I had half way done with the keyboard hack because six months before the starting of this project I built a control panel prototype just to know if it would work.
I got an old PS keyboard and removed the printed circuit board (PCB). Then I soldered a wire to each pin of the PCB (if I had to do it again, I would use a ribbon cable instead). With the maylar and the keyboard layout I was able to determine the definition of each key.
I needed to handle 24 keys:
4 keys for player 1 joystick 4 keys for player 2 joystick 6 keys for player 1 buttons 6 keys for player 2 buttons 1 key for escape button 1 key for insert coin button 1 key for one player start 1 key for two player start
Most games will work OK with this setup.
One of the most difficult things with the keys selection was to avoid the "ghosting" and "blocking" got in some cases:
What this means:
Ghosting: when you press 2 keys you got a third Blocking: pressing certain keys combinations they blocks others
I created a reference sheet as a helper to choose the right keys as you can see in the picture.