Trouble in reading from Serial port using QSerialPort

Question

I have to develop a C++ program for an embedded FriendlyARM-based processor system. I use Qt Creator 3.0.0 (based on Qt 5.2.0) for desktop computer. My program should be able to read from serial port at Mini2440 FriendlyARM processor.

Before going to target system (embedded system), I tried to read and write from/to a serial port on my Laptop. My main problem is how to read from serial port. As you know, new computers and laptops don't have serial port so I try to simulate serial port programming using hand-made USB-to-serial adapter cable. When the USB serial cable is plugged in, it is recognized as "/dev/ttyUSB0" on Ubuntu. It seems to work well. Please note, the other end of cable (the serial port) isn't connected to anything.

My first question is: Is it OK to configure cable like this, or I have to connect it to other device? I try to wrote to /dev/ttyUSB0 each 10 seconds and read the data. I ended up the following code:

void MainWindow::refreshNotificationArea()
{
    generateNotifAreaData(); // a typical random data-generator
    QList<QSerialPortInfo> L = QSerialPortInfo::availablePorts();
    for (auto e : L)
        qDebug() << e.portName() << '\n'; // it prints 1 serial port: :ttyUSB0
    // write to the port
    QSerialPort notifAreaPort;
    // 1. set properties
    notifAreaPort.setBaudRate(QSerialPort::Baud9600); 
    notifAreaPort.setStopBits(QSerialPort::OneStop);
    notifAreaPort.setParity(QSerialPort::NoParity); 
    notifAreaPort.setDataBits(QSerialPort::Data8);
    notifAreaPort.setFlowControl(QSerialPort::NoFlowControl); 
    QObject::connect(&notifAreaPort,SIGNAL(error(QSerialPort::SerialPortError)),
    this, SLOT(errorReport(QSerialPort::SerialPortError)));
    notifAreaPort.setPortName(serial_device.c_str());
    // 2. open port
    notifAreaPort.open(QIODevice::ReadWrite);
    if (!notifAreaPort.isOpen())
        qDebug() << "Open failed"; // open is OK, no error message printed

    string s  = convertNotifAreadData2Str();
    qDebug() << "Generated data " << s.c_str(); // OK
    int a = notifAreaPort.write(s.c_str()); // write done
    qDebug() << "Write count" << a; // OK

    // now read the info

    QByteArray ba = notifAreaPort.readLine(3); // read failed
    QSerialPort::SerialPortError err = notifAreaPort.error();
    qDebug() << "Error code" << err;
    qDebug() << "What? " << notifAreaPort.errorString();
    qDebug() << "Read count " << ba.size(); // 0

    notifAreaPort.close();
}

void MainWindow::errorReport(QSerialPort::SerialPortError error)
{
    if(error!=0)
        qDebug()<<"ERROR:"<<endl<<error; // nothing printed
}

Writing to serial port is OK. but Reading issues sometimes "No such file or directory"! sometimes "File temporarily unavalable! The strange thing is notifAreaPort.error() returns 0, and it means no error occured!

Thoughts?

-- Saeed Amrollahi Boyouki

Answer 1

There are a couple of issues in your code, but I will highlight the most important of those:

• You are setting the parameters before opening. This should happen after opening. That is how the API was sadly designed, but we are in the process of revamping it.

• You should command line examples for reading that I added in 5.2? It seems that you do not know how to read and those would give you a simple example. In short: you are basically trying to read before the write potentially even finished.