RS-485 Idle Failsafe Is it really needed?
Normally the master device controls the bus, and has its output driver enabled so it can drive the bus to an "idle" state when there is no traffic. When the master sends a request to a slave it must immediately release the bus (by turning off its output driver) so the slave can reply. It will take the slave a finite time before it can enable its output driver, which may be just microseconds or milliseconds. During this time the bus could float to levels which look like a logic 0, ie a start bit, which may confuse the master which is looking for a start bit. By having failsafe biasing of the bus you eliminate this possibility since the bus is always pulled to voltages which represent an idle condition.

Note: a lot of newer RS485 transceivers have the failsafe biasing built in.