OK I confirmed I cannot run Modbus using the RJ45 on the WJ200. Works perfect on the terminals though. This is kind of nice because you can run the op panel on the RJ45 and still control via modbus on the terminals
I also did not need a fancy modbus controller. In fact mine was less than $6 shipped. Search for "New 9 PIN RS-232 to RS-485 Adapter Interface Converter" from seller id eherebuy in Roselle Park, New Jersey, United States if you want to see the exact one I bought.
I have it working off a built in serial port on one computer and a PCI Rosewill RC-03 2serial1Parallel card from amazon installed in a second computer. Both work great.

A link for as long as it will work to the RS485 adapter I confirmed worked with a WJ200
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=190539776031