PDP-11/44 problem - RUN stays on

Gooijen, Henk henk.gooijen at oce.com
Wed Nov 15 04:53:01 CST 2006

Don wrote:

> By first memory board I infer that you mean the MS11-M M8722 256KB.
> This board requires both +5V for logic and +-12V for the memories.
> By second memory board I infer that you mean the MS11-P M8753 1MB.
> This board is 5V only; it does not require either +-12V or +-15V.
> I would have expected it to be the other way around (MS11-M 
> does not work but the MS11-P does) but hey. It could be a 
> stuck driver on the MS11-P that is pulling some memory/unibus 
> control signals to ground, hanging the bus and cpu.

Yes, correct!
I checked the voltages, they are all just fine. Phew!
I removed the M9302 in slot 14 a-b, and plugged the M9202 in it
to add the next backplane to the total system.
In that backplane is a TUK50, RUX50, UDA50 and the RL11.
Power on ... all is perfect, for what I could see in a glance.
We had a family visit, so I did not have much time, but as in a
miracle, the RL02 FAULT light remained OFF too!
One quick check: I installed the MS11-P in the 2nd memory slot.
Turn on the system ... yep, a hang condition and the FAULT light
on the RL02 goes ON immediately!  Removed the MS11-P again, and
everything looks healthy!

It definitely looks like a defective MS11-P. The effect that it has
on the system (especially the RL02's FAULT light) gives me hope to
be able to repair that module one day!

- Henk.

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