Another question about core sense amplifiers

Mike Loewen mloewen at
Wed Mar 11 13:43:33 CDT 2015

On Wed, 11 Mar 2015, Paul Koning wrote:

>> On Mar 11, 2015, at 1:45 PM, Rick Bensene <rickb at> wrote:
>> OK, while we're on the topic of core memory sense circuitry, in some of
>> the old calculators that I've come across (a good example being the
>> Casio AL-1000 -- ) there
>> are transformers (pulse transformers) in the core memory sense
>> circuitry.  What purpose would these serve, and why are they used in
>> some core memory applications, and not in others? In the case of the
>> AL-1000, similar transformers are used in the X-Y drivers as well.
>> Core memory sense amps and drivers are that mystical analog stuff that I
>> don't understand very well :-)
>> Can someone enlighten me about these transformers?
> I found a very good detailed description of high speed core memory in the CDC 6600 training manual ? copy available on bitsavers at .  Chapter 4 describes the memory. It includes schematics, and a pretty detailed description of how the circuits work.
> Those memories are a bit more complex than usual; there are two inhibit wires rather than one, and the inhibit does not cover the whole core plane but only a part of it (i.e., there are several independent inhibit current paths).  It doesn?t say why.  My best guess is that this is to keep the inductance of the inhibit wire similar to that of the X and Y wires, so the same driver design works with roughly the same timing.  This memory is much faster than most other memories of the same era: 1 microsecond full cycle, in 1964.

    There's an article online from Byte Magazine, July 1976, about 
coincident current core memory:

Mike Loewen				mloewen at
Old Technology

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