So the Arduino is an interpreter between the full-duplex RS232 and the half-duplex RS485?
The lost cable must have had some smarts, I don't think it could have been a simple cable unless a handshaking signal was used to control some hardware in the cable.
The Arduino cannot talk to RS232 without some extra hardware.
The communications is bi-directional.
If the above is true I cannot see any way to do this without hardware to convert both the 232 and the 485 levels. The best way to do this is with transceivers for both, the 232 can often be fudged with a few components and the 485 can partly be fudged as we've talked about.
It's possible that the A signal coming from the controller could be used directly into an Arduino pin, but it's far from guaranteed and it depends on the driver chip they are using and the value of the terminating resistor (if any).
All I can suggest is that you try it, run the A signal into a pin and use software serial to send a command then turn off the TX pin and listen on the Rx pin.
You then have to interface with the RS232 from the PC, at minimum this requires a diode (A to GND, K to signal with a small resistor (220R maybe) in series with the signal) because the -12v will kill the Arduino. If you are lucky that should be all that is needed on this end.
RS232-USB adapter
Where does this come into it? If you can talk USB none of the RS232 stuff matters.
Rob