The differential receiver is “failsafe” to invalid bus states caused by:

• open bus conditions such as a disconnected connector,
• shorted bus conditions such as cable damage shorting the twisted-pair together, or
• idle bus conditions that occur when no driver on the bus is actively driving

In any of these cases, the differential receiver outputs a failsafe logic High state, so that the output of the receiver is not indeterminate.

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 V_ID is more positive than +200 mV, and must output a Low when the V_ID is more negative than -200 mV. The receiver parameters which determine the failsafe performance are V_IT+ and V_IT- and V_HYS. As seen in the Receiver Electrical Characteristics 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 V_IT+ threshold, and the receiver output is High. Only when the differential input is more negative than V_IT- 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 V_HYS (the separation between V_IT+ and V_IT- ) as well as the value of V_IT+.