If you can live with +-50 V isolation, why not use a chip such as MAX3157 @$3.50 it provides galvanic isolation and differential output. If you need higher isolation voltage, it is available, but at a price. No book or professsor ever said that 422/485 chips can only be used for data transmission, they work well for jusr signals also. Such a differential setup, properly terminated, would make your cable virtually immune to noise.

Erik