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MAX3485 Fail-Safe Biasing Resistors Not Working?

Category: Hardware
Product Number: MAX3485EESA+t
Software Version: n/a

MAX3485EESA+T data sheet claims to have internal fail-safe biasing resistors that will force the output logic high when the A and B lines are both open circuited.

However, when we implemented this part into a RS-485 communication system, we found that this was not occurring as promised. External pullup and pulldown resistors were added to force a voltage >200mV across the A and B lines to cause an output of logic high when the lines were idle. We already had the 120 Ohm resistors between the A and B lines at each end of the system. Doing this solved our issues. 

The resulting question is therefore why does the datasheet state that this is an internal feature when it does not seem to be? My understanding was that we could implement a circuit as shown below from the datasheet and have no issues. 

Summary: Datasheet claims internal fail-safe feature that guarantees logic high output when both inputs are open circuited. However, we had to add external pullup and pulldown resistors to our design to achieve a logic high output when the inputs were open circuited. Is there some specific case or setup that needs to be met to achieve this feature?



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