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About termination resistors on ECUs

William Tu1
Prodigy 210 points

Hi all,

I want to know whether the ECU termination resistor(red resistors) does not need.

The very left and right side ECU has termination resistors for the CANH and CANL line, I could see that it's not used resistor on the ECU and make the line short, I want to know why red resistor does not put.

Thank you

  • 0
    TI__Genius 15345 points

    For a CAN bus the two termination resistors (the resistors you circled in blue) are necessary for impedance matching. Since the CAN bus wires between the beginning ECU and the last ECU are significantly long we have to impedance match in order to avoid reflections of the signal. These wires have a characteristic impedance of 60 ohms so we use two 120 ohm termination resistors in parallel in order to impedance match. Any reflection of the signal will ruin the integrity of your signal.

     

    For the ECUs in-between the two termination ECUs, in general if their wire lengths (stub lengths) are less than (0.3*critical length) no resistors are needed.

     

    In order to calculate the critical length look at section 4.8 (pg. 9) of this application note where it details the math on how to calculate critical length. https://www.ti.com/lit/an/slla270/slla270.pdf?ts=1592506067187

     

    However, if the stub lengths are longer than (0.3*critical length) then larger value resistors can be used in order to mitigate any reflections of the signal. It is important to use larger value resistors (1k-10k) so as to keep the overall impedance of the bus at 60 ohms.

    Let me know if you have any other questions.

    Thanks,

    Chris Ayoub