The main parts of a PC keyboard are the keyboard controller and the contacts for each key.

There are 2 sets of wires for the contacts, forming a matrix. The contacts are connected to one wire from each set. When a contact is presesd, the circuit is completed between the two wires. The keyboard I hacked had 16 wires in one set and 8 in the other, giving a maximum of 16x8 = 128 keys.

The controller checks for key presses by passing current through each wire in the first set, in turn, and seeing which wires in the second set have current. It then translates the info into something the PC can understand. The limit to the number and combination of keys that can be held down together comes from the controller.

The 'keyboard hack' involves removing the controller from the keyboard and wiring up the cabinet controls to your own matrix. The keyboard I used was an old IBM ps/2 model. I had several of them sitting in boxes in the garage, so there were plenty of backups in case I killed one. Inside the keyboard, the matrix was printed on 2 sheets of plastic. The contacts were seperated by a third sheet. The controller was on a small PCB, hidden underneath a sheet of metal supporting the matrix.

I mounted the controller together with a hobby PCB to make it easier to work with. The first photo below shows the state of the controller before I started working on it. The three black connectors on the top, from left to right are for the 1st set of 16 wires, the 2nd set of 8 wires and the keyboard LEDs. The LEDs are still plugged in at this stage and you can just make out the tracks on the plastic that connect them to the controller. In the second photo, I've removed the connectors and wired the controller to the hobby PCB with some old floppy-drive ribbon cable. It took a while to solder all the wires in place, but it made the rest of the work easier.

I placed the controller between player 2's controls and the cabinet controls. The mounting is hinged, and swings open, for easy access. The next job was to find out where the necessary keys were on the matrix. To do this, I connected the controller to an old 386 running the same program I used to wire up player 1's controls. The 386 was used instead of the cabinet's PC as it's fairly easy to blow up the controller and/or the keyboard interface on the PC. The matrix I came up with is shown below. It's actually using MAME's standard settings for player 1, as the player 2 standard key assignments have always caused problems when multiple keys are being held down. The 5, 1 & 2 keys are used for the coin-mech, 1up and 2up respectively.

This is the final result. If you look carefully, you can see that the ribbon cable for the set of 16 wires in the matrix is now wired underneatch the PCB. I had to replace the keyboard controller as it broke sometime when I was either fitting or testing it (oops!). The keyboard LEDs for Num-lock and Caps-lock are wired to the 1up and 2up buttons on the front panel. The buttons light up when enough credits are inserted in some games.