ADM1485 datasheet p.3 Table II specify when differential input A-B is <Inputs Open>, RO output becomes to logic High.
Here, are you talking Inputs Open by defining A-B actual differential voltage is from -0.2V to 0.2V, isn't it ?
By another word, when it knows differential voltage is between -0.2 and +0.2, ADM1485 recognizes it is open and put RO output to logic high ?
Could you please confirm.
Another confirmation and maybe double check same as above, that even when it is terminating the extreme ends of line using resistors, and opposite node becomes to be high-impedance, the feature of fail-safe still be working good ?