How scarce (valuable) is core for the PDP-8?

From: allisonp_at_world.std.com <(allisonp_at_world.std.com)>
Date: Tue Apr 20 14:28:23 1999

> I would have thought that transformer steel is _not_ a good material for cores.
> Transformer steel is designed for making the hysteresis as small as possible, so
> as to minimise core losses, etc. Whereas for core memory, you need a good sized
> hysteresis because this corresponds to stored energy, which will drive the pulse
> on the sense line.

It happens to be adaquate, it does exhibit hysteresis.

> (As I understand it, when you _don't_ flip a bit, the pulse on the sense line is
> roughly that from the transformer effect in the core. If you _do_ flip a bit,
> you get the transformer effect, plus a pulse of stored energy from the core.)

Correct, but the pulse that indicates the core switched is delayed in
time Hence the critical slice time. Core size affects this as does
the select current.

> Rather than using steel nuts - which may be very inconsistent in their magnetic
> properties - would ferrite beads, as used for interference suppression, work?
> Or to they have too small a hysteresis like transformer steel?

Actually the nuts have an adaquate hysteresis to make a demo core but it
would not work useably well. Ferrites used for beads have low
permability. But since they are available I'd try one and see.

> Nice Description, Allison. I like it.

Core is mysterious but when broken down the mechanism is easy enough to
understand. then you need to understand the interactions... ;)

Allison
Received on Tue Apr 20 1999 - 14:28:23 BST