You are correct about the SCSI rules.  The first and last devices must be terminated.

Before writing the statement that you disputed, I searched the HP web site.  I could not find anything on the 9x8 or the MFIO cards
there.  The web site seems to have been scrubbed clean of any "legacy" HP 3000 information.  So, I went to the Adaptec web site and
searched for the documentation on one of their oldest and most prolific product, the AHA-2940.  We have just such an SE SCSI device
in one of our very old NT servers here.  The 2940 is about 2 million years old and, I believe, even predates the 9x8 systems.

At any rate, the 2940 is the SCSI HBA I used as a reference.  This ancient model has auto-termination.  This means is if you only
have devices attached to the internal ribbon cable, you do not need to terminate the external string as long as you do not attach a
cable to that connector.  The reverse is also true, if you have only external devices, you do not need to terminate the internal
string.

However, if you have both internal and external devices, you do indeed have to terminate both ends.  That is usually accomplished
with a jumper on the internal device and either a jumper or a plug terminator on the external string.

I figured that surely the MFIO card in the 9x8 which probably costs at least a thousand dollars when the 928 was new, had at least
the same capabilities as a card costing about $200 when it was introduced.  Auto termination is not magical by any means.  But who
knows?  This is HP we are talking about here, where hardware innovation was rarely applied to the HP 3000 after 1990.

I checked our 9x8s and 9x7s SE controllers here and they indeed to not have any terminator and have never had terminators when no
devices are connected.  We have never had a problem.  However, "we have always done it this way" is not a proper argument.  So last
night I went back to the HP web site and this time I expanded the search to even include HP 9000 models like the E, D and R class in
the hope of finding any type of manual.  That did the trick.  I found the Operator's manual for the D and R class.  On page 1-23, it
does indeed state that the terminator must be used if there is no external device on that SE SCSI connecter.

By the same logic, if the internal string is empty, it would also have to be terminated. We have some 9x8 that have an extra SCSI
card so that we can connect external devices to them, such as tape libraries.  The manual is mute about terminating the internal
string if there is nothing on there.  We have never terminated the internal string when it is empty and have never had problems
there either.


Denys

-----Original Message-----
From: HP-3000 Systems Discussion [mailto:[log in to unmask]]On Behalf Of Gavin Scott
Sent: Friday, February 28, 2003 6:42 PM
To: [log in to unmask]
Subject: Re: 918LX question

Denys writes:
> But as for termination itself, if there are no external devices on the SE
> port, it does not need a terminator.

I believe the SE SCSI rules are that the first and last devices on the cable
must be terminated.

I don't believe there's any termination *on* the MFIO card, therefore there
*must* be a SCSI terminator on the external connector to meet the SCSI
requirements.  HP has always shipped such a terminator with these systems
for this purpose.

It seems to be true that one can often get away *without* the terminator in
place, but I have seen one or two failures that were solved by providing the
missing terminator.

It's therefore my belief that running a system without a terminator on the
external SCSI connector is asking for trouble.

It's important to remember that the external SE SCSI connector is *not* a
separate interface, but is simply the other end of the SCSI chain that is
internally connected to your (internal) disk and tape devices.  Therefore if
you have no terminator on the external connection, you're probably running
with your LDEV 1 device and your backup device (DDS/LDEV 7) in an
electrically unstable configuration.

Obviously anything you plug into the external connector needs to be at a
SCSI address that does not conflict with any internal SCSI devices in the
system.

G.

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