Keypad Errors - Mainboard Component?

There has been a common issue with the Passport 2 Monitors where all 3 buttons under "Print" are not registered in the Keypad Diagnostic Test, and there is a double beep whenever those three buttons are pushed. I know it's a mainboard issue and think it might be the communications processor specifically -- has anyone come across this problem and know whereabouts on the mainboard the issue is occurring?