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.

In the HVD17xx family of RS-485 devices, receiver failsafe is accomplished by offsetting the receiver thresholds so that the “input indeterminate” range does not include zero volts differential. In order 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 HVD17xx receiver parameters which determine the failsafe performance are V_IT+ and V_IT– and V_HYS. In the Electrical Characteristics table, V_IT– has a typical value of –150 mV and a minimum (most negative) value of -200 mV, so 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, because the typical value of V_IT+ is -100mV, and V_IT+ is never more positive than -10 mV under any conditions of temperature, supply voltage, or common-mode offset.

When the differential input signal is close to zero, it will still be above the V_IT+ threshold, and the receiver output will be 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+.

For the HVD17xx devices, the typical noise immunity is typically about 150 mV, which is the negative noise level needed to exceed the V_IT– threshold (V_IT- TYP = –150 mV). In the worst case, the failsafe noise immunity is never less than 40 mV, which is set by the maximum positive threshold (V_IT+ MAX = –10 mV) plus the minimum hysteresis voltage (V_HYS MIN = 30 mV).