I'm not directly familiar with the IOB, but most industrial I/O is simply very high-impedance signals that flip between states of 0V or 24V.
The 0V "ground" (not earth ground, but the 0V reference for the DC signals) needs to be linked between both units, and the inputs work by detecting the difference between the 0V reference and the individual input terminal voltage.
As long as the output can drive to VMax and VMin, and the 0V reference is shared between both, there's normally negligible amounts of current to be concerned with. I don't recall ever running into a fully-rated industrial I/O where an NPN/PNP issue made a difference.
Of course, there can be cheap units out there that break the rules. I once encountered an "industrial" motor controller that turned out to be all 5V TTL logic, and whose interface document didn't mention that... I found out the hard way after connection my 24V I/O as usual and smoking the controller.
So, probably part of your answer involves what brand PLC you have, and what the ratings of the I/O are.
If you're concerned, you might be able to add a high-value resistor in line for current limiting. If you're really worried, and can't afford even a small chance of smoking a component, the safest alternative is to build a relay board in between the two units. The robot outputs would drive the coils of the relays, and the PLC inputs would be wired to the relay contacts. Opto-isolators of the correct voltage would also serve the same purpose.
you can... but you still need to reference them to same potential.
IOB has 16 digital inputs and 16 digital outputs.
both of them can be PNP or NPN.
manual for optional interfaces has example connection diagrams.
