How do you monitor communications on a positive-grounded device from a negative-grounded desktop?
There are isolated USB-RS-485 converters you can buy. They allow data flow while keeping the grounds (indeed, all signals) on both sides galvanically isolatd from each other. They have their limits to how much isolation they can provide between both ends though (usually limited to ONLY a few hundred or thousands of volts).

If the adapter requires a power input on both sides, ensure that the respective power supplies used are also isolated (like a wall-wart with a transformer inside and the like), especially on the end your your device is on or else you'll run into the same issue as before.