PDP-8 core memory problems.

Mattis Lind mattislind at gmail.com
Tue Sep 6 04:11:30 CDT 2016

> I'd tend to be more pessimistic about this working.
> There are different requirements in winding a wire for purposes of inhibit
> and sense.
> In the 3-wire arrangement the winding of the combined wire has to meet both
> sets of requirements.
> Specifically, for this case, in a 4-wire mem, where the inhibit wire was
> woven just for
> the purposes of inhibit, I don't anticipate it's going to have the noise
> cancellation topology
> needed to function as the sense wire.
> Sense wires were woven as a floating loop feeding a differential
> amplifier. The loop is kept
> quite closed or otherwise woven in a very balanced manner so that the
> magnetic fields from the
> large select currents (and other influences) will cancel out or be
> rejected as common-mode influence at the
> differential amp inputs.
> The select current magnetic fields trigger the magnetic field reversal of
> the core, you want to sense the latter distinct from the former.
> Unless you have circuitry with the wherewithal to distinguish those
> induced currents in the sense wire in time
> (delay of the core field reversal from the select field), you otherwise
> have to minimise the influence of the select current fields on the sense
> wire.
> If you look at the diagrams in my article you can see the sorts of
> differences in weaving topologies between 3 & 4 wire arrangements,
> as well as examples of the tortured topologies resorted to to balance the
> sense loop.
> In the 3-wire example there you can see how the S/I wire was split in half
> with a special resistor network at one end to allow inhibit current flow
> while at the same time configuring it as a balanced loop for the sense
> function.
> My article certainly isn't the last word on the variety of
> implementations, I believe there were 4-wire designs with sense wires
> parallel to
> select wires as in the 3-wire designs for example, so you never know until
> you examine the specifics at hand,
> but I think it unlikely you'd have much success getting the inhibit wire
> to function for sense, not without going to as much trouble
> messing with the stack as if you tried to fix the sense wire.

Thanks. I think that this information will rule out the idea of using the
inhibit wire instead. It is likely to be not viable.

> If the sense wire is open I'd guess there's a good chance it's at one of
> the end points where it's soldered to a terminal or at an existing splice
> from manufacture time and might be repairable if access could be had.

Access is of course the problem. I tried to check at the terminals and it
looked like the wire was ok there, but it is in an extremely tight place. I
tried to apply some small amount of solder in the hop that it was just bad
contact at one of the terminals but no difference unfortunately.

> Might the stack have a parity bit array that could be redirected to
> replace the faulty bit array?,
> perhaps foregoing the parity checking on that board-set if the parity
> logic is present.

I think I have checked the existence of a parity plane. It appears that
there is none present. The writing on the core module show no parity plane
and there are only twelve sense and inhibit terminals.

So what are the other options?

* Trying to repair the unit. Every plane is soldered together with the ones
nearby to convey the X/Y signals. This can probably be undone with a
patience and soldering braid. But what are the chance that the X/Y wires
gets lose then? Are those soldered or welded into place?

Then it would be quite tricky to just identify where it is actually broken.
Any ideas for how to do this? A microscope of course. Any other ideas?
Applying an electrical field between the wire and something else and try to
detect it?

Repair. If the wire is broken in the mat it is probably not to difficult to
pull out the broken parts. But then the new wire has to be spliced in. What
is t he best technique to do that?

How to push in the new wire in the matrix? I now that Anders was able to do
this with a broken X-wire in a PDP-8/L stack.

* Use a PDP-15 MM15 stack and sense/inhibit boards.

I have several off these. Adding a small backplane, put the X/Y drivers,
sense amp/inhibit drivers and level converters there and then adapt to the
existing slots for the memory module. It would be a horrible mixture of TTL
and transistors. But it would still be core memory.

* Use solid state technology. Possibly inside the memory box so it looks
real but emulates the actual core memory module.
Any ideas how this could be done in the best way?


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