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ISO1050 CAN Bus stub length

Other Parts Discussed in Thread: ISO1050

Hallo,

1) Would you please let me know how to calculate maximum stub length in CAN bus network?
Is there any method to estimate maximum stub length in order to avoid the reflections (using the ISO1050 at 500Kbps. Maximum rise/fall time of ISO1050 is 50ns in the datasheet.)?
Though the ISO11898 Standard specifies a maximum bus length of 40m and maximum stub length of 0.3m at 1Mbps.

2) In the customer application, the ISO1050 output has two branch output lines. One output line is optional and ordinarily not use.
Does this optional line need any termination, though I think it’s okay to leave it open?

Best Regards,
Kazu Ogawa

  •  

    Hi Ogawa-san,

    The rule of thumb I have always heard is to limit the length of the stub so that the propagation delay down the stub is less than 0.3 of the rise time. At a typical propagation delay of 5ns per meter of twisted pair cable, this would result in 0.3*50ns/5ns/m = 3 meters.

    I have seen that adding partial terminations on the ends of long stubs will help the reduction of reflections (for example 2kΩ), but I do not have a hard a fast rule for how to calculate this. It will be system dependent, and the trade-off will be lower VOD vs. smaller reflections.

    Please let us know if you have any other questions.

    Thanks,

    John

  • Kazuhiko Ogawa said:
    2) In the customer application, the ISO1050 output has two branch output lines. One output line is optional and ordinarily not use.

    Optional?  Why do you think it is optional?

    There are single wire CAN setups but they use a different transceiver and lower baud rates AFAIK.

    Robert

  • Hi Robert,

    I read that as the bus is split and goes to two places. Not that only one of the bus wires is being used. Ogawa-san, can you clarify which you meant?

    The ISO1050 will require both bus lines to be used as a differential pair to be operational.

    Thanks,

    John

  • That would be different, I had read the grey lines on the diagram as optional but I think your interpretation is probably the correct one).  Basically it makes the stub longer.  It would have to be quite restricted in length.

    Robert

  • John-san, Robert-san,

    Thank you very much for your reply.

    1) - In the calculation, which should the propagation delay of the line be used, one way delay (5ns/m) or down-and-back delay (5ns/m x 2)?
    I found the below application report (slla270).
        http://www.ti.com/lit/an/slla270/slla270.pdf
        This mentions using down-and-back delay in 4.8 section. So, this is my question.
       - What are different between 0.3*50ns/5ns/m=3m and 0.3m at 1Mbps specified in the ISO11898 Standard?
         I would not be able to explain the answer to this question to the customer.
    2) Sorry my poor explanation.
        The customer intend to draw two branch pattern from one output pin on the PCB. This branch is used for another optional application and ordinarily not use. Maybe the length of this branch pattern should be shorter than 0.3m.

    Best Regards,

    Kazu Ogawa

  • Hi Ogawa-san,

    1) I apologize for my mistake when calculating the stub length; I should have taken in the two way delay not just one way. This was a mistake.

    The application note that you attached is very similar to what I was saying, the author just presents it a little differently. The author calculates the maximum stub by first calculating what he refers to as the "critical length" of the bus which is based on the rise time of the transceiver and the two-way propagation delay of the signal on the bus medium. Then he says that a stub needs to be one third or less of that critical length. This is very similar to what I had heard before with keeping the stub length to less than 0.3 of the two way prop delay of the stub. (0.3 verses 1/3)

    2) If the cabling is not connected when that portion of the bus is not in use, then you are fine. But if the cabling is attached at all times, then you need to take that length of cable into account when doing your stub length calculations to estimate the reflections.

    Thanks,

    John

  • John-san,

    Thank you very much for your support.

    Best Regards,

    Kazu Ogawa