[SOLVED] JLinkOB - Serial Passthrough Payload Size Limited to 64 Bytes?

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  • [SOLVED] JLinkOB - Serial Passthrough Payload Size Limited to 64 Bytes?

    Hi all,

    I am working on writing some integration testing scripts for BLE devices running Mbed-OS. The integration test scripts are written in python and use pyserial to interact with a RPC server allowing arbitrary control of BLE clients/servers over a serial port.

    The boards I am using are nRF52840DK's that have JLink On-Board debuggers. I have been encountering a strange issue where some commands time out because incorrect bytes are inserted into the serial stream.

    It appears the USB-to-UART bridge presented by the JLink OB is overflowing or otherwise unable to handle single transfers above 64 bytes. If so, this is a somewhat annoying limitation!

    See associated GitHub issue here:


    The JLink OB's I'm using report themselves as the following:

    Source Code

    1. J-Link[0]: Connection: USB, Serial number: 683717356, ProductName: J-Link OB-SAM3U128-V2-NordicSem
    2. J-Link[1]: Connection: USB, Serial number: 682564699, ProductName: J-Link OB-SAM3U128-V2-NordicSem
    3. Response of both to 'f' command:
    4. Firmware: J-Link OB-SAM3U128-V2-NordicSemi compiled Mar 17 2020 14:43:00
    5. Hardware: V1.00
    I think I've encountered this issue before when sending large packets through the virtual UART with JLink OB. It would be nice to know if this is a bug or an "official limitation" :)

    Any comment would be appreciated!
  • Hi,
    Thank you for your inquiry.

    In general, we do not provide support for the J-Link OBs.
    The support is provided by the silicon vendor (in this case Nordic).
    You would have to contact Nordic support about this.

    As far as we know, Nordic is very thoroughly testing the VCOM feature on their boards
    and we have not yet received a report of such an issue.

    We will close this thread now.

    Best regards,
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