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Thread: QModbusClient: all subsequent modbus requests fail once a request had timedout

  1. #1
    Join Date
    Nov 2006
    Dresden, Germany
    Thanked 6 Times in 6 Posts
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    MacOS X Unix/X11 Windows

    Question QModbusClient: all subsequent modbus requests fail once a request had timedout

    Hi there,

    got a quick question about QModbusClient / QModbusDevice.

    I'm using
    Qt Code:
    1. QModbusReply * QModbusClient::sendReadRequest(const QModbusDataUnit &read, int serverAddress);
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    to send out modbus requests to a (successfully) connected device. When the modbus data unit is configured correctly, this works well and I can retrieve several registers one after another by sending a request and processing the reply.

    When I send a request to a non-existing register (either due to configuration failure, or if the modbus hardware was configured differently), I will get a timeout on the request. This is to be expected and not problematic.

    However, when afterwards a request is sent to a correctly configured register, this will also result in a timeout.

    Example sequence:

    - read register 30201 -> success
    - read register 30230 -> timeout (register 30230 does not exist)
    - read register 30201 -> timeout (why????)

    I didn't find anything in the documentation (e.g. about clearing the error flag on the QModbusDevice or else).

    Does anybody have an explanation or pointer to some overlooked part in the docs? Or maybe this is a bug?

    Thanks for any ideas!

  2. #2
    Join Date
    Jan 2006
    Graz, Austria
    Thanked 1,523 Times in 1,474 Posts
    Qt products
    Qt3 Qt4 Qt5
    Unix/X11 Windows

    Default Re: QModbusClient: all subsequent modbus requests fail once a request had timedout

    I haven't worked with this part of the API myself, but in general with cases such as this it is often most useful look into the code itself.

    Usually the most efficient way to do that is to look for the class that is your entry point at the Woboq Code Browser.

    For example here QModbusClient and then following the method that is being called.


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