A poltergeist in my machine?

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Sep 7 17:09:29 CDT 2015

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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