SN65HVD12: Question about failsafe biasing resistors

Hi Chris,

Several of our RS-485 devices feature this internal failsafe biasing that is meant to keep the receiver circuitry in a known state when no fail-safe network is in place on the bus. However, this biasing is only done internally and does not provide external biasing to the bus. Thus, any other nodes on the same bus will not be fail-safe biased by this feature. Any remaining nodes that are not internally biased may still require an external fail-safe network to be present to retain this stability in an open- or shorted-bus state. 

Apart from biasing other nodes without this feature, an external fail-safe network also provides greater stability to an idle system by increasing immunity to noise. The internal biasing keeps the output in a known state when the input differential remains close to zero volts. In a noisy system, this differential may exceed this small threshold and propagate noise through the receiver circuitry in and idle or disconnected bus state. External biasing will keep this idle differential well above the actual negative threshold and thus increase the noise immunity of the system in these states. 

If only SN65HVD12 (and similar devices with internal fail-safes) are used on the same bus, it is possible to remove external failsafe biasing resistors for simplicity, cost, efficiency, decrease total loading, or other purposes. It is up to the system designer to determine how these reasons weigh against potentially reducing system flexibility to add other non-internally biased nodes and increased noise susceptibility. 

Let me know if this makes sense and if you have any more questions.

Regards,
Eric