THVD1550: Pull up and pull down resistor

Hi James,

Resistor power ratings will depend on the voltage/current across them in an application.  To be conservative, you could estimate the maximum differential output from an RS-485 transceiver to be its VCC voltage.  Therefore, if you had 120-Ohm resistance then the power dissipation would be (5 V)^2 / (120 Ohm) = 0.2 W. If you used two 60-Ohm resistors in series instead, then the power rating required would be about half as much.

External pull-up and pull-down resistances are optional on RS-485 buses and are sometimes used so that there is a defined differential level even if no transmitters are active on the bus.  (This is especially common in multi-point half-duplex applications where there could be some idle time between when each node transmits on the bus.)  This is often referred to as failsafe biasing.  You can read more about it in this blog:

https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/12/06/rs-485-basics-two-ways-to-fail-safe-bias-your-network?tisearch=e2e-sitesearch&keymatch=RS-485%252525252525252520basics%25252525252525253A

If you have any further questions please let us know.

Regards,
Max