Does anyone have experience with Modbus over RS232 via wireless (900MHz)?
We're in search of either a full time or contract engineer to assist with design improvements on our product. We're using PLCs with a HMI as a remote control unit.

What kind of radios are you using?. The most radios in 900 MHZ band are really "transparent" so the only you should have present is the cabling between the radio and and PLC and radio and HMI. The RS 232 ports in the radios are generally DCE and the ports in the other devices are DTE so the cable should be direct connection (RX - TX).

1) If communication is established then you have connected correctly i.e. as Diana mentioned (RX to TX) and Control like RTS and CTS.
2) Your devices (PLC and HMI) are Modbus complaint hence RS232 Port are using Half duplex Protocol where Radio Modem is Point to point which may be configured for Full duplex protocol.
3) Modbus Read and write data blocks must not be more than 62 words (16 Bit) and unidirectional means can not be initiated simultaneously.
4) Update rate must be slow i.e.matching with RS232 Baud rate (typically minimum 1 sec0
4) Have you tried Prosoft radio modems
Please visit my profile, I am available immediately on either full time or contract (employment or job based) but have no authorization to work in USA.

P.S. -- Please make a dedicated portion of Data table (or Tags) in PLC for HMI avoiding scattered dat