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RS485 terminal resistor select

Hi TI,

How to decide the resistor in the RS485 receiver?And its power and value?

Best regards,

Frank

  • Hi Frank,

    The termination resistors on the RS-485 bus should match the cabling characteristic impedance value, which typically is 120ohm.  Having two terminations on the bus will put a 60ohm load on the bus, which is typical.  

    In normal operation, RS-485 drivers will drive a maximum of 5V differential signal onto the bus lines, which create a P=V^2/R power dissipation value of 25/120 = 208mW.  

    Therefore, a 0.25W rated resistor will work for normal operation; however, during a fault condition on the bus, the power dissipation may rise to higher levels.  The worst-case fault condition would occur if one bus pin is shorted to -7V and the other bus pin is shorted to +12V.  Under these extreme voltage conditions, the power dissipation could rise to 19*19/120 = 3W.

    Choosing termination resistors at 3W is pretty extreme.  Usually a system designer will choose a trade-off with bus fault conditions and lower power-rated resistors.  If you choose to protect against a single pin shorting to -7V or +12V individually, then you could choose a resistor in the 0.5W to 1.5W range.  

    Does this information help?  Please let me know if I can help answer any more questions you have.

    Best Regards,
    Max Megee

  • Hi MAX,
    Thanks a lot.Can you give me some typical RS485 application circuits to understand your nice replay.I want to know why the bus pin can be shorted to -7V,how does it occur?
    Best regards,
    Frank
  • Frank,

    One of the advantages of the RS-485 interface is the ability to tranfer data over long distances with large ground potential differences (GPD) between nodes.  This is the reason that the RS-485 standard specifies transceivers that operate across a common mode range from -7V to 15V.  If it happens that one of the bus lines are shorted to ground or supply, then this common mode voltage short can create the large amplitude fault condition.  The figure that I have attached below offers some advice on proper grounding techniques to reduce ground loop currents when GPD is high.  I include it here just to show how ground potential differences can contribute to higher voltage fault conditions in RS-485.

    Does this make sense?

    Regards,

    Max