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ISO1050: CANH to CANL voltage overshoot

Sheldon He
Expert 8585 points
Part Number: ISO1050

Hi expert,

My customer is using ISO1050 as CAN transceiver, their communication is good but get some unexpected voltage overshoot between CANH and CANL as shown below. Voltage is 1V/div and communication speed is 500K.

Hope you can give some advice to us.

Thanks

Sheldon

  • 0
    TI__Mastermind 49445 points

    Hi Sheldon,

    Thanks for reaching out to us with the issue.

    Could you please confirm if the waveform shared is a CANH, CANL or the difference between them (VOD)?
    Could you please also confirm what are the other CAN devices connected on the CAN bus?
    Is it ISO1050 on both ends of CAN bus?

    It is possible that the dominant output voltage for different devices to be different levels. In the above waveform it is possible that all the pulses with one voltage level might in from on device in one direction while the other pulse with higher voltage could be from the other device in the other direction. I can't say this for sure without knowing the devices being communicated and the direction of data being transmitted. Please do share above requested information and we will be able to comment better. Once you share the data, one of our team members will get back to you. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Mastermind 49445 points

    Hi Sheldon,

    Could you please share the above information requested? Can you please also confirm if the overshoot that you are referring to happens only on the last bit or if it happens for any bit in the stream?

    Please do also share more details related to the application, number of CAN nodes, devices used and anything else that you think is important. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 8585 points

    Hi Koteshwar Rao,

    Could you please confirm if the waveform shared is a CANH, CANL or the difference between them (VOD)?

    Shared waveform is the difference between the two.
    Could you please also confirm what are the other CAN devices connected on the CAN bus?

    They are all ISO1050 and has a maxima notes number of 4.
    Is it ISO1050 on both ends of CAN bus?

    Sure.

    Can you please also confirm if the overshoot that you are referring to happens only on the last bit or if it happens for any bit in the stream?

    It happens at the end of each frame and not been eslewhere.

    Thanks

    Sheldon

  • 0
    Guru 243120 points

    I notice that the 'normal' rising edges have a little bit of overshoot, while the last one appears to be rounded, as if the voltage is passively pulled up. Can you zoom in on these edges?

    Can you show the individual CANH/CANL voltages? Are both affected, or only one of them?

    The isolators work on the electrical level and do not know about frames. So I suspect that the last bit is driven by multiple transmitters. Can you show all four TXD signals?

  • 0 in reply to Sheldon He
    TI__Mastermind 49445 points

    Hi Sheldon,

    Thank you for sharing the requested information, this helps.

    Upon my interaction with other folks in the team, I understood that in a proper CAN implementation at the end of each frame there will be an acknowledgement bit (ACK) which is dominant will be driven by all the nodes except for the one that has transmitted the frame. When multiple nodes drive the bus dominant, it is expected to see higher voltage on bus than with just one node driving the bus. This explains the reason why you are seeing higher swing on the CAN bus than the normal voltage only at the end of each frame.

    Hence, this is normal and not be considered as an issue. Let me know if you or customer has any questions, thanks.

    Regards,
    Koteshwar Rao