The differential receiver is fail-safe 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
• Idle bus conditions that occur when no driver on the bus is actively driving

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

Receiver fail-safe 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 fail-safe performance are V_IT+ and V_IT– and V_HYS. As seen in the Electrical Characteristics table, differential signals more negative than –200 mV will always cause a Low receiver output and differential signals more positive than +200 mV will always cause a High receiver output.

When the differential input signal is close to zero, it is still above the maximum 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. 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+.