Nascom 2 keyboard connector

[Tony Duell]

> 
> On Fri, 2005-06-10 at 00:09 +0100, Tony Duell wrote:
> >
> > Alas I don't have a Nascom 1 schematic...
> 
> Well the Nascom 1 *keyboard* schematic is online on a Nascom site along

I've now taken a look at that....

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 is 4. Moreover, I will guess that there are 2 pairs of pins that 
are dead shorts to each other, but one pair is insulated from the other. 
And that this doesn't change if you press the key.

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.

Each of the lines on the matrix in the schematic is a set of these loops 
in series. Horizontal lines are the sense loops, vertical lines are the 
drive loops. That would explain the cryptic comment on the schematics 
that the order of keys in the matrix may not be the electrical order on 
the PCB.

Now for an amazing coincidence (if my guess is right). As you probably 
know, I've had an HP9845B on my bench for the last few months [1]. Last 
weekend I finally finished working out the rear section with the 
processor and memory boards, the PSU, etc, and started on the keyboard 
assembly. Tge keyboard is exactly as I've just described. The electronics 
is somewhat different to the Nascom 1 keyboard, but the 'switches'[2] may 
well be the same. Hmmm...

[1] My advice to anyone with a non-working 9845 is to find some other mug 
to fix it. If you're not that easily put off, I will try to answer 
hardware-related questions.

[2] Most of them have 4 pins, as described. The arrow keys have 6, one 
drive loop and 2 sense loops. The reason is that said keys contain 2 core 
as they kave 3 postions -- up, pressed gently (single cursor movement) 
and pressed hard (repeated cursor movement). Well, it _is_ an HP...

> Not that there appears to be much difference between the two; the Nascom
> 2 one just has the extra row (and no doubt totally different key matrix)
> and this keyboard sense line (which as you say probably isn't used by
> the software anyway).

It can't be a keyboard detect line. It goes to a TTL input, which would 
float high anyway. It was probably easier to use an 8 bit buffer on the 
CPU board, and there was no good reason not to wire all 8 inputs to the 
keyboard connector.

> 
> > > In all it uses 13 out of the 16 pins on the keyboard PCB. Pins between
> > > the keyboard and CPU board *do* match up 1-1, it's just a case of
> > 
> > WHich 3 signals are not used? I would guess at Q2 and Q5 and one other.
> 
> Yep, Q2, Q5, and /NMISW 

The first 2 could be used as general-purpose outputs...

NMISQ/ is somewahat like the reset input, but it produces an NMI (Duh...) 
I wondered if it was used for the break key or something, Apparently not.

-tony