Modbus write_register problems

Hi there all.

There has been some questions asked about this issue (ie: having trouble with the write_register Service) back in 2017 and 2016 but I have found no positive resolution for this problem which I am also having.

In the components entry for modbus it states:-
write_register Requires unit, address and value fields. value can be either single value or an array.
My question is this…What is the ‘unit’ and ‘address’ values it is referring to? Could it be that they are looking for ‘slave:’ and ‘register:’ values really?

I ask because I am also having trouble writing a value to a register. I can easily read and write coil values (switches) no problems at all and likewise reading sensor values is fine but writing to a setpoint held in an analog flag on my PLC is not working at all.

Thanks in advance for any help with clarifying this issue for me!