I'm programming a Concentrator that queries as a master a set of automats using Modbus. This concentrator acts too like a Modbus Server for a HMI/Scada. Its memory is like a copy of the automats' memories.

If the communication with an automat fails, what's the best practices to show to the HMI/Scada these issue using Modbus protocol? Maybe reserving 0xFFFF value for an "error value" in registers? But in coils I think I have to reserve an address to indicate that values from N to M are incorrect...

How do you solve it using Modbus?

 

If you can map a register to a clock second counter (or any counter), your master could then determine loss of communications if the register value were to freeze.