Advanced Gamepad Hack Wiring Question

[Tiger-Heli]

I am considering two gamepads to hack, a gravis pro USB (14 inputs, no common ground), or a Wal-Mart USB (13 inputs, common ground).  Problem is I would really like 15 inputs: 4 Directionals, 6 buttons, Start 1, Coin 1, Start 2, Pause, and Escape.

My thinking: In MAME, I can map Escape to ?Joycode_UP AND Joycode Down?  and Pause to ?Joycode_RT and Joycode Left? and this should allow me to exit the game (or Pause) with no chance of accidentally activating the functions.

Questions -

For the common ground, I assume I could just wire a button with one terminal connected to both the UP and Down inputs and the other connected to GND and the button will activate both directions and exit MAME, correct?  (Or do I need to do something with diodes to make this work?)

For the non-common ground, let?s say UP is Terminals A and B and DN is Terminals C and D.  I don?t think I can wire a button to A and C on one side and B and D on the other, or if I did, would MAME interpret this as 5 Buttons pressed (A-B, A-C, A-D, B-C, B-D) and still Escape, or would it fail.  Is there a way to make it work with diodes, perhaps?

[RandyT]

I am considering two gamepads to hack, a gravis pro USB (14 inputs, no common ground), or a Wal-Mart USB (13 inputs, common ground).  Problem is I would really like 15 inputs: 4 Directionals, 6 buttons, Start 1, Coin 1, Start 2, Pause, and Escape.

My thinking: In MAME, I can map Escape to ?Joycode_UP AND Joycode Down?  and Pause to ?Joycode_RT and Joycode Left? and this should allow me to exit the game (or Pause) with no chance of accidentally activating the functions.

Questions -

For the common ground, I assume I could just wire a button with one terminal connected to both the UP and Down inputs and the other connected to GND and the button will activate both directions and exit MAME, correct?  (Or do I need to do something with diodes to make this work?)

This should work with diodes, but may depend on the way the pad is designed.  There may have been some shortcuts taken, as this would never occur when used as it was designed.

For the non-common ground, let?s say UP is Terminals A and B and DN is Terminals C and D.  I don?t think I can wire a button to A and C on one side and B and D on the other, or if I did, would MAME interpret this as 5 Buttons pressed (A-B, A-C, A-D, B-C, B-D) and still Escape, or would it fail.  Is there a way to make it work with diodes, perhaps?

If this works at all, diodes would definitely be in order.  Certain combinations of those lines could be very bad depending on the design of the circuit.   Even diodes might not isolate things well enough if too many random lines get crossed that shouldn't be.

Without knowing how the circuit works, any activity like this is a crap shoot.  

RandyT