What are the negative PINs for on the IndustrialShield M-Duino 58 Analog
It looks like those inputs are opto-isolated. That means that the "input" goes through a resistor and the internal LED of an opto-isolator, which then comes back out of the "-" pin:







simulate this circuit &ndash; Schematic created using CircuitLab

So "-" connects to the ground of the remote device, and the "input" connects to the signal.