What would be a reliable and simple tranceiver interface/protocol for board to board serial communication at about 100ft. [9600bps or lower is fine.]
The two-lead communication wiring cannot be changed and is un-twisted, unshielded wire [~25pf/ft] in commercial buildings.
12VDC rail is already available.
Specifically, I've looked at Modbus/RS485 and RS232. Most available and low cost [~$0.50] chips seem to either run 3V3 or use capacitor charge pumps to level shift to 6V.
What's typically used in these scenarios?