First question: where are you putting your ground lead of your meter when you are measuring those voltages?
So no 110VAC and no USB. Presumably you still had your motor drivers connected when you measured that 2nd set of voltages on the PMX-424 connectors, right? If that is not the case, exactly what did you have connected to the PMDX-424? And what motor drivers are you using and how do you have them wired (both power and step & dir signals)?
If you really did have 15V on the step and dir pins, that might possibly have fried whatever output buffers the PMDX-424 has on it. But first check the wiring.