So, I decided to hook up a DB9 terminal, so I could plug in some controllers to a DMG. Now, like the color, or sp, I was hoping that just connecting one end of some kynar to ground, and poking a bunch of stuff with the other would let me find some places to tap controller signals. Boy was I wrong. I found up and right, but right always hit's b, and then if you hold it doesn't do it again. Up is normal.

Odd things happen too, like start plus ground = select, down plus ground= select, it looks like select, and right, and tied to basically everything. I tried looking at the signals from the connector to where the top board connects to the bottom one, and ran into signals that hit a few buttons at once.

So if anyone has a pinout, or possibly some advice, I would appreciate it.

Tried this, the DMG does not have a full common ground system, sadly. I have used the DMG board as a gamepad board for a GBA (Very close to what you are doing, pretty much just the reverse), and to do it, I had to cut a lot of traces, remove a lot of parts off the board, and wire it all together to make it a common ground setup. Would not work in a DMG, though.

You COULD do what I just said, but hook it up to a GB Pocket or GB Color mobo instead, it would look stock on the outside.

Or, do what I did, and use a GBA SP, as I did in my "Power Gameboy" last year.

Yeah, that's what I feared. What I might just do is use a neo geo controller port, and just wire up every trace. Worst case scenario I will need two wires per button, but already I got two working with the same ground, so I think it will work alright.