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The differential receiver is failsafe to invalid bus states caused by:
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 VID 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 VIT+, VIT–, and VHYS (the separation between VIT+ and VIT–). 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 VIT+ threshold of –20 mV, and the receiver output will be high. Only when the differential input is more than VHYS below VIT+ 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, VHYS, as well as the value of VIT+.