This is a wide open question.  I don't know about other people but for me
to be able to answer your question correctly, I need to know a little bit
more.  First off, is this card on an HP 3000 or on something else.  If it's
something else, what brand is the card?

When you talk termination on SCSI, it can be a complex subject if you want
to make it so, or it can be a simple and straightforward subject.  On a
SCSI chain, you have an adapter card and you can have both and internal and
an external SCSI chain.  If you have only an internal SCSI chain, then the
adapter and the last device on the internal cable need to be terminated.
 If you have only an external SCSI chain, then the adapter and the last
device on the external SCSI chain need to be terminated.  If you have both
an internal and external SCSI chains, then the last device on the internal
chain and the last device on the external chain need to be terminated, the
adapter must not be terminated.

What is termination?  Good question.  The SCSI bus is a chain of devices,
it is not a loop.  Therefore the two ends of the chain must be terminated.
 The SCSI bus has an impedance (resistance to the passing of electrical
signals), however when the signals reach the end of the cables of the bus,
they hit the air, which has very high impedance leading to infinite
resistance.  It's a good thing too or else your wall outlets would be
sparking like mad and you would be an ash if you were in front of one.

Now when a signal hits this "wall" of infinite resistance, it is reflected
back in the other direction.  This is not a good thing.  So one uses
termination.  Termination is an electrical requirement that must be met in
order to prevent the reflection of signals when they reach the end of the
SCSI bus.  The terminator is a circuit attached to the physical ends of the
SCSI bus which provides an impedance that matches the cable's impedance,
thus preventing the signal from being reflected back.

The terminators use power and this power comes from the SCSI adapter card
via the TERMPWR wire on the SCSI bus.  There are 3 types of termination
methods:  passive, active and forced perfect termination.

Active termination is accomplished through a voltage regulator which
controls the impedance at the end of the SCSI bus.  It regulates the power
coming from the SCSI adapter card.  Active termination is better than
passive.

Passive Termination is accomplished by simply having a circuit on the bus
to provide an impedance that is close to the impedance of the cable.

FPT is more complex and we won't go into it here.

I hope this explains what the issue with TERMPWR is and why you need to
terminate your SCSI bus.

Nowadays, many devices are self terminating and so is the adapter card.  If
your system is working fine as it is, then do not worry about termination
since the devices do that for themselves.  On the other hand, if you do not
see all the devices on the SCSI bus, or the system is having "unexplained"
problems, then you need to address the termination issue further.

A light on the card probably simply means that TERMPWR is being generated
by the card.

Kind regards,

Denys. . .

Denys Beauchemin
HICOMP
(800) 323-8863  (281) 288-7438         Fax: (281) 355-6879
denys at hicomp.com                             www.hicomp.com


-----Original Message-----
From:   Dave Knispel [SMTP:[log in to unmask]]
Sent:   Monday, January 17, 2000 3:19 PM
To:     [log in to unmask]
Subject:        SCSI and terminators

Could someone point me in the right direction regarding SCSI and
termination?  Specifically I have a card where the TERM POWER light is on.
Does this mean it is terminated internally and I don't need another
terminator?

Thanks in advance.

David Knispel
[log in to unmask]
Phone: 513-248-5029
Fax: 513-248-2672