PDP-8 core memory problems.
elson at pico-systems.com
Mon Sep 5 18:36:22 CDT 2016
On 09/05/2016 05:46 PM, Mattis Lind wrote:
> måndag 5 september 2016 skrev Jon Elson <elson at pico-systems.com>:
>> On 09/05/2016 01:59 PM, Mattis Lind wrote:
>>> I have now concluded that the fault is in the core memory module itself.
>>> The sense winding is broken on bit plane 7.
>>> Have you actually ohmed out the sense/inhibit wire?
> This is by the way a four wire stack. Separate sense and inhibit wire.
OH! I didn't know any PDP-8 had 4-wire planes. Very
interesting. Hmmm, you could rig a pulse transformer to the
inhibit wire to bring signals over to the sense amp.
> Come to think of it, since the inhibit wire is OK, would it be possible to
> arrange it as a three wire stack somehow. Change the sense amp and inhibit
> drivers so that they use the same wire?
Ah, you already thought of that. Well, it SHOULD be
possible. You'd probably put a resistor in series with the
pulse transformer so that all the select current went
through the inhibit wire. Then, the only problem would be
that the sense amp gets hit with a big pulse during the
writeback. As long as the sense amp recovers from the
overload before the next read cycle, it ought to work. Some
4-wire planes were set up so the polarity of half the sense
wires were opposite to the inhibit, so the coupled inhibit
current balanced out to zero in the sense winding. Any
mid-sized 1:1 pulse transformer should do the coupling, the
currents might be pretty high, but the duration isn't very
long. (Minor nit, the inhibit wires only get pulsed one
direction, so you might need a resistive path across the
secondary to discharge the flux build up so it doesn't
> Maybe Brent can come in with some advice if this is possible or just
It is NOT stupid, I think it could really work!
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