When using protocols such as MODBUS or OMRON it should be totally transparent to the PLC operation. The HMI will send commands to read or write to the I/O or internal memory of the PLC without the need to modify the PLC program at all. The only TBASIC command that you may need to execute once is the SETBAUD command which let you set the comm port to the same baud rate and format as the HMI if the HMI's comm baud rate and format cannot be changed to match the PLC's default of 38400,8,n,1.
The few most important points about serial communication between two devices are:
1) the correct communication baud rate and format (how data bits, what parity and how many stop bit).
2) the correct cable - there is no universal cable here. It really depend on how the HMI manufacturer wire up their RS232 port. Our T100MD PLC only uses pin 2,3,5 of its comm port. But the HMI may demand that its RTS be tied to its CTS be connected together and its DTR be tied to its DSR.
3) The correct ID address has been defined in the HMI that matches that of the PLC. OMRON default ID is 00. Try to change that and reset the device.
To ensure that that the HMI is sending out correct serial commands, try to connect the HMI serial port to a PC via a null modem cable and run the HyperTerminal on the PC, select "Properties -> Direct to COM1" and define the parameters as 38400, 8 data bit, 1 stop bit, no parity, and none for hardware flow control. If wired correctly the Hyperterminal will display the serial data from the HMI, especially for the OMRON protocol it should be in the form of "@01xxxx...". This also offers a way for you to examine whether the HMI is addressing the correct ID, with the correct baud rate and the correct comm format. If you get garbage then something is wrong.
We have experienced some brand of HMI that even though we changed the baud rate setting on the panel it simply refuses to change without some eloborate reset - it is very annoying. How do we know? We connected to a Hyperterminal and saw that the baud rate never change despite change of the settings on the HMI itself.
Many users have used our PLCs with SCADA packages such as National Instrument Lookout, Wonderware and Intellusion and HMI such as Maple System without trouble. It is a matter of getting to know your HMI well enough to get a successful communication.