I've noticed that I have two USB PS2 Style controllers and a Dreamcast controller that don't appear to use a common ground for the digital control pad.
Can anyone shed some light on this?
Apparently the XBOX pad doesn't either.
It wouldn't surprise me to find that some controllers use smaller/cheaper chips that can handle fewer inputs, so they poll different parts of the controller separately. The PC Engine for example polls UDLR/I,II,SEL,STA/----/III,IV,V,VI (four passes, 12 buttons) and the Saturn polls similarly.
There's no need to have a common ground. It's weird, but probably seemed like a good idea at the time.
Makes 'em a PITA to mod tho.