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ISO1044: ISO1044BD: CAN Bus Stops Working When Any One Node is Powered OFF in a 10-Node Network

Mohith Kommuri
Prodigy 10 points

Part Number: ISO1044

Hi TI Team,

I am using the ISO1044BD isolated CAN transceiver in a multi-node CAN network and am facing an unexpected issue.
 

Network Topology

  • CAN Transceiver: ISO1044BD
  • Total Nodes: 10
  • Bus Topology: Linear CAN Bus
  • CAN Controller: (Mention MCU/controller part number)

Issue Observed

  • When all 10 boards are powered ON, CAN communication is stable and works as expected.

    However, when any one board is powered OFF (irrespective of which board), the entire CAN network stops communicating.

    While debugging, I observed:

    • CANH ≈ 0 V
    • CANL ≈ 0 V

measured on the bus after powering OFF one node.

The communication resumes only after powering the board back ON or disconnecting the powered-off node from the CAN bus.



Attached Schematic
image.png

  • 0
    TI__Genius 16070 points

    Hi Mohith,

    Thanks for reaching out.

    When you say board Powered OFF - Do you mean powering OFF the VCC1 or VCC2 or both on the board?

    Also, can you double confirm that both CANH and CANL are 0V at the same time?

    This cannot be a device behavior at all in any of the Power Condition for VCC1 and VCC2 as ISO1044 cannot drive the CAN bus to 0V in any of the power OFF condition:

    If it indeed is 0V for both CANH and CANL, then you need to double check the boards on CAN side of all the boards that might be shorting to GND2 in powered off condition.

    Additionally, one comment form schematics that I observed - Please add an additional bulk capacitor on VCC2 (like 10uF) for optimal operation of device. This is mentioned in the datasheet as well in section 10.

    Regards
    Varun

  • 0 in reply to Varun Kumar
    Prodigy 60 points

    Hi Varun,

    In our case, when both VCC1 and VCC2 are powered OFF for any one of the boards in the CAN network, will it impact the CAN bus communication for the entire network?

    We expect the CANH and CANL pins of the powered-off board to go into a high-impedance state so that the board does not affect the bus communication. Can you please confirm if the transceiver will behave this way in the power-off condition?

  • 0 in reply to Mallikarjun Gajulapalli
    TI__Genius 16070 points

    Hi Mallikarjun,

    Yes, that is correct.

    When both VCC1 and VCC2 are OFF, the CANH/CANL will be in Hi-Z state (High Impedance) - which means that it will not affect the bus communication.

    Regards
    Varun