Yes, 485 is balanced and most likely would need additional wires.
CAN is kinda balanced and you need two wires - but you _may_ be able to
re-purpose your TX and RX lines and use them as CANH and CANL, then send
and receive on the same pair. That probably falls apart because your higher
protocol needs a collision management mechanism... maybe :-)
232 is probably the only one you can implement without regard to the wiring
or protocol. Essentially you'd just be changing signaling levels. (typical
MAX232 would give you about -9V for a logic one and +9V for logic zero)

-Denny
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