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RS-485 Pull up resistor

hj
Intellectual 650 points

I read in order to maintain a solid reading from the pins A and B of the RS-485, I need to attach pull up and pull down resistors for "fail safe biasing".

Therefore, I have attached a 360 ohm resistor from Vcc to A and a 360 ohm resistor like I saw in an example, and from B to Ground and I am unable to read the output.  How did I incorrectly apply this?

  • 0
    TI__Genius 14015 points

    Hello,

    I responded to your other post about this, and so I believe 360 Ohms may be too strong of a failsafe value. It violates the equation 2 from this app note:

    http://www.ti.com/lit/an/slyt324/slyt324.pdf

    The maximum common mode resistance from the RS-485 standard is 375 Ohms. This may be why you are having issues, I am not sure. 

    Try re-calculating the failsafe values, or simply remove them.

    Thanks,
    Jason