S7-200, 224xp and modbus

dear siemens gurus, I am going to use the a PID controller which communicates on RS485 (MODBUS RTU, ASCII), and wishing to use it with s7 200 224xp PLC, Since i never done it...so is any body have used it ?. can it be done. as the 224xp manual says it has two RS485 ports.
any help on this regards will be highly appreciated.

Thanks I think the below thread can help me out..

http://www.plctalk.net/qanda/showthread.php?t=51162&highlight=pid+rs+485