Hello
In my project i will have 8 digital outputs from arudino which will be used by an OMRON PLC (supplied by 24VDC).
I have a board with 8 relays:
My idea is that:
- Supply the relay board from arduino +5V and GND
- IN1-IN8 are arduino digital outputs. ( I tested that the relay signal is okay for Due's 3,3V).
And now there is a part for the output of the relay. I can use the +24VD from PLC so let's say the +24V (one wire) comes in from PLC and i split it in relay (COM and NO) for example the wire comes to COM and comes out from NO to the output like this
:
BUT because my project will be basically an IO module for PLC I am thinking, what with the GND from PLC ? 
I am not sure if i should connect it somewhere in my module or not and if I should connect it should I just connect it to the output so the user can take and use this GND to some other parts.. BUT I do not know how I should connect the GND and where.
Do i need to use the GND and if I need then in what way ?
PS I think I also need some fuse at the +24VDC wire and maybe some resistors, can someone provide me some values?
Or I should add the GND wire like this :
