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THVD2419: differential load resistors

Ethan Ku
Prodigy 120 points

Part Number: THVD2419

hi 

we have started testing the RS-485 communcition and observed the communication is randomly lost . the originally assigned RD has resigned already . we understand that RS-485 commucnaiton worked before.but it si currently not functioning . could your please confirm whether placing 120ohm differential termination resistors on both ends of the bus is correct for the THVD2419,given that datasheet specifies a 60 ohm?

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  • 0
    TI__Guru* 79531 points

    Hi Ethan,

    The datasheet uses 60 ohms because two 120 ohm resistors between A/B is an equivalent of 60 ohms (120//120=60). 

    You ideally choose the termination resistor based on the characteristic impedance of the cable you are using. You need 120 (or whatever the characteristic impedance of the cable is) on the furthest node away from the driver node. If there are multiple nodes that can drive, you ideally should have two 120 on the two furthest nodes.

    Ethan Ku said:
    we have started testing the RS-485 communcition and observed the communication is randomly lost .

    Do you have scopeshots of the issue you are seeing? 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 120 points

       thanks , we found where the issue is from