Some more 360 controller mod details

I received the following email about this post:

Hey,
you got any secrets for a novice modder? I tried making an Atari 2600
(no Kempston) port for an Xbox 360 controller I’d torn apart for
experiments, but haven’t had any luck making it work… it just makes
the Xbox 360 go haywire. Would I need resistors to make that mod work
properly, and if so, which ones do I use? Do I need optocouplers as
well, or can I just bypass those? Where do I feed the grounding wires? Thanks much for your
time!

Here’s my response, in case it’s useful for anyone else:

The way that an Atari 2600 and Kempston joystick work is to
have a single pin as a common ground, and another pin for each
direction (and one for ‘fire’). One or more pins is connected to ground
when you press a direction (or ‘fire’).

If you try to connect
a kempston joystick directly to the pads inside the 360 controller, you
run into the problem that you have to connect one side of all the pads
together as a common ground. Also, when you press more than one
direction (plus ‘fire’), you are making all sorts of electrical
connections that the 360 is not expecting.

To solve this
problem I used the cheapest simplest component I could find that
electrically isolates the joystick from the 360 pads. I bought five
Opto-couplers for about $1 each, and drove them with power from the +ve
and ground straight from where the USB connects to the board. I used
five 1K resistors to limit the current through each opto-coupler to
avoid burning it out (there’s a small LED inside each one, and you
always need to limit current with an LED).

So each direction and
fire button on the joystick will cause a different opto-coupler to
switch on, which will make a different and isolated electrical
connection across each of the 360 controller’s pads. There are probably
many other solutions, this was just the simplest that I could think of.

Hope this helps, I’ll try to put a circuit diagram up on the site sometime soon.