Just threw this together to show my friend. Made this several years ago from scratch. It used to be part of a full size arcade unit Chris made, but it was taking up too much space, so now i just have the controller.
The Ipac board plugs into a computer via usb/ps2 and is basically just a keyboard. The buttons and joysticks are real arcade controllers.
It is setup with full 6 buttons on players 1 and 2 for "street fighter" style games. Players 3 and 4 only have 3 buttons as most 4 player games use only 3 buttons (TMNT!).
The middle 5th joystick is a special old style joystick for games where you do not want more than one direction at a time. In newer games, when the up button and the right button are both on, the games sees your joystick in the top right corner, but old games like pacman dont know what to do with corners on joysticks.
The Pod of 5 buttons are for "enter/esc" buttons and the other two are for selecting the special functions inside of arcade games so you can adjust settings.
The wiring looks crazy, i probably could have cleaned it up, but basically the Ipac board has a spot representing every key on your keyboard. One wire from every button (joysticks have 4 buttons) is screwed down to one of these terminals. Than you just daisy chain a ground wire around to the other side of every button and back to the Ipac.
Since it shows up as just a regular keyboard you can use it for any piece of software that runs on your computer. You could take it and make up your own custom controller that has nothing to do with arcade games.