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Different voltage on last bit CAN message SN65HVDA1050A-Q1

Markus Flueckiger
Prodigy 40 points

Hello,


when viewing the CAN messages on our product (J1939 protocol) we discover that the last bit has a different voltage:

design:

CAN_Tx, CAN_Rx : to Piccolo DSP

CAN_High_inp, CAN_Low_inp : to connector of the product

setup:

product connected to USB/CAN device from National Instruments (ref: USB-8473) with 1m cable and 120 Ohms resistor close to USB/CAN device

we have different products using the same CAN interface design and this phenomenon is obervable on all products.
Do you have an explanation for that? could it be critical?

thanks in advance for your reply,

Benoit

  • 0
    TI__Guru 59962 points
    This post has been moved for more suitable support.

    Industrial interface team,

    Can you help out Markus?

    Regards,
    Eric Hackett
  • 0
    TI__Mastermind 20820 points

    Hi Markus,


    I do not have an explanation for this behavior. Can you provide me a schematic to look over and possibly a screen capture from the scope of this phenomena? This might help me determine where the problem is originating.


    This may be critical to your system but I am not sure. Are you getting errors in your communication or does all traffic pass error free?

  • 0 in reply to Michael Peffers
    Prodigy 40 points

    Hello,

    thanks for your quick reply, I'm sorry, it seems the pictures have not been correctly uploaded...

    all the traffic is error free.

    scopes (CAN_HIGH):

    design:

    another thing, In your application note, it seems there is the same phenomenon:

    and another thing again: on some products the last bit is not at an higher level but at a lower level...

    thanks for your time !


    Markus

  • 0 in reply to Markus Flueckiger
    TI__Mastermind 20820 points
    Hi Markus,

    The over sized bits at the end of your CAN packets is the ACK bit being transmitted by the nodes on the bus back to the transmitter. When there are many nodes on a network, every single node needs to transmit this ACK bit back to the transmitter so that bit gets bigger and bigger with more and more nodes. You should see the same behavior on the bus during the arbitration state.

    For the cases where the bit is smaller, the driver transmitting the ACK bit back probably has a slightly weaker driver than the other nodes on the bus causing a lower voltage.

    Please let me know what other questions you have?
  • 0 in reply to Michael Peffers
    Prodigy 40 points

    Hello,

    Thanks for your quick reply and your explanations, it's all right for me.

    thank you again!

    Markus