A poltergeist in my machine?
jsw at ieee.org
Mon Sep 7 18:00:18 CDT 2015
Check if the -5v charge pump is stable and supplying the correct voltage.
I recall a problem with a PDP 11/34 and a non-DEC memory board in
which the on board pump went slowly bad. Depending on the
data content and tolerance margins for the memory chips, we saw
very erratic problems.
> On Sep 7, 2015, at 5:09 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
> So I've mentioned how I've seen this wierd behaviour where QBUS memory boards
> that hadn't been used in a long time didn't work when first plugged in, but
> started working later.
> I just had something even weirder happen, and am curious if anyone has an
> plausible explanations.
> So I had a dead M8044 (MSV11-D), symptom was that you could write -1 to any
> location, it read back as 0. Quite repeatable, I can power cycle the machine,
> take the card in and out, etc, etc.
> So I throw it on an extender, and start chasing. I have a two instruction
> loop (write location 0, loop), and I'm watching the data going into the
> memory chips on the card, and it all looks good. So I add a third instruction
> (read location 0, after the write), and continue chasing.
> Data looks good coming out of the chips; then it goes to an octal latch. So I
> look at the latch enable, and that doesn't look so hot - just a tiny little
> ugly spike. So I look at the source of that, and it's a D flop. So I look at
> the D flop's clock input, and it's also a nasty little spike. So that comes
> from the output of a triple-AND, and so I start looking at the inputs of the
> 3-AND. And when I put my 'scope lead on the second input... the memory
> suddenly starts working!
> Well, I could see that - the added resistance or capacitance or whatever of
> the probe might have had some effect on a circuit that was right on the edge.
> But here's where the ghost enters the machine.
> I pull the 'scope probe ..... and the memory keeps working!
> I can power cycle the machine, leave it off for 15 minutes, power it back on
> - and the memory still works fine!
> Does anyone have _any_ idea WTF is going on here?!?!
> I feel like I'm in some sort of AI koan...
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