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RS-485 Rineq Value using 8ft to 100ft of wire

hj
Intellectual 650 points
Other Parts Discussed in Thread: SN65HVD72

I am trying to pick pull up and pull down resistors for the RS-485, SN65HVD72.  I am connecting the RS-485 to devices ranging from 8 feet of wire to 100 feet of wire.  According to this document, using the 100 foot long cable, I calculated that the maximum value of the pull up resistor needed is 375 ohms.  Why am I finding examples with resistances larger than 375?

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

To my understanding Rineq is the value that changes depending the length of the wire used.

  • 0
    TI__Genius 14015 points

    Hello,

    The failsafe resistor, Rfs, value is determined from equation 6 in that app note. It is based on the worst case (minimum) supply voltage, Vs, and the worst case (maximum) required Vab. 

    Rfs = (Vs_min / (Vab_threshold + noise) + 1) * 27.8

    Rfs = (4.75V / (200mV + 50mV) + 1) * 27.8 = 556 Ohms.

    Rineq is found after the failsafe resistor size is determined. This value will tell you how many nodes you can have on the bus. It is not a characteristic of the wire chosen. 

    That being said, SN65HVD72 may not need external failsafe resistors. It has a built in failsafe which will account for open circuit, short circuit, and idle-bus conditions. It does this by offsetting the receiver thresholds so that during failsafe conditions, the part will be receiving a High. If you are operating in a very noisy environment, however, then additional failsafe resistors may be required. 

    Does this help?

    Thanks,
    Jason