Fail-safe is a technique used in RS-485 interface transceivers which forces the output to a predefined state in the event of a line short or open circuit.
Maxim's RS485 tranceivers features fail-safe or true fail-safe. The difference is as below,
True fail-safe receivers guarantee a logic-high receiver output when the receiver inputs are shorted or open, or when they are connected to a terminated transmission line with all drivers disabled. This guaranteed logic high is achieved by setting the receiver threshold between a minus voltage (like -10mV to -50mV) and -200mV.
Fail-safe receivers guarantees a logic-high output if the input is open circuit.