The differential receiver is failsafe to invalid bus states caused by:
Receiver failsafe is accomplished by offsetting the receiver thresholds so that the input indeterminate range does not include zero volts differential. To comply with the RS-422 and RS-485 standards, the receiver output must output a High when the differential input VID is more positive than +200 mV, and must output a Low when the VID is more negative than –200 mV. The receiver parameters which determine the failsafe performance are VIT+ and VIT– and VHYS. As seen in the table, differential signals more negative than –200 mV will always cause a Low receiver output. Similarly, differential signals more positive than +200 mV will always cause a High receiver output.
When the differential input signal is close to zero, it will still be above the VIT+ threshold, and the receiver output is High. Only when the differential input is more negative than VIT– will the receiver output transition to a Low state. So, the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis value VHYS (the separation between VIT+ and VIT– ) as well as the value of VIT+.