This question is an exact duplicate of:
In my current project I have to communicate with 10 different devices with PLC.
PLC is the master. When input comes from PLC to a device, the Arduino controller through RS485, I want to check the input and perform an action accordingly and once completed need to send acknowledgement back to PLC.
Communication protocol is MODBUS can some one suggest something on it.
I am completely new to Arduino and PLC.
If there is any sample program which I can check, it will be more helpful