PLC Modbus communication

[huntleight]

How can i communicate with plc (Plc as a master)? How plc can change my parameter ? For example plc wants to change or read my maxhumidity parameter, how can plc know maxhumidity modbus adress(for example 4x2002) ? Can you explain all phase communication, i dont know maybe assign modbus address, if plc change my parameter how can i catch and change parameter, ..? I hope it was understandable.