Terminal Example for QSerialPort doesn't work correctly

oBOXPOH

Hello everyone!

I still trying to use QSerialPort for reading data from some device. Now I think, that QSerialPort has really many bugs. But, maybe, developers don't know about it, so I am writing here.

I am trying to get data from some device with Terminal Example for QSerialPort: if I send somy byte, device will send me back some set of bytes. Device works normally.

In Terminal Example I connect to port with needed options (default) and then press some keys in console, but no any data.

When I write key, there will be executed next method:

void MainWindow::writeData(const QByteArray &data)
{
    qDebug() << data;
    m_serial->write(data);
    qDebug() << "WRITTEN" << endl;
}

and I really get first time next correct messages:

"1"
WRITTEN

Then I must to get readyRead signal, when data will be ready:

connect(m_serial, &QSerialPort::readyRead, this, &MainWindow::readData);

void MainWindow::readData()
{
    qDebug("Reading");
    const QByteArray data = m_serial->readAll();
    m_console->putData(data);
}

but always no data (no Reading message), i.e. no readyRead signals:

"1"
WRITTEN 

"1"
WRITTEN 

"1"
WRITTEN 

"1"
WRITTEN 

"1"
WRITTEN 

"1"
WRITTEN

Ok, example doesn't work, but I tried to fix it. I added to writeData() method waitForBytesWritten() function:

void MainWindow::writeData(const QByteArray &data)
{
    qDebug() << data;
    m_serial->write(data);
    qDebug() << "WRITTEN" << endl;

    if (m_serial->waitForBytesWritten(30000))
    {
        qDebug() << "WRITTEN REALLY" << endl;
    }
}

and I begin to get data from putdata() method, but only previous data with new message:

void Console::putData(const QByteArray &data)
{
    insertPlainText(data);

    for (int item : data)
    {
        QString str = QString("%1").arg(item, 0, 16);
        qDebug() << str;
    }

    QScrollBar *bar = verticalScrollBar();
    bar->setValue(bar->maximum());
}

"1"
WRITTEN 

WRITTEN REALLY 

"2"
WRITTEN 

Reading
"31"
"ffffffffffffffe0"
"ffffffffffffffc0"
"ffffffffffffffbb"
"ffffffffffffff87"
WRITTEN REALLY 

"2"
WRITTEN 

Reading
"32"
"ffffffffffffffe0"
"ffffffffffffffc0"
"ffffffffffffffbb"
"ffffffffffffff87"
WRITTEN REALLY

so, for first message "1" I got nothing, because no previos data, and for next message "2" I got data for message "1" (after Reading message "31", although must be "1", by the way) before waitForBytesWritten().

It works really strangely, incorrectly. Maybe, I understand something badly.

I already don't know, how to get data correctly, like every message -> correct data.

Hope for your help!

mostefa

Hi @oBOXPOH

I don't think that QSerialPort is buggy, I think that your using it wrong:

AFAIK data comes asynchronously from QSerialPort, so you should consider using QSerialPort::waitForReadyRead :

for example you should write something like that inside your readData() function:

void MainWindow::readData()
{
    qDebug("Reading");
    while (m_serial->waitForReadyRead()
   {
     ....
    const QByteArray data = m_serial->readAll();
     ...
   }
....
}

I hope that this can help you,

Thank you and best regards!

J.Hilk

@oBOXPOH
It's already reported and fixed:

https://bugreports.qt.io/browse/QTBUG-78086

Qt versions 5.12.6 and 5.13.2 are working normally

aha_1980

@mostefa and @oBOXPOH: Please don't use waitForReady... if you don't know what you are doing. In 99% of case you use it wrong.

Using the correct signals is the way to go.

Regards

oBOXPOH

@J-Hilk said in Terminal Example for QSerialPort doesn't work correctly:

@oBOXPOH
It's already reported and fixed:

https://bugreports.qt.io/browse/QTBUG-78086

Qt versions 5.12.6 and 5.13.2 are working normally

Yes, thank you very much! Now Terminal Example works correctly on 5.13.2 Qt version. I had 5.13.1 or 5.12.9 versions before.

aha_1980

@oBOXPOH So please mark this topic as SOLVED too. Thanks!

oBOXPOH

@aha_1980 said in Terminal Example for QSerialPort doesn't work correctly:

@oBOXPOH So please mark this topic as SOLVED too. Thanks!

By the way, maybe, I found another strange thing. Also in Terminal Example for QSerialPort.

    connect(m_serial, &QSerialPort::readyRead, this, &MainWindow::readData);
    connect(m_console, &Console::getData, this, &MainWindow::writeData);

//! [6]
void MainWindow::writeData(const QByteArray &data)
{
    qDebug("ReadyWrite");
    m_serial->write(data);
    qDebug("EndOfReadyWrite");
}
//! [6]

//! [7]
void MainWindow::readData()
{
    qDebug("ReadyRead");
    QThread::msleep(10000);
    const QByteArray data = m_serial->readAll();
    qDebug() << QString::number(data.count()) << endl;
    m_console->putData(data);
    qDebug("EndOfReadyRead");
}
//! [7]

When I press a key ('1'), I enter into readData() method 2 times:

ReadyWrite // Before write() method
EndOfReadyWrite // After write() method
ReadyRead // Before readAll() method
"3" // After 10 seconds I get only 3 bytes

EndOfReadyRead // And of readData() method
ReadyRead // Again readyRead() signal
"2" // After 10 seconds I get 2 last bytes

EndOfReadyRead // End of readData() method again

The question is how do I need to get all data with 1 call to readData? Just if I use even reading in another thread, I get also 2 calls to readyRead() signal.

Thank you in advance!

J.Hilk

If you remove the QThread::msleep(10000); call, you'll get the 2nd package much much faster.
That is normal behavior, you get noticed each event loop cycle, if new data arrived at the Serial port. -> getting partial data is normal and expected.
It is your task as a developer to collect and store the data and decide when the answer is complete.

oBOXPOH

@J-Hilk said in Terminal Example for QSerialPort doesn't work correctly:

If you remove the QThread::msleep(10000); call, you'll get the 2nd package much much faster.

That is normal behavior, you get noticed each event loop cycle, if new data arrived at the Serial port. -> getting partial data is normal and expected.
It is your task as a developer to collect and store the data and decide when the answer is complete.

Thank you very much for quich answer! QThread::msleep() I used only to check how readyRead() signal works.