The signal should be above +/-400mV if I recall, and bellow +/-200mV it is undefined. The special fail safe transceivers... wait is that why it works? Yep, the MAX3085 is one of the failsafe parts, it sees everything above -50mV as a HIGH. Are there resistors between A and +5V as well as between B and 0V. I guess that would do it.
What I am doing is just as outlandish, I turn everything off to send a HIGH so on the pair, 0V is a HIGH bit and -3V is a LOW bit. The transceivers seem to be cool with the HIGH (so to speak) and so is the battery.