A poltergeist in my machine?
bqt at update.uu.se
Mon Sep 7 18:05:23 CDT 2015
On 2015-09-08 00:09, Noel Chiappa 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...
A bad solder. Physically affecting it could make ti get a good connection.
As for memory starting to work better after a while, other things that
can have effect are bad capacitors that improve performance after a
while. Or another case of bad solder. Heat expansion can also affect bad
If you've never actually build electronics this might sound strange, but
anyone who have been soldering stuff have seen plenty of this.
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