THVD2450: Recommended biasing resistors

Hi David,

Thanks for reaching out on E2E!

For that E2E post you are referencing, Parker's comment highlighted in green is the way to go.

So if you want true failsafe for THVD2450, you can add resistors to the bus lines. Just note that you want to keep the overall bus impedance matching, and adding the failsafe resistors will affect this. 

This app note explains it in full detail, but you can just use this equation:

Here R_T2 is the termination resistor at that particular node, and R_FS is the failsafe resistors (multiplied by 2 for both the pullup and pulldown resistor). Also keep in mind that if you have multiple failsafe resistors on the RS-485 bus, these would technically be in parallel. So you will want to increase the R_FS value for each node that has biasing resistors. For example, if you have 3 nodes with failsafe resistors, then you will need each node to have 3*R_FS resistance. 

Let me know if that makes sense.

Best,

Ethan