The differential receiver of the SN75HVD17xx family 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
• Idle bus conditions that occur when no driver on the bus is actively driving

n any of these cases, the differential receiver will output 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. 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 VID is more negative than -200 mV. The receiver parameters which determine the failsafe performance are V_IT(+) and V_IT(-). As shown 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 of -10 mV, and the receiver output will be high.