Well you might be able to get a card for the Omron PLC that talks Modbus. There's a company called Prosoft that makes one for Allen Bradley PLCs, so maybe they have one for Omron. Otherwise, Omron has written a protocol macro for the C200H alpha and the CS1 which will allow it to make Modbus RTU communications out the communications card.
If all else fails, try NR&D for the QUCM module. It can be configured to communicate with any RS232 or RS422 device. They will even do all the programming and setup for you if you provide your specificaitons. Hunter Farris
We have used an EXOR operator terminal to connect an Omron PLC (Host Link) and a Modicon PLC (Modbus). We simply connected one op.terminal to each of the PLC:s, they have more than 100 different drivers, and then the terminals can be connected in a fast network. It works very well.