Thanks for ALL the answers!!! 
… still, I find extremely hard to pull out some details from you 
So, if I understand you right:
-
You have a PLC. (But it’s a secret what kind of…
) -
OH is not running on that, but somewhere elsewhere. (It’s probably a secret too?
) -
There is one cable starting from the PLC >> running around your place and there are 5 Arduino boards attacked to that bus. (Or is it a star topoligy width 5 cable running in 5 direction?)
… this remains a mistery forewer. -
I guess they are connected with these pins: RX0=30 + TX1=31 ?
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and the sensors (which type and brand can not be ever reviled
) are attacked to those Ardu boards in a short distance. -
Or i2c ?
Well … if you don’t like sarcastic humour > you won’t ever answer to these question and we’ll never know. 
-
You don’t really use any of the digital ports for switching things, only the analog pins to read the sensors. Right?
Oh noooo !! Apr.1. past away. I’m a bit late.