A true fail-safe RS485 receiver such as MAX14946 can ensure that receiver output is logic high when the bus is short or open. They do not require external pull-up or pull-down resistors.
For parts that are not true fail-safe, RS485 network requires pull up (A) and pull down (B) resistors to guarantee that the receiver output is at a known logic state (logic high) when it is disconnected or the bus is open. Since receiver's differential threshold for logic high is 200mV (for components that are not true fail-safe), the pull-up, and pull-down resistors need to be selected such that the voltage between A and B is larger than 200mV when there is an open.