Reverse engineering of a industrial controller

I would like to create a software, which read and sets parameters to a industrial controller(rs232). I have the orignal software, it also needs a usb dongle. But I think the dongle is just to unlock the software. Because after software start up I can take off the dongle and read and write to the controller. Without dongle I can t start the software. The best for me, if possible, would be to "sniff" the controller and use this information to create a C sharp software. My problem is, that I don t have any idea how to or even if my plan is possible. Can you help me?