Another question about core sense amplifiers

[Paul Koning]

> On Mar 11, 2015, at 1:45 PM, Rick Bensene <rickb at bensene.com> 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 -- http://oldcalculatormuseum.com/al1kck10l.html) ) 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 http://bitsavers.trailing-edge.com/pdf/cdc/cyber/cyber_70/fieldEngr/60147400A_6600_Training_Manual_Jun65.pdf .  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.

	paul