Here is another possibility, assuming the motor controller does indeed have RS-422: I have used a module(Micromint Domino) that had RS-422 and it would talk to an RS-232 port on a PC by tying the TX- pin of the module to the RX pin of the PC, the RX- pin of the module to the TX pin of the PC, and the ground pins together. I do not know if the module had any pull-up or pull-down resistors on any of the RX or TX pins. This setup does not give you true RS-232 signaling, but I used these modules with several computers and none of them had trouble talking to the modules.