JAJS488J January 2008 – March 2023 SN65HVD1785 , SN65HVD1786 , SN65HVD1787 , SN65HVD1791 , SN65HVD1792 , SN65HVD1793
PRODUCTION DATA
The differential receiver is failsafe to invalid bus states caused by:
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 VID is more positive than 200 mV, and must output a Low when the VID is more negative than -200 mV. The HVD17xx receiver parameters which determine the failsafe performance are VIT+ and VIT– and VHYS. In the Electrical Characteristics table, VIT– 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 VIT+ is -100mV, and VIT+ 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 VIT+ threshold, and the receiver output will be High. Only when the differential input is more negative than VIT- 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 VHYS (the separation between VIT+ and VIT– ) as well as the value of VIT+.
For the HVD17xx devices, the typical noise immunity is typically about 150 mV, which is the negative noise level needed to exceed the VIT– threshold (VIT- 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 (VIT+ MAX = –10 mV) plus the minimum hysteresis voltage (VHYS MIN = 30 mV).