3 button JAMMA conversion. Getting really confused with the controls!

[TedZed]

Pi Model or other hardware: Pi 400
Power Supply used: Official Pi 400
RetroPie Version Used: 4.8
Built From: Pre made SD Image on RetroPie website
USB Devices connected: IPAC2
Controller used: 2 player, 3 button JAMMA controls.
Emulator: MAME 2003

Hey all,

I've just finished the restoration/conversion of an old two player, 3 button JAMMA cab (as in: each player has a stick, three "action" buttons and a start button) connected to an IPAC2, with the action buttons hooked up to the IPAC's SW1, 2 and 3 respectively. I also have 1A, 1B, 2A, and 2B wired up to discreet buttons under the panel. My plan is to use this exclusively for MAME, which makes sense from the perspective that the default IPAC inputs are setup to match MAME's defaults, but I'm getting my head in a spin over all the different places that controls are defined.

Firstly: Emulation Station. I can't remember exactly what order it wants me to define controls, but it seems like I should be skipping either X or Y - whichever comes first? All it seems to do is arbitrarily jump around the list of games (changing sort order maybe?) whereas the other button at least does add/remove from favourites.

Am I also right in thinking that the ES controls are exclusive to this program and don't affect anything else? Except for maybe the Hotkey that I could assign to one of the under-panel buttons.

RetroArch: I think this is where my main confusion lies. It seems like it's overriding or perhaps adding to/doubling up MAME's controls? One of P2's buttons was affecting P1 and I think I tracked it down to a conflict with MAME's defaults in RetroArch's cfg file. What should I be doing here? Set everything to "nul" and let MAME handle inputs or should I set everything to exactly match the IPAC/MAME's defaults?

MAME: Same question as above I suppose. The odd thing I found with this is that when I tested the build with Golden Axe, a 3 button game, my inputs were doing attack, magic and the special move that's supposed to be done with jump and attack together. Jump itself was missing and seemingly assigned to input 4? Again, I can't tell if this is due to MAME's default setup being odd/broken here or if it's RetroArch overriding things.

It always helps me to type things out and now that I have, I think that maybe RetroArch is what's causing my confusion, but some clarity on what to configure and what to leave alone would be amazing if available! I did think about adding a forth button per player but the vast majority of games I'm interested in only use 1-3 buttons anyway. Should I perhaps re-wire SW3 to SW4 for simplicity?

Thanks a lot for any help. :)