Actually, the PC has an RS232 port, so I had:
PC --- 232<->485 --- controller
What I want is, as far as the "DriveManager" (motor controller software) sees it, the exact same thing. I want it to use the virtual COM port emulated by the ATmega16U2, then I want the micro controller to blindly pass the bits to and from the RS485 port of the device. The only thing I see standing in my way is knowing when to switch from sending to receiving.
My idea is to switch to send mode when receiving a start bit from the virtual RS232 port (pin 0 on the arduino). Switching back to receive mode after a specified number of silent bits is a good idea, I'll probably try that.
Another problem that may surface is if the UART of the micro controller shows some activity, but I haven't started researching this.