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

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Tue Apr 20 12:01:15 1999

>> Can you translate that to a size? I can imagine the size of a #4 nut
>> (I have several). I can't clearly picture how big the O.D. of a #2 would be.
>>
> Roughly .1375 across the hex faces.
>
>> > ...A good material for this is hypersil commonly used for transformers.
>>
>> Used for the windings or the core laminations?
>
> Hypersil is a silicon steel alloy and is used for laminations. Copper, is
> the wire.


I seem to have lost the attributions of much of the above, but...

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.

(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.)

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?


> That is only part of the picture. Core size affects switching speed and
> current needed to switch. Compounding this is more wire means
> resistance, heat and inductance all influencing how fast you can switch.
> Core is where magnetics, analog and digital intersect.


Nice Description, Allison. I like it.

Philip.
Received on Tue Apr 20 1999 - 12:01:15 BST