The differential receivers of the SN65HVD178x-Q1 family have receiver input thresholds that are offset, so the receiver output state is known for the following three fault conditions:

• 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 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 200mV, and must output a Low when V_ID is more negative than –200mV. 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 the Electrical Characteristics table, differential signals more negative than –200mV always cause a Low receiver output, and differential signals more positive than 200mV always cause a High receiver output.

When the differential input signal is close to zero, the signal is still above the maximum V_IT(+) threshold of –35 mV, and the receiver output is High. Only when the differential input is more than V_HYS below V_IT(+) does 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(+).

Figure 8-5 Noise Immunity Under Bus Fault Conditions