23 Mar
2018
23 Mar
'18
10:48 p.m.
On 3/23/2018 9:55 PM, Tony Duell via cctalk wrote:
On Sat, Mar 24, 2018 at 4:00 AM, Josh Dersch via
cctalk
<cctalk at classiccmp.org> wrote:
Ah ha! -- thanks for pointing that out, I'd neglected the +12 in this
scenario.? Turns out all of the boards I'd used to load the system use
only +5 by some stroke of luck.? I put a DHV11 in the backplane and the
supply now works fine without a CPU.? I suppose I could use a few extra
serial lines anyway, so I guess I'll leave it in there :).
- Josh
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.
My first thoughts are
one of two things :
Insufficient load on one of the supply lines (may not be the +5V one),
the CPU board does load that line.
Lack of pull-ups on ACLO, DCLO. Possibly pulled up (or at least loaded)
on the CPU board)
I would start with tryng to get the power supply running on its own.
Disconnect it from the backplane. Add dummy loads. Put a pull-up
from 5V to ACLO and DCLO.
Once the PSU will run on its own, connect the backplane and try
again.
-tony