At 21:11 1/13/2000 , Brad Loomis wrote:
>The Seagates have a jumper for Term. Power. If they aren't
>terminated, do I leave this jumper on? There are three
>choices for settings. I can find no explanation for their
>function. They are Term. Power from drive (default),
>Term. Power to SCSI bus, Term. Power from SCSI bus, and
>Term. Power to SCSI bus and Drive.


Hi Brad

Think of the termination circuit on the drive as a voltage
regulator module that will hold voltages at the end of the
bus to their specified values. This will prevent signals
from reflecting back from the end of the bus and causing
interference with the real signals. A properly terminated
bus will act as if the SCSI bus just goes on forever.

The termination circuitry needs power to operate. The
SCSI controller normally supplies this power via the
ribbon cable. But if the cable is very long (say 20 feet
long) or winds around a lot, the power might dissipate
enough (due to the cables own resistance or due to
electronic impedance) to cause problems. This should not
be a factor with your internal cable. Ultra SCSI specifies
that the total length of all your cables be no more than
1.5 meters. (Actually, some people say that if you only
have one or two devices, your Ultra cable can be longer.
But other official sources say to follow the 1.5 meter
(5 feet) rule all the time.)

You can set your hard drive to supply termination to its
on board termination circuitry but not supply power to
the rest of the SCSI bus. This setting could not cause
harm to anything by overloading the SCSI bus's termpower
line. The drive would not take power from the SCSI cable
for its own terminator. Please note that this setting
does not actually enable the drive's own termination. You
must do that additionally by setting the TE (Termination
Enabled) jumper.

You can set the drive to get its power from the SCSI bus.
This would be necessary if the drive were going to be
powered down at times but was at the end of the bus and
hence needed to terminate the bus. (A scanner or external
removable drive at one end of the bus would be a good
example of this.)

If you had a device at the end of the bus that might be
powered down and was at the end of a long or windy cable,
a good setting would be to have a device that was always
powered up and was nearby the last device supply power to
the bus. This is the last TP option that you mentioned.

If your drive does not have its termination enabled
(the drive not being at the end of the cable), the
TP (TermPower) setting becomes a mute point...as you
suggested.

So in summary, I would follow Mary Wolden's advice
"I have always used the "Term Power from Drive" with
no problems." I have always used the setting "Power
from the bus" without problems but I think I will
follow Mary's advice from now on. Finally, if the
drive is not at the end of the bus and won't be
terminated, it doesn't matter what TP setting you use.

Regards,
Bill

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