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
