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THVD8000EVM: Are failsafe resistors required on the THVD8000?

Yixing Qie
Prodigy 70 points
Part Number: THVD8000EVM
Other Parts Discussed in Thread: THVD8000

Tool/software:

I see that there are pads for failsafe resistors on the THVD8000 EVK, but they are not populated.

I have a single master which communicates to multiple nodes on a bus. 

Do I need to add failsafe resistors to all my nodes? Are there any issues with adding these fail safe resistors? 

  • 0
    TI__Guru 51882 points

    Hi Yixing,

    No they are not necessary - and not as commonly used in real applications as they would be in more standard RS-485. 

    If you do want to add a fail-safe to the THVD8000 - you can -  but it wouldn't be setup as shown in EVM (EVM gives you an option to add pull-ups or pull-downs depending on what you want). What I'd suggest if you are looking to add them is to populate R11 and R9 on EVM and make them the same value - on EVM they show like 15k which is pretty weak but can be used. The reason for using two pull-downs is that typically for idle fail-safe you want the "R" pin to output a high level when bus is idling - a differential voltage of 0V is a logic high - so pulling both A and B down to ground through pull-down resistors will act as a way to keep the differential input to 0V during bus idling. 

    If you do add them - they will load the bus and will impact sizing of inductors on the line. Typically we don't see them used in most applications - they can be - but they will add some loading - along with the transceivers themselves (96k) and the coupling network (inductors). 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson