Hi all --
I've got an 11/03 backplane here (designated as an "11/03 EA" originally
but marked as "Model changed from EA to AA") inside a small chassis
w/power supply, model "OH780-A." (So it was at some point in its life
the front-end to a VAX-11/780.) As far as I can tell, it has a standard
H9270 backplane (4-slot, quad width) in it.
The chassis appears to work fine when I have an actual LSI-11 in there
(I've been experimenting with an 11/23, but I do have an 11/03 boardset
that works in it).
What I want to do with the chassis is use it with a UNIBUS->QBus bridge
I have, as a Qbus backplane for a SCSI controller (and maybe other
things) for my VAX-11/750. I have encountered an oddity in doing so,
and I've looked at the documentation and I remain puzzled. The oddity
is this: If I don't have an LSI-11 CPU board in place in the chassis,
the power supply won't come up properly. (The +5V appears to pulse, the
kind of behavior I'd expect with a shorted or heavily loaded supply rail
somewhere).
Here's a basic rundown of what I've observed:
- If I put an 11/23 or 11/03 CPU in, everything works fine.
- Doesn't matter what slot the CPU's in either, as long as there's a CPU
board somewhere in the thing, all's well.
- Doesn't matter how heavily or lightly loaded the backplane is; I've
tried it with just a QBus SCSI controller, I've tried it with the
backplane stuffed full of boards.
Since I want to use this as a simple QBus backplane, I don't want to
have a CPU board in place. I haven't seen this behavior before (but
then, I can't recall trying to run a Qbus backplane without a CPU in it
either). I thought I knew QBus fairly well, so I assume I'm overlooking
something obvious. Or perhaps there is a fault here with the chassis,
somehow. I figure someone here will have the answer immediately and
I'll slap my forehead...
Thanks as always,
Josh
