26 Jun
2005
26 Jun
'05
3:40 p.m.
> Initially I was puzzled. It makes no sense at all
if the keys are simple
> switches.... Tell me (as I can't get to my Nascom at the moment), how
> many connections does each key have?
>
> My guess (again) is that these keys are actually magnetic. There are 2
> loops of wire trough a little magnetic torroid core. Pressing a key brings a
> magnet against the core, changing its magnetic properties, anf thus the
> coupling between the loops.
I've spent some more time on the HP9845 keybaord. There's one correction
to that dsecription, pressing the key moves the magnet away from the
torroid, thus desaturating it and increasing the coupling between the
loops.
The 9845 electronics (and it seems to be similar to that used in the
HP250, the printset for which is on bitsavers). It's quite different to
the Nascom 1 schematic, but I suspect the keys may well be similar.