The RS-485 differential receiver of the THVD4431 is failsafe to invalid bus states caused by the following:

• 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 failsafe logic high state so that the output of the receiver is not indeterminate.

Receiver failsafe is accomplished by offsetting the receiver thresholds such 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 V_ID is more negative than –200 mV. The receiver parameters which determine the failsafe performance are V_TH+, V_TH–, and V_HYS (the separation between V_TH+ and V_TH–). As shown in the Receiver Function table, differential signals more negative than –200 mV always causes a low receiver output, and differential signals more positive than 200 mV always causes a high receiver output.

When the differential input signal is close to zero, it is still above the V_TH+ threshold, and the receiver output is high. Only when the differential input is more than V_HYS below V_TH+ does the receiver output transition to a low state. Therefore, the noise immunity of the receiver inputs during a bus fault conditions includes the receiver hysteresis value, V_HYS, as well as the value of V_TH+.