What are the negative PINs for on the IndustrialShield M-Duino 58 Analog I am working as an internship on an Arduino based PLC. It is the M-Duino 58 Analog from IndustrialShields. I try to figure out how it works; Reverse Engineering. But i dont know why it has negative Digital Inputs like normal Digital Input I1.0 also has a connection -I1.0.
It is probably a simple question but i cant figure it out. Hope you guys can help me.



I guess i figured it out. Seems to be the GND connections. Should read the Manual better next time.
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.