Date   : Mon, 05 Jan 2015 21:23:25 +0000
From   : bbc@... (Dave Curran)
Subject: Serial console active at power-on

The additional circuitry on the BBC keyboard is just a way of reducing the
standard matrix scanned keyboard to only needing one port 8 bit port on the
system VIA. Four pins send a column address, via a latch controlled by
another pin to the keyboard matrix. The row is selected via another three
pins, and the value is read back on one final pin indicating if that key is
pressed or not. The three chips that do this are simple logic chips, the
keyboard scanning is all controlled in the MOS.

Emulating the keyboard is actually quite simple, just read in the 4 bit
column address and 3 pin row address and set the output according to
whether the key is pressed. So you could build something with a few logic
gates to simulate a key being held down for a short while after power on.

I think it was with the early PCs, RISC PCs and the 16 bit Amiga and ST
generation that they started to use dedicated keyboard controllers, most
things before then were all row / column matrix scanned for a couple of IO
ports (or in the case of Sinclair computers, the address bus and part of an
input port).

Dave


On 5 January 2015@..., Daniel Beardsmore <public@...> wrote:

> (Quoting myself! ;)
>
> > I assume that the keyboard controller code is actually inside MOS
> > itself, which uses the system VIA to communicate with the chips on
> > the keyboard PCB.
>
> Having read that page John posted, it's both more and less complicated and
> strange. The keyboard is capable of figuring out for itself *when* a key
> was pressed, but then forgets what it was and makes the computer figure it
> out for itself :P
>
> I don't know quite honestly when dedicated controller chips came into
> being, that just did all this for you in a single chip. Something I need to
> research one day.
>
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