The MODBUS RTU master sends out binary data. It starts with the 8 bit ID of the slave, followed by the function code (01,02....0F) and then the data. The MD1616+ will send back response in similar fashion. The most important thing to get communication working is to make sure that the host is using the same baud rate and data format as the PLC, also the ID must be defined to be the same as the PLC (default is 01). If you are using RS232 cable and the HMI expect hardware flow control, then it may be necessary to short the DTR to DSR and RTS to CTS on the touch panel side. You will have to refer to the touch panel documentation for details. The only wires need to be connected to the PLC is pin 2,3,4 and 5.
We have some application notes for using the Maple System and Taian HMI MODBUS setup that you can download from our website:
http://www.tri-plc.com/dataentry.htm - these should serve as good reference even if your touch panel is not Maple System or Taian. Some customers used the Automation Direct touch panel and reported smooth operation.
