Communication Problem between MODBUS RTU and Siemens PLC


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i'm trying to communicate a siemens S7 200 to PC software master (mb master from National Instrument) using MODBUS RTU. i'm a beginner on this so i need some help. my problem is the hardware. why does not work the PPI/PC cable (not multimaster) it is only i have. what kind a rs232/485 converter must buy? what type of cable (UTP, Profibus, other)?

i have the libraries installed; speed: 9600 baud; non parity; 1 stop bit; 8 data bits.