Not at all, fail safe biasing helps a lot, remember what I wrote in my first post:


Let's discuss a RS485 loop consisting of one driver and one reciever:
If the cable isn't connected to the reciever then you get a potential difference of U(a-b) = 0.48V due to the fail safe biasing, assuming that the fail safe biasing network is located at the reciever input and consists of two 560 Ohm resistors connected from the lines to supply rails and a 120 Ohm resistor between the lines. In this case no relevant noise exists at input of reciever and the reciever provides a stable, non-toggling output, telling the user that something is wrong with the R485 loop, means that the cable is disconnected from the reciever.
If the master is powered off, but cable is still connected, on the other hand, then you get a potential difference between the lines of only U(a-b) = 0.25V, due to the second 120 Ohm resistor at the driver. And, noise can be superimposed and it's not guaranteed that output of reciever keeps non-toggling. Then, the user might not reliably see that something is wrong with the RS485 loop.

Or by other words:
Regular signal of RS485 loop is about U(a-b) = +2V and U(b-a) = -2V. But what you plan to do is to provide a signal of U(a-b) = +0.25V and U(b-a) = -2V. Now take into consideration that input threshold voltage of reciever can range between -0.2V and +0.2V, with an hysteresis of only 70mV. So, what if input threshold voltage of your individual reciever chip is just +0.2V? How much noise margin will you have with the fail safe biasing "signal" then? Only terrible 0.25V - 0.2V = 0.05V! What noise margin will you have with regular signal? 2V - 0.2V = 1.8V!
So, with your driving methode you will decrease the standard noise margin from 1.8V down to 0.05V, means you end up with only 2.8% of standard noise margin!

Kai