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Problem with data read from USB port

  • Hello All,

    I am new to Qt and I need your suggestions for my problem. I have a sensor connected to USB port and I want to read the data from the sensor. I tried with QSerialPort and I am getting some random values. Could anyone please advice me where I am going wrong. Below is my source code

    #include "dialog.h"
    #include "ui_dialog.h"
    #include <QtSerialPort/QSerialPort>
    #include <QtSerialPort/QSerialPortInfo>
    #include <QDebug>
    #include <QString>
    #include <QMessageBox>
    Dialog::Dialog(QWidget *parent) :
        ui(new Ui::Dialog)
        sensor = new QSerialPort(this); //sensor is included in header file as QSerialPort
        serialBuffer = ""; // included in header file as QString
        qDebug() << "Number of ports: " << QSerialPortInfo::availablePorts().length() << "\n";
        foreach(const QSerialPortInfo &serialportinfo, QSerialPortInfo::availablePorts())
            qDebug() << "Description: " <<serialportinfo.description() << "\n";
            qDebug() << "Has vendor id: " <<serialportinfo.hasVendorIdentifier() << "\n";
            qDebug() << "vendor id: " <<serialportinfo.vendorIdentifier() << "\n";
            qDebug() << "Has product id: " <<serialportinfo.hasProductIdentifier() << "\n";
            qDebug() << "prodcut id: " <<serialportinfo.productIdentifier() << "\n";
        bool sensor_is_available = false;
        QString sensor_port_name;
        foreach(const QSerialPortInfo &serialportinfo, QSerialPortInfo::availablePorts())
            if(serialportinfo.hasProductIdentifier() && serialportinfo.hasVendorIdentifier())
                if((serialportinfo.productIdentifier() == sensor_product_id) && (serialportinfo.vendorIdentifier() == sensor_vendor_id))
                    sensor_is_available = true;
                    sensor_port_name = serialportinfo.portName();
            qDebug() << "Found the sensor port\n";
            QObject::connect(sensor, SIGNAL(readyRead()), this, SLOT(readSerial()));
            qDebug() << "Couldn't find the correct port for the sensor.\n";
            QMessageBox::warning (this, "Serial Port Error", "Couldn't open serial port to sensor");
        delete ui;
    void Dialog::readSerial()
        serialData = sensor->readAll(); // serialData included in header file as QByteArray
        serialBuffer += QString::fromStdString(serialData.toStdString());
        qDebug() <<serialBuffer;

    The output which I am getting is as below :


    I am still figuring out where I went wrong. I read somewhere that data from USB port can be read through serial port functions. Could anyone advice me on the above code?

    Thank you for your time and knowledge.

  • the problem is here: serialBuffer += QString::fromStdString(serialData.toStdString());

    how does your sensor encode the data?

  • @VRonin The sensor is based on capacitive technology. Any change in capacitance gives corresponding values.

  • @pj0909 said in Problem with data read from USB port:


    This still does not answer my question. how are those values represented in binary format to the serial output? is it a 32 bit integer, a 64 bit double precision float, an ASCII encoded string?

  • @VRonin Sorry, I misinterpreted your question. The answer to your question is 64 bit double precision float.

  • Lifetime Qt Champion

    @pj0909 Then you should use http://doc.qt.io/qt-5/qbytearray.html#toDouble without converting to string:

    bool ok = false;
    serialBuffer += sensor->readAll().toDouble(&ok);
    if (!ok) // error

  • To add to @jsulm, you should also check that serialData contains 64 and only 64 bits of data (i.e. serialData.size()==8) otherwise use QDataStream

    void Dialog::readSerial()
    QDataStream sensorStream(sensor);
    QVector<double> readValues;
    double readValue=0;
    sensorStream >> readValue;
    qDebug() << "data read: " << readValues;

    If your sensor sends the data in little endian order, you'll have to set the QDataStream byte order http://doc.qt.io/qt-5/qdatastream.html#setByteOrder

  • Moderators

    @pj0909 said in Problem with data read from USB port:

    The answer to your question is 64 bit double precision float.

    Then you need to read the data as binary, not as text! Something like this (warning: the code isn't really portable):

    QDataStream in(&serialData, QIODevice::ReadOnly);
    double value;
    in >> value;

  • I will implement the suggestions in the code and will see the output from the sensor. Will let you all know if its working correctly or still there is a problem. Thank you for the suggestions.

  • Finally I solved my problem. I set the floating point precision in addition to the suggestions given above and now I am reading the data as expected.

    Thank you all for the suggestions.

  • Hi all!
    I have the same problem, I need to communicate with a USB card reader, but I don't recognize it when i plug it with my PC ( Win 7)
    serialportinfo.hasProductIdentifier() and serialportinfo.hasVendorIdentifier() all return false;
    Who can help me? Thanks!

  • Qt Champions 2020

    What's relation between the serial ports and the USB card readers?

  • Reader Card connect directly to the computer using the USB port
    Are there any libraries supporting this?

  • Lifetime Qt Champion

    Hi @namduc,

    there is exactly one: libusb


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