PDP-8 core memory problems.

Jon Elson elson at pico-systems.com
Mon Sep 5 22:03:38 CDT 2016

On 09/05/2016 09:28 PM, Brent Hilpert wrote:
> I'd tend to be more pessimistic about this working.
> 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.
Well, older core memories had larger cores, which slowed 
everything down as well as gave much bigger pulses when a 
core flipped.  That might make this exercise a bit easier.
> 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.
But, he has to split the whole stack to at least access the 
bad plane from one side.  Already a pretty daunting 
exercise.  Still, fixing the bad wire, if the break is at an 
exposed point, would be preferable than re-engineeering the 
whole sense-inhibit circuitry. If the wire can be fixed, 
there would be no engineering required.

I saw our old IBM 7094 memory, which had a combination of 
tube and transistor circuitry, and one inhibit (or maybe 
sense) wire had been burned up by a failed circuit.  The 
cores kept the wire cool, so the only places it opened was 
where the wire looped around the plane. There were dozens of 
bits of wire soldered to fix the places where it opened.  
Sheesh, I sure wouldn't have wanted to be the CE who had to 
do that repair!


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