Problem with data read from USB port

[pj0909]

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) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);

    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();
            }
        }
    }

    if(sensor_is_available)
    {
        qDebug() << "Found the sensor port\n";
        sensor->setPortName(sensor_port_name);
        sensor->open(QSerialPort::ReadOnly);
        sensor->setBaudRate(QSerialPort::Baud9600);
        sensor->setDataBits(QSerialPort::Data8);
        sensor->setFlowControl(QSerialPort::HardwareControl);
        sensor->setParity(QSerialPort::NoParity);
        sensor->setStopBits(QSerialPort::OneStop);
        QObject::connect(sensor, SIGNAL(readyRead()), this, SLOT(readSerial()));
    }

    else{
        qDebug() << "Couldn't find the correct port for the sensor.\n";
        QMessageBox::warning (this, "Serial Port Error", "Couldn't open serial port to sensor");
    }
}

Dialog::~Dialog()
{
    if(sensor->isOpen())
        sensor->close();
    delete ui;
}

void Dialog::readSerial()
{
    serialData = sensor->readAll(); // serialData included in header file as QByteArray
    serialBuffer += QString::fromStdString(serialData.toStdString());
    ui->textBrowser->setText(serialBuffer);

    qDebug() <<serialBuffer;
    qDebug("stopped");
}

The output which I am getting is as below :

"\r\u0014f\u0000@\np?>??>?V?>S&?\r\u0014f\u0000@\n?=?'?>?\u0005?>?$??\r\u0014f\u0000ve?????>?f|>????\r\u0014f\u0000ve?=??i>??C>????"
stopped

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.

[VRonin]

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

how does your sensor encode the data?

[pj0909]

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

[VRonin]

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

values

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?

[pj0909]

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

[jsulm]

@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

[VRonin]

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;
for(;;){
sensorStream.startTransaction();
double readValue=0;
sensorStream >> readValue;
if(sensorStream.commitTransaction())
readValues.push_back(readValue);
else
break;
}
qDebug() << "data read: " << readValues;
}

P.S.
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

[kshegunov]

@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;

[pj0909]

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.

[pj0909]

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.

[namduc]

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!

[kuzulis]

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

[namduc]

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

[aha_1980]

Hi @namduc,

there is exactly one: libusb

Regards