Core memory

[Curious Marc]

You don’t strictly need an inhibit wire to write cores. You can write the core with just the addressing lines. The inhibit wire is just there to simplify the addressing electronics logic in early core memory, so you don’t need to have per core control of the current in the address lines when writing. You just do it wholesale: scan all address lines with current in one direction during reads, scan once again with current in the other direction during writes, with inhibit when needed.

You could certainly use the sense wire as an inhibit. But that gets impractical with a diagonal weave, because you’d have to know in which direction the inhibit works and reverse the inhibit current accordingly. And it’s different for each core depending on the direction the sense wire crosses the core. So it defeats the purpose of simplifying the control logic with an inhibit wire. Therefore I don’t think that scheme is used.

In practice, 3 wire systems that use the same wire for inhibit and sense (like some IBM core planes) have the sense go along the address lines, and use a half plane column shift to provide cancellation of the unwanted signal induced from the address line it is running along. We demonstrate such a 3 wire plane here: https://youtu.be/AwsInQLmjXc .

So in your plane, the sense wire is likely just used for reads, and the writes are done uniquely with the address wires, like I demonstrate here: https://youtu.be/7ozNMgx7WtQ

Marc

> On Apr 1, 2022, at 2:14 PM, Brent Hilpert via cctalk <cctalk at classiccmp.org> wrote:
> 
> On 2022-Apr-01, at 11:51 AM, Paul Koning wrote:
>>>> On Apr 1, 2022, at 2:38 PM, Brent Hilpert via cctalk <cctalk at classiccmp.org> wrote:
>>>> On 2022-Apr-01, at 6:02 AM, Paul Koning via cctalk wrote:
>>> 
>>>> When I looked at that ebay listing of "glass memory" it pointed me to another item,https://www.ebay.com/itm/265623663142 -- described as "core rope memory".  Obviously it isn't -- it's conventional core RAM.  Interestingly enough, it seems to be three-wire memory (no inhibit line that I can see).  It looks to be in decent shape.  No manufacturer marks, and "GC-6" doesn't ring any bells.
>>> 
>>> Well, it would still work for 1-bit-wide words, so to speak. One wonders what the application was.
>> 
>> I wonder if the sense wire was used as inhibit during write cycles -- that seems doable.  It would make the core plane simpler at the expense of more complex electronics.  With that approach, you have regular memory, not limited to 1 bit words.
> 
> Maybe I'm being overly cautious, but offhand I'm initially skeptical without doing the math or some good vector diagrams, or seeing an example. With the diagonal wire you're changing the current/magnetic sum vectors in direction and magnitude. The question is coming up with a current that reliably performs the cancellation function on the selected core of a bit-array while reliably *not* selecting another core, while accounting for all the variation tolerances in the array. 
> 
> While there's probably some value by which it would work in theory, I wonder whether the diagonal wire would narrow the operating margins. From some stuff I've seen, the hysteresis curves for cores weren't spectacularly square. With the usual 3D-3wire scheme of a close parallel inhibit wire you have 'cancellation by simplicity', you maximise the difference (cancellation) influence on one wire while minimising it's sum influence on the other.
> 
> A related issue is the normal diagonal sense stringing (which this looks to have) has the wire entering the cores from both directions relative to the address wires, which is why sense amplifiers respond to pulses of both polarity. If this diagonal wire is put to use as an inhibit wire, some logic is needed to decide the direction of the inhibit current from the address, though that may not be very difficult.
> 
> Some history of the 3-wire development might tell, whether inhibit was first applied to a diagonal sense stringing or whether sense was first applied to an adapted parallel-inhibit stringing. The real benefit of the 3-wire development was getting rid of the diagonal stringing for manufacturing ease.
> 
> 
>>> There are a couple of Soviet core-rope memories up right now:
>>>    https://www.ebay.com/itm/294558261336
>>>    https://www.ebay.com/itm/294851032351
>> 
>> Neat looking stuff.  It doesn't look like core rope memory in the sense of the AGC ROM, nor in the sense of the Electrologica X1.  It looks more like the transformer memory used in Wang calculators that you documented in your core ROM paper.
> 
> Yes, (I was, perhaps lazily, slipping into the habit of referring to both forms (of woven-wire ROM) as rope).