The buttons and joysticks on the control panel as well as the service buttons located inside the coin door and the coin mechanism switches are all hooked up to a keyboard encoder salvaged from an old keyboard. In the world of MAME cabinet building this is known as a keyboard hack. When I started working on the hardware for SpiffMAME, I found this was the easiest way of hooking up all the controls to the computer.
The image above shows the keyboard hack. The IC on the bottom of the circuit board is a 4066 quad analog switch, which is used to allow the 1UP and 2UP buttons to generate coin events (selectable by a switch on the service panel inside the coin door). The harness of colored wires go to two connectors inside the coin door. On the right you see the keyboard wire, which goes to the connector inside the coin door.
Inside the coin door, A DB-50 connector hooks up to the control panel. For the coin switches and the service panel inside the coin door, a separate DB-25 connector is used. Both connect to the keyboar controller shown above. Note how the keyboard connector is looped back through the coin door, and from there to the PC mainboard. This allows the keyboard hack to be disconnected and a regular keyboard to be connected instead (for maintenance).
The monitor is also looped through here, allowing the connection of a regular PC monitor in case the graphics adapter is set to a resolution that the arcade monitor cannot handle. The switch next to the monitor connector goes to the power relay, allowing the monitor to be turned off.
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