PDP 11/24 - A Step Backwards

Noel Chiappa jnc at mercury.lcs.mit.edu
Wed Mar 30 21:52:25 CDT 2022

    > From: Rob Jarratt

    > I found these two signals and ACLO is low (-15V)

'Good news, bad news'...

Bad is that something is seriously wrong there; 'allowed' values are 0v
(asserted) and +3V (un-asserted). I'm worried that the -15V will have taken
out some of the semiconductors that are 'listening' to ACLO (like E70, page
K2 of the CPU prints, lower right corner) - and possibly some of the things
that are connected to _them_.

Good news is that i) this would definitely cause a problem, so we're closing
in, and ii) even better, the machine doesn't actuallly _need_ the ACLO (or
DCLO) signal from the P/S to function properly. Just disconect them (which may
be a bit tricky; IIRC you've got a BA11-A - but you can pull the pin in the
connector shell of the power harness from the backplane, details of that here:


or, worst case, just cut the yellow wire to pin 4 of the 6-pin connector). At
that point the pullups on ACLO (on the M9302 and the CPU - page K3 of the CPU
prints - that's odd, there's a pullup to +5V there, but a cap to ground; the
M9302 does indeed have the pull-up/down resistor pair on both ACLO and DCLO)
should pull ACLO high and the clock should now run (CLK LED on) - unless the
-15V killed something.

If the machine then runs, it's up to you as to whether you get the P/S
repaired so that ACLO work properly - your call. (I wonder how the -15V got
to ACLO - I suspect a solder bridge from the prior repair - but knowing the
answer is not important to getting the machine running.)

   > DCLO is high and the DC ON light is illuminated


    > the CPU doesn't do anything presumably because ACLO is asserted.

Yes. As long as the CLK LED is off, the machine will definitely be totally
dead. If you can get it on, ODT should run (modulo issues yet to be sorted
about the minimal functional machine - I'll post on that in a moment).


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