Thanks for your helpful replies.
After investigating RS485 transmitters and receivers, I agree that they would make a good solution; and it is useful to have learned about them. However, I have decided to try a different approach in this particular project - merge the two boxes into one and eliminate the communication problem entirely. If this works, I will be relying on up to 5m USB connection between the PC and the USB controller, but then there is no need for so much twisted pair communication from USB controller to other parts of the system.
Regards
Grevillea