If your just using the RS485 link to collect and monitor data from the DCS you shouldn't have a problem. However, if it is used to do critical control (such as safety or interlocks) you probably shouldn't use it at all and just stick with discrete wiring.
On another note, RS485 is pretty...
You might want to consider looking at a different software package that can do that. Most likely you'll need to find a 3rd party that has developed something custom or have a developer write you one. However, the cost of doing that will most likely outweigh purchasing an off-shelf-package...
Your best bet is to call your local Rockwell supplier that sold you the PLC. They usually have a tech onsite that can help get you going. Sometimes they'll come onsite to help out if you're in jam - especially if you are a customer that purchases alot of equipment!
