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THVD1449: Is RFS needed on A and B pins of THVD1449

Basit Ali Khan
Intellectual 370 points
Part Number: THVD1449

Tool/software:

Hi,

Is the RFS pull up or pull down needed on A and B pins of THVD1449DR keeping in view the full speed grade and the full length it supports?

  • 0
    Guru 316780 points

    The THVD1449 itself has fail-safe inputs and does not require external fail-safe resistors.

    You need fail-safe resistors (once, somewhere on the bus) if there are transceivers without fail-safe inputs on the bus, or if you want to increase the fail-safe noise margin.

  • 0 in reply to Clemens Ladisch
    Intellectual 370 points

    Actually I am concerned about the pull-up pull-down resistors on the A and B pins of THVD1449. Generally these resistors are placed but these were not mentioned in THVD1449 datasheet, so if these are needed or not and they are internally done.

  • 0 in reply to Basit Ali Khan
    Guru 316780 points

    The "FS" stands for "fail-safe".

  • +1 in reply to Basit Ali Khan
    TI__Guru 52182 points

    Hi Basit,

    So they are not required for operation - so you don't need them. 

    In general designers may add them to devices with an offset fail-safe to increase noise margin - i.e. both VIT+ and VIT- are < 0V so a 0V differential results in a high output - however this device doesn't  use an offset fail-safe; it uses a timer based failsafe that takes ~10us to recognize that the bus is in-between -40mV and 40mV before it forces a high on the output pin R - but could result in undefined behavior during that 10us.

    So you would add the RFS resistors to set the idle differential to a little above 200mV to bypass the internal failsafe - as not all designers like the timer based failsafe. You don't need to do this - but if you are going to place them they should be setting the the idle point above 200mV. You still would have to rely on the timer based fail-safe for short circuit and open circuit conditions - but the idle condition can be bypassed with this network. 

    Adding the properly sized RFS resistors here (when I say properly sized they are probably going to be < 1k to set the idle that high) will not impact max speed or max distance - but it does impact loading. RS-485 can only support 32 unit loads on 1 bus - the RFS resistors reduce the number of transceivers you can use on the bus. 

    Please read application note on RFS resistors here (we don't really include anything in datasheets because they aren't required) as it explains how to properly size and how much loading the resistors will add to the bus: https://www.ti.com/lit/an/slyt324/slyt324.pdf?ts=1738078285523&ref_url=https%253A%252F%252Fwww.google.com%252F

    Best,

    Parker Dodson