As per the Data sheet the SN65HVD08D having Fail safe,
but in practical the network is it is giving problem with out Fail-safe bias resistors, after connecting the Fail-safe bias resistor the network is working fine .(connected nodes 9, distance 200meters )
Please clarify really the SN65HVD08D require the fail- safe bias resistors.
Thanks& regards
-BVRK Prasad
In short: your differential noise on the bus is probably larger than the failsafe range provided by the HVD08.
In the left diagram you see that the HVD08 has only a limited range of failsafe. The data sheet gives the hysteresis only as a typical value. Assuming a symmetrical noise signal, its maximum peak-to-peak amplitude would be twice negative input threshold (so 2 x 45mV) until the receiver output state would change due to differential noise. Bare in mind that the minimum hysteresis could lower (i.e. 25mV), thus reducing the failsafe range even further.
By implementing external faislafe resistors you create a failsafe voltage on the bus that first needs to be overcome by the differential noise. In the right diagram I assumed using the typical 523W failsafe biasing resistors which in combination with two parallel 120W termination resistors create a failsafe voltage of approximately VFS = 250mV. Now the noise voltage has to be significantly larger (almost 300mV) before it causes the receiver output to change states.
regards, Thomas
