An RS485 transmission line enters an indeterminate state if no nodes are transmitting on it. This indeterminate state can cause the receivers to receive invalid data bits from noise picked up on the cable.

To prevent a line from receiving invalid data bits, force the transmission line into a known state. To force a known state, install two bias resistors at one node on the transmission line. This action creates a voltage divider that forces the voltage between the differential pair to be greater than 200 mV.
Note 200 mV is the threshold voltage for the receiver.