Hi Alexander,
Thanks again for your assistance.
Yes, when i looked up the corresponding meanings of Modbus status codes 4 and 5, i saw that it was an issue of not being able to connect to the target slave controller.
Further to what you're saying, the Modbus RC operation is successful for the 2 controllers that are actually connected in this test setup.
What i need though is for the master controller to keep looping through reading the coils on each of the slave controllers after a say 5-second interval. It's important for the master controller to be able to skip over communicating with an unreachable slave on each iteration and just carry on. Any thoughts on how to achieve this?
Many thanks again,
James