The IPAC supports two full sets of key mappings.
The first set is the normal mapping.
The second set is accessed by holding down the P1 Start button.
So for example, if you push down on the P1 Button 1, it normally sends the key LEFT-CTRL to MAME.
But if you hold down the P1 Start button, and then hit the P1 Button 1, it sends the key "5" to MAME, which adds coins.
By default, only a few of the buttons have shift mappings, but you can map more. The default key mappings are here: http://ultimarc.com/ipac2.html
Do any of the default shift mappings work for you?
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