i'm trying to communicate a master software (from National Instrument) on PC with a s7 200 Siemens PLC using MODBUS RTU. the problem is that cannot get it. the s7 200 has the libraries installed. i bought a rs232 to 485 converter but the master software say "the port is busy" and not connected. what kind of cable do have to use? between converter and PLC. UTP? profibus? other?