Controllers That Don't Use A Common Ground

[Blaine]

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?

[NFG]

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.