Perhaps worth considering an established standard. The problem with the proposed scheme is that if you're just testing for
anything positive or negative compared to the ground line, then what's the point in driving to +-15V?
Surely you should be testing for anything (say) greater than +10V or less than -10V?
If you use an established standard, you'll have lots of info on bitrate possible, distance achievable and
lower cost design because you can use off-the-shelf driver ICs.
Plus, more people can interwork to it.