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.

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

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  • Looks like the cathode of an opto isolator maybe? Would need a full schematic not a pinout diagram. The manual should tell you. – Majenko Nov 8 '18 at 9:45
  • Thx, i guess these are the GND connections. Did not see it when i read the manual the first time. – Holger Nov 8 '18 at 10:04
  • Yeah, I think the inputs are all opto-isolated - so the input is actually a resistor and LED, with the "input" being the anode and the "-" being the cathode. It's hard to find any real information though, documentation seems to be a little poor. – Majenko Nov 8 '18 at 10:07
  • The Manufacturer's documentation is at industrialshields.com/… – Dave X Dec 29 '20 at 14:33

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 – Schematic created using CircuitLab

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

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