This is probably very easy but I am very confused right now.

I have a card (machine) that has both inputs and outputs. Outputs are NPN (Sinking) and I want to record what they are doing on a different card (all inputs)(PC). I'm confused with how to hookup the "commons'. Since the machine card is NPN, then I would assume that the output signal is a 0V and therefore the common for that output is also 0V. Or is it VCC? Okay, whatever that signal is, it reaches my input card. What should the common of that input be? 0V?

Thanks for your help.

 

Yeah, connect 0V to 0V. You may need a pull-up resistor to VCC on the outputs if they are open collector (from your description it sounds as if they might be).

 

Be careful if there is more than one power supply involved. Current can flow from one to the other through the ground wiring. If there's any doubt, use a 1k resistor to connect them first and see if there is a voltage across that resistor.