Odd memory error in PDP-11/04

Paul Koning paulkoning at comcast.net
Wed Sep 7 11:12:52 CDT 2016

> On Sep 7, 2016, at 11:44 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>> From: Ethan Dicks
> Let's look at this one first, this is probably the easier to solve.
>>>> 2) Setting D10 in location 000000 results in D10 set in all the
>>>> locations
>>> Sorry, didn't follow that? Did you mean that if you store 02000 in
>>> location 0, all other locations now report the 02000 bit set?
>> Only 04000, but, yes.
> Ah. That's D11. :-)
>> If I set that bit in location 0, or other locations, it gets set in all
>> locations. If I clear that bit, it clears.
> So that's likely in the memory (although I suppose it could be the CPU,
> _somehow_). It sounds like there's a latch somewhere in the output path
> (because it affects all locations right away, not just once you've written to
> them) that's getting set one way or the other, and and then, won't change. I
> suspect the problem is with the flop for that bit, not in the circuitry
> that's clearing/clocking that flop, since it only affects that one bit.

Semiconductor memory, right?  A possible reason would be that the address drivers for that bit, or the address decoders in that chip, are busted.  The result would be that reads and writes always touch the same address in the chip.

The fact that other bits repeat every 20 also suggests issues with addressing logic.  I'd suggest checking the address signals on the memory card.


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