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ISO1042: Bus terminating resistor

AK
Intellectual 870 points
Part Number: ISO1042

Hello Experts,

This might be a very basic question:

We have 3 CAN nodes hardwired on the same PCB, and the ISO1042 are spaced about 3 inches apart from each other. We have not put in the 120 ohm terminating resistors(we forgot). Will this cause problems in CAN communication over such short distances?

Secondly , we plan to solder external resistor on the bus, will a single 60 ohm resistor anywhere on the bus be sufficient, or do we need to have two 120 ohm resistors and all the nodes be physically in between them? 

Thanks,

AK

  • +1
    TI__Expert 8030 points

    Hi AK,

    Thanks for reaching out.

    AK said:
    We have 3 CAN nodes hardwired on the same PCB, and the ISO1042 are spaced about 3 inches apart from each other. We have not put in the 120 ohm terminating resistors(we forgot). Will this cause problems in CAN communication over such short distances?

    Based on the information that you shared, I'm assuming that the entire CAN bus is on a single PCB, and that there are a total of three nodes connected to the bus, where all nodes are about 3 inches apart from each other. If this is not the case, please let me know.

    Since all of the nodes connected to the CAN bus are very close to each other, I do not think you will experience CAN communication errors if you do not connect termination resistors to the ends of the CAN bus. For this application, since the bus is very short in length, there will likely be only a small amount of impedance mismatches introduced to the CAN bus. The signal reflections generated by this small amount of impedance mismatches will likely not be significant enough to cause CAN communication errors. Therefore, I believe that it is likely that this specific application will not require termination resistors to be connected to the CAN bus. 

    AK said:
    Secondly , we plan to solder external resistor on the bus, will a single 60 ohm resistor anywhere on the bus be sufficient, or do we need to have two 120 ohm resistors and all the nodes be physically in between them? 

    The CAN standard (ISO 11898) recommends for each end of the CAN bus to be terminated with resistors equal to the characteristic impedance of the cable/bus. Typically a CAN cable/bus will be designed to have a characteristic impedance of around 120 Ω, which means that each end of the CAN Bus should be terminated with a 120 Ω resistor. The purpose of connecting two 120 Ω termination resistors to the ends of the bus is to reduce impedance mismatches, which will result in less signal reflections and better signal integrity. Here is an example of how it is recommend for a CAN bus with multiple nodes to be terminated.

    In order to minimize signal reflections, RL in the diagram above should be equal to the characteristic impedance of the CAN bus. 

    If you are planning on connecting termination resistors to the CAN Bus for your application, connecting one 120 Ω resistor to each end of the PCB CAN Bus (a total of two 120 Ω resistors) should be enough to minimize reflections and maintain signal integrity. 

    For more information about this, you can refer to this E2E blog post: https://e2e.ti.com/blogs_/b/industrial_strength/posts/the-importance-of-termination-networks-in-can-transceivers.

    Regards,

    Kenneth