General Instruments Capacitive Keyboard Encoder

[Paul Birkel]

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.

 

It evidently is not a relabeled simple variant of the documented AY-3-4592
as it does not multiplex the input side of the matrix (sense lines), there
are fewer output data lines, and the power pins are non-standard (Vcc = pin
37; GND = pin 16).

 

Reverse engineering identifies this M2406-054-02 as supporting an 8 column
by 16 row (3 unused in my case) matrix plus 8 output data lines. I can
identify analogs of several pins on the AY-3-4592.  My interpretation of the
pin uses is that the necessary key-scanning behavior is generated using the
8039 ALE line (pin 11) as the encoder clock input.

 

Proper documentation for this IC would be nice to come by!  Pointers and
suggestions appreciated.

 

Thank you,

paul