General Instruments Capacitive Keyboard Encoder

[Tony Duell]

On Tue, May 4, 2021 at 10:33 AM Paul Birkel via cctalk
<cctalk at classiccmp.org> wrote:
>
> I'm currently reverse-engineering an AMPEX keyboard that uses capacitive key
> switches.  The basic design employs a GI encoder coupled to an 8039 MCU
> supplemented by a 2K EPROM and 74LS373 (used to latch the ROM address set
> from Port A while Port A is then used to read data back from the ROM).  The
> 8039 MCU drives a bit-banged serial interface.  The PCB identifies itself as
> AMPEX on the coper foil, although the key switch mounting-plate actually has
> a "General Instruments Quality Accepted" sticker.  The EPROM is labeled
> "3512663-03 Copyright 1983 AMPEX CORP".
>
>
>
> The GI encoder is a DIP-40 labeled as "321239007  M2406-054-02  GI 8233 CBU
> TAIWAN".  I seek technical documentation for this IC.

You might take a look at the manuals here :

http://cpu-ns32k.net/Whitechapel.html

I am pretty sure there's a keyboard techincal description in 'binder
1' and a reverse-engineered schematic in 'binder 2'. While it's not
quite the same IC, it's related and the power pins are in the right
place :-)

Alas there is no real description of what that IC does or how to talk
to it from the 8039. It is designed to sit on the 8039 bus, it takes
in the multiplexed address/data bus, ALE, rd/ and wr/

-tony