Building Core, Stage 2

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Sat Apr 24 23:23:40 1999

Ok, I'm getting to the point where I can _almost_ start taking pictures of
the waveforms on the oscilloscope.

Now, if you are wondering, this does have a point. I've got a 4K core stack
for a PDP-8 that I've been loath to get into because I didn't feel I had a
strong enough understanding of the basics in order to figure out just what
the heck was wrong with it, once this project is complete and I understand
core memory, theory and operation, I'll be ready to tackle fixing that stack.

I've been reading and re-reading the theory section in the PDP-8a manual
and some parts are starting to seep into my consciousness. This is where we
are at.

1) I'm using copper clad board to provide a ground plane "under" the core.
After my last message it became obvious that this was the other half of the
winding "through" the core.

2) I'm using "magnet" wire (enameled copper wire) for the core wire. I'm
guessing it can take a couple of amps for a short period of time. I'm not
sure that is sufficent to "switch" the #2 nut I'm using but we'll see.

3) Allison referenced a waveform that looked like this:

           +--+
           | |
           | |
 ----------+ +-------------+ +--------- Gnd
                            | |
                            | |
                            +--+

Now in the ToO section in the manual it was written that the first pulse
"reads" the core, and the second pulse "writes" the core. A light bulb went
on because of course core is "destructively" read, and has to be rewritten.

4) The diagram in the 8a manual shows the sense wires going through one set
of cores, looping and coming back through another set. The sense wire is
also used as the 'inhibit' wire when one wants to write a zero. This is
done by reversing the X or Y current so that the selected cores don't get
full write current (and thus don't switch.)

5) So this is the plan:
        put three wires through the nut and wire them up as follows:
                                 --
        1/h select ----------\ | | /----- GND
        1/h select ---------------| |-------------- GND
        sense + ----------/ | | \----- sense -

   The scope will display the current on the select lines on channel 1
   the sense lines on channel 2 (floating ground)

6) When the current is sufficient, I should see a pulse on the sense line.
   (This is the hypothesis part, now to apply the scientific method)

We'll see ... if there are obvious errors in the above please let me know!

--Chuck

                                 --
Received on Sat Apr 24 1999 - 23:23:40 BST