extrange behavior of serial port using Qt

Question

I am performing an app with QT for communicate my PC with an 8 bits microcontroller through rs-232. I am using the QtSerialPort library and the communication is working fine, but each time I write something from the PC to the micro and I receive the response, I have to close and open the serial port or I can't continue communicating.. My configuration is: 10500 bps, 8 bits, 1 stop, no parity, no flow control.

The code used for the configuration and the lecture/writting of the port is the next:

bool DriverS::configure(int port, int baudRate)
{
    if(port!=22)
        return false;

     serialPort->setPortName("COM22");

 if (serialPort->open(QIODevice::ReadWrite)==true){

        if (!serialPort->setBaudRate(baudRate)) {
            return false ;
        }

        if (!serialPort->setDataBits(QSerialPort::Data8)) {
            return false ;
        }

        if (!serialPort->setParity(QSerialPort::NoParity)) {
            return false;
        }

        if (!serialPort->setStopBits(QSerialPort::OneStop)) {
            return false;
        }

        if (!serialPort->setFlowControl(QSerialPort::NoFlowControl)){
            return false;
            }
    };

    return true;
}



bool DriverS::read(QByteArray & rxData, int * size)
{

    Sleep(200); 
    *size = 0;
    if (serialPort->waitForReadyRead(TIMEOUT_SERIAL)) {
        rxData = serialPort->readAll();
        *size = rxData.size() ;

    if (!this->checkCRC(rxData))
        {
            qDebug()<< "Rx Checksum Error";
            return false;
        }

    return true;
    }
    qDebug()<< "Rx Timeout";
    return false;
}

bool DriverS::write(QByteArray txData)
{

    unsigned int chk = 0;
    int ret ;

    for(int i = 0;i<txData.size();i++)
    {
        chk+=txData.at(i);
    }

    txData.append(chk);
    ret = serialPort->write(txData);
    return (txData.size()==ret);

}

Can you show your code that carries this issue? Have you checked the errorString? What did it say? It is not possible to tell you what would be wrong based on this information, unfortunately. Also, are you just using a dsub-9 serial port, or there is some usb-serial dongle involved? What operating system is running on your PC? Which Qt version are you using? Which QtSerialPort version? Is it 32 or 64 bit? Does it only work with a 8 bit microcontroller? What microcontroller is it? — lpapp
Hi, and sorry for my late response, I have been very busy these last days. Answers: - Error string says : "The requested device operation is not supported or prohibited by the running operating system". - Operating system : Windows 8.1 64 bits - 8 bits microcontroller (the only one I'm using) : mc9S08AW60 - My QT version: 4.8.1 - I am not using a dsub9 connector but a Usb to TTL-232 converter (FTDI) — Cristina
Cristina, just in case, have you tried to run the application as administrator if there is such a thing on W8? — lpapp
Another intriguing issue is that I only can work with COM22, the rest of the ports are not detected :-S — Cristina

DiBosco DiBosco · Accepted Answer · 2014-02-02T15:28:35

I just found myself with exactly the same situation. I was getting stuck in the read routine. Try opening the port in unbuffered mode.

port->open(QIODevice::ReadWrite | QIODevice::Unbuffered)

It was just on one computer I found this, I've never had to do it before, but it might get you out of a hole.

extrange behavior of serial port using Qt

2 Answers