I recently picked up a numeric keypad from a thrift store. I couldn't help myself because it was new in the box, has an RS232 connection and has nice clicky mechanical buttons. Mhmmmmm. I just love electronics from the 80s and 90s that is new in the box. It's like someone saved it for me for 25 years.

Anyway, it has a 9 pin serial pass through that it claims can coexist with another serial device like a mouse. First question: how does a keypad and mouse use the same COM port simultaneously? Wouldn't the driver for each one want exclusive access to the same COM port? And how does each driver know what data is for it? Or maybe the keypad doesn't use Rx and instead uses a handshaking line.

Second, when I opened the unit (because why wouldn't I) I was shocked to not see a microcontroller. But maybe that's because I tend to use micros for lots of things because they're convenient. It only has four 4000 series logic ICs. A 4009 inverter, two 4015 shift registers, and a 4098 multivibrator. Ok, so I can start to see how a couple shift register chips could be used for this. Perhaps the keypad driver is toggling one of the RS232 lines as a clock? And not even using RS232 in a standard way, but as I mentioned earlier, using handshaking lines in a non-standard way to transfer data.

I did take photos of the top and bottom of the board and might do a half-assed reverse engineering just to satisfy my curiosity.