TIDA-01365: TIDA-01365

Hi Larry,

Those resistors are often used in RS-485 networks so that the default bus level is "high" (>200 mV per EIA-485 spec) when the line is idle (no transmitters enabled). This is not needed for the SN65HVD3082E device, though, since that device features internal failsafe biasing. That means that it would interpret a 0 V input as a logic high level, and that external biasing to a positive differential level is not needed. The resistors could still be installed, though, if the system were expected to interface to other transceivers that do not feature internal failsafe biasing.

I hope this is all clear - please let me know if you have further questions.

Regards,
Max

Larry,

Yes, in this case I would install them. Ideally, though, this would be something that is defined for the network (similar to how the placement of termination resistances is determined). That's because it is usually best to place failsafe biasing just at one node. This allows the effective pull-up/pull-down values to remain fixed rather than reduce with each additional node connected in parallel to the bus.

In general 750 Ohms would be a reasonable value. If you expect that there is additional pull-up/pull-down resistance placed elsewhere on the bus, though, you could choose to increase these values (depending on how much additional external loading is expected).

You can read more on RS-485 failsafe biasing in this blog:

e2e.ti.com/.../rs-485-basics-two-ways-to-fail-safe-bias-your-network

Regards,
Max