SCSI-2 and were given the name "Alternative 2" to distinguish
them from passive terminators.

Recommendations and requirements: In SCSI-2 when the fastest defined speed
was 10 MHz, passive terminators were allowed, but active terminators were
recommended. In SCSI-3, the "alternative X" terminology has been
discarded, and the SPI-2 standard only allows active termination for
single-ended buses regardless of speed. My personal recommendation is not
to buy any new passive terminators. If you want to use up the old ones you
have lying around, on older systems, with short buses and no more than 4
devices, that don’t have any devices faster than 10 MHz, I can’t argue with
that, but ONLY BUY ACTIVE (or preferably LVD) terminators for any new
systems. If you run into problems, switching to an active terminator might
well solve them. Other people will tell you that only active terminators
are ever acceptable at any speed. I leave the choice up to the individual
at Fast10 and below, above that, active is absolutely the only acceptable choice.

A final nit to pick: As I was reminded in looking back at the standards, technically SCSI-2
did not sanction Fast10 on single ended buses. It was only spec’d for differential. However,
as was the case with WIDE SCSI using the 68 pin P cable, the industry latched onto it and
it later became standardized in SCSI-3 SPI.

What is terminator power (TERMPWR)? Why do I need it? Where does it come from?
TERMPWR is the power source for the SCSI terminators. Terminators (both active and passive)
require power because in addition to providing the correct impedance to prevent reflections
on the SCSI bus, they source pull-up current to the SCSI signals. The SCSI spec. allows for
multiple devices to supply power, but also limits the maximum current that should be
available. The "rule" is that "initiators shall supply TERMPWR". Hence a SCSI controller
(host adapter) should supply TERMPWR, and on longer buses it is worth having a device near
the end to also supply it . However, no more than about four devices should supply it,
because in the event of a failure (shorted cable etc), there could be dangerous currents
available. Not all devices are designed to be able to supply TERMPWR, but many can.
Usually this is done by setting one or two jumpers to select where TERMP WR will go.
For example:

TERMPWR to on drive terminator only TERMPWR to SCSI bus On drive terminator
gets its TERMPWR from SCSI bus

Even though the spec. says that host adapters should supply TERMPWR,
PCMCIA type host adapters do NOT do it. This is because PCMCIA cards are
generally plugged into laptop computers that run on batteries and can’t
afford the extra current drain. Another reason is because the contacts in
a PCMCIA connector are so tiny that the 1 Amp TERMPWR current load is
beyond their ratings. This being the case, at least one of the devices
that you wish to attach to a PCMCIA host adapter needs to be able to supply
TERMPWR, or you must provide a special terminator that has a power
connection for this purpose.

Leave a Reply

Your email address will not be published. Required fields are marked *