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QSerialPort: problem with waitForReadyRead()



  • Hello everyone!

    Now I try to use QSerialPort to get data from some device. In order to get data I need to send any symbol (1 byte) and then I will get 5 bytes data word.

    The problem is next: sometimes I write symbol to port and don't get data with waitForReadyRead(), but with next writing I get double size data (10 bytes instead of 5 bytes). In waitForReadyRead(time) I can use even long time, but anyway I can't get data.

    I have been trying many times to solve this problem, but no success. I only found, where the problem is.

    My code:

    while (1)
    {
           QByteArray input, output;
    
           input.clear();
           input.append('0');
           deviceControl_SerialPort->write(input); // <-- WRITE
           count++;
    
           if (deviceControl_SerialPort->waitForBytesWritten(30000)) // <-- WAIT FOR WRITE
           {
                  QThread::msleep(500); // <-- SLEEP FOR COMFORTABLE DEBUGING
                  qDebug() << "WRITTEN " << count << endl;
    
                  if (deviceControl_SerialPort->waitForReadyRead(3000)) // <-- AFTER WRITE WAIT FOR READ
                  {
                        qDebug() << "READING..." << endl;
                        output = deviceControl_SerialPort->readAll(); // <-- READ
                        while (deviceControl_SerialPort->waitForReadyRead(10)) // <-- READ AGAIN
                            output += deviceControl_SerialPort->readAll();
    
                        if (output.count() >= 5)
                        {
                            deviceControl_List.at(0)->setAngleEncoderData(output.at(1), output.at(2));
                            deviceControl_List.at(1)->setAngleEncoderData(output.at(3), output.at(4));
                            //qDebug("Data");
                        }
                  }
    
                  else
                  {
                        qDebug() << "I AM NOT READY!" << endl; // <-- SOMETIMES (OR OFTEN) I AM HERE
                  }
            }
           else
           {
                  qDebug("SOMETHING WRONG!");
           }
    }
    

    Some Debug Information during run:

    WRITTEN  1 
    
    READING... 
    
    WRITTEN  2 
    
    READING... 
    
    WRITTEN  3 
    
    READING... 
    
    WRITTEN  4 
    
    READING... 
    
    WRITTEN  5 
    
    READING... 
    
    WRITTEN  6 // <-- DATA WAS WRITTEN
    
    I AM NOT READY! // <-- BUT EVEN AFTER 3 SECONDS NO DATA
    
    WRITTEN  7 
    
    READING... 
    
    WRITTEN  8 
    
    READING... 
    
    WRITTEN  9 
    
    READING... 
    
    WRITTEN  10 
    
    READING... 
    
    WRITTEN  11 
    
    READING... 
    
    WRITTEN  12 
    
    READING... 
    
    WRITTEN  13 
    
    READING... 
    
    WRITTEN  14 // <-- HERE - SAME PROBLEM
    
    I AM NOT READY! // <-- AND THEN ONLY MORE OFTEN
    
    WRITTEN  15 
    
    READING... 
    
    WRITTEN  16 
    
    I AM NOT READY! 
    
    WRITTEN  17 
    
    READING... 
    
    WRITTEN  18 
    
    I AM NOT READY! 
    
    WRITTEN  19 
    
    READING... 
    
    WRITTEN  20 
    
    I AM NOT READY! 
    
    WRITTEN  21 
    
    READING... 
    
    WRITTEN  22 
    
    I AM NOT READY! 
    
    WRITTEN  23 
    
    READING... 
    
    WRITTEN  24 
    
    I AM NOT READY! 
    
    WRITTEN  25 
    
    READING... 
    
    WRITTEN  26
    

    Realy hope for your help! I can't find useful information to solve this problem in Internet.

    I think it's not problem in device, because in another application I can't see my problem.

    Thank you in advance!



  • @oBOXPOH said in QSerialPort: problem with waitForReadyRead():

                    output = deviceControl_SerialPort->readAll(); // <-- READ
                    while (deviceControl_SerialPort->waitForReadyRead(10)) // <-- READ AGAIN
                        output += deviceControl_SerialPort->readAll();
    

    readAll() only reads whatever is available at the instant it is called. This can be anywhere from 1 byte to the total number you expect. Your while (waitForReadyRead(10)) may return false, and so exit the loop, when there is no data right now but there will be later on, meaning that you will miss the data this time round and pick it up only next time.

    I don't know much about serial ports, but at least try upping that 10 to a bigger number and see if it makes any difference?

    Make sure the other end is flushing whatever it writes immediately.

    You're not really "supposed to" use waitForReadyRead(10). You're "supposed to" use the readyRead() slot with signals. You could try that and see if it makes a difference.


  • Qt Champions 2020

    What is OS, and what is Qt version?



  • @JonB said in QSerialPort: problem with waitForReadyRead():

    @oBOXPOH said in QSerialPort: problem with waitForReadyRead():

                    output = deviceControl_SerialPort->readAll(); // <-- READ
                    while (deviceControl_SerialPort->waitForReadyRead(10)) // <-- READ AGAIN
                        output += deviceControl_SerialPort->readAll();
    

    readAll() only reads whatever is available at the instant it is called. This can be anywhere from 1 byte to the total number you expect. Your while (waitForReadyRead(10)) may return false, and so exit the loop, when there is no data right now but there will be later on, meaning that you will miss the data this time round and pick it up.

    I don't know much about serial ports, but at least try upping that 10 to a bigger number and see if it makes any difference?

    Make sure the other end is flushing whatever it writes immediately.

    You're not really "supposed to" use waitForReadyRead(10). You're "supposed to" use the readyRead() slot with signals. You could try that and see if it makes a difference.

    In order to get data with readAll() I need to go through waitForReadyRead(), but sometimes I wait even 10 seconds - and no data anyway. When waitForReadyRead() returns true, then readAll() works fine.


  • Moderators

    while loops and sleeps... why don't you simply use the asynchronous signals that QSerialPort provides ?

    Also connect the error signal to an output see if something is triggered there



  • @kuzulis said in QSerialPort: problem with waitForReadyRead():

    What is OS, and what is Qt version?

    OS: Windows 10
    Qt Version: 5.13.1
    Building: MinGW 32-bit



  • @oBOXPOH
    Like I & @J-Hilk have said, suggest you at least try it with readyRead signal instead of waits and see if that makes the difference?


  • Moderators

    @oBOXPOH
    I would suggest to use a different Qt version QSerialPort has some serious issues in 5.13.1

    see this bugreport
    https://bugreports.qt.io/browse/QTBUG-78086



  • @J-Hilk said in QSerialPort: problem with waitForReadyRead():

    @oBOXPOH
    I would suggest to use a different Qt version QSerialPort has some serious issues in 5.13.1

    see this bugreport
    https://bugreports.qt.io/browse/QTBUG-78086

    Tried Qt 5.9.8.

    In this case waitForReadyRead() always returns false with accepted write command.



  • @JonB said in QSerialPort: problem with waitForReadyRead():

    @oBOXPOH
    Like I & @J-Hilk have said, suggest you at least try it with readyRead signal instead of waits and see if that makes the difference?

    I tried next (all is inside the class):

    connect(deviceControl_SerialPort, &QSerialPort::readyRead, this, &Class::readData_slot);
    
    void Class::readData_slot()
    {
        qDebug() << "Ready Read" << endl;
        deviceControl_readData.append(deviceControl_SerialPort->readAll());
    }
    

    In this case I can't reach readData_slot() method.

    Then I tried next:

    connect(deviceControl_SerialPort, &QSerialPort::errorOccurred, this, &Class::serialPortError_slot);
    
    void Class::serialPortError_slot(QSerialPort::SerialPortError error)
    {
        qDebug() << error << endl;
    }
    
    

    and got some error (see log below):

    WRITTEN  1 
    
    READING... 
    
    QSerialPort::TimeoutError 
    
    WRITTEN  2 
    
    READING... 
    
    QSerialPort::TimeoutError 
    
    WRITTEN  3 
    
    QSerialPort::TimeoutError 
    
    I AM NOT READY! 
    
    Ready Read 
    
    Ready Read 
    

    and, by the way, message for readyRead signal. Strangely, that I got it only after 2 real readings...


  • Lifetime Qt Champion

    Hi @oBOXPOH,

    Qt is an event-driven framework. By using endless loops, wait routines you create problems if you don't know what you are exactly doing.

    You should use QSerialPort with signals and slots. An example is shown here: https://doc.qt.io/qt-5/qtserialport-terminal-example.html

    Regards



  • @aha_1980 said in QSerialPort: problem with waitForReadyRead():

    Hi @oBOXPOH,

    Qt is an event-driven framework. By using endless loops, wait routines you create problems if you don't know what you are exactly doing.

    You should use QSerialPort with signals and slots. An example is shown here: https://doc.qt.io/qt-5/qtserialport-terminal-example.html

    Regards

    Terminal Example also works badly -> https://forum.qt.io/topic/108848/terminal-example-for-qserialport-doesn-t-work-correctly



  • @oBOXPOH said in QSerialPort: problem with waitForReadyRead():

    @aha_1980 said in QSerialPort: problem with waitForReadyRead():

    Hi @oBOXPOH,

    Qt is an event-driven framework. By using endless loops, wait routines you create problems if you don't know what you are exactly doing.

    You should use QSerialPort with signals and slots. An example is shown here: https://doc.qt.io/qt-5/qtserialport-terminal-example.html

    Regards

    Terminal Example also works badly -> https://forum.qt.io/topic/108848/terminal-example-for-qserialport-doesn-t-work-correctly

    Solution is to install 5.13.2 or more version of Qt.


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