10.2.1.4 Receiver Failsafe

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 will output a failsafe logic high 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_IT+, V_IT–, and V_HYS (the separation between V_IT+ and V_IT–). As shown in Electrical Characteristics, 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 of –20 mV, and the receiver output will be high. Only when the differential input is more than V_HYS below V_IT+ will the receiver output transition to a low state. Therefore, the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis value, V_HYS, as well as the value of V_IT+.