This PCB encodes a keypad to output a binary value DCBA (D is the MSB and A is the LSB). The DATA output briefly toggles HIGH when a keypad button is pressed.
Pinout
Note that the empty header pin (all the way to the right in the diagram) has no connection.
Hardware
This PCB uses a 12-key keypad and the 74C922 keypad encoder in asynchronous mode.
Schematic
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