Simple serial port not outputting data

starfire151

Apologies to all if this is not the correct place to post this...

I'm a very newbie to the Qt environment but I am excited about the possibilities. I've looked through most of the example serial port apps related to this and tried to make a very small app that just writes a test message to a serial port. I've just created a simple main window which has a combobox populated with the available serial ports and a send command button which starts the action of sending the data to the port. I tried to make this very small and streamlined (maybe I made it TOO small and streamlined??). I'm including the mainwindow.h here:

@
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QtSerialPort/QtSerialPort>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private slots:
void on_pbSendCmd_clicked();

private:
Ui::MainWindow *ui;
QSerialPort *serial;
};

#endif // MAINWINDOW_H
@

The mainwindow.cpp is included here:

@
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QComboBox>
#include <QString>
#include <QMessageBox>
#include <QtSerialPort/QSerialPortInfo>

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

// change window title
setWindowTitle("Widget Testbed");

// do serial port combobox population
foreach(const QSerialPortInfo &info, QSerialPortInfo::availablePorts())
    ui->serialPortComboBox->addItem(info.portName());

// create serial port
serial = new QSerialPort(this);

}

MainWindow::~MainWindow()
{
delete ui;
}

void MainWindow::on_pbSendCmd_clicked()
{
// set parameters for com port
serial->setPortName(ui->serialPortComboBox->currentText());
serial->setBaudRate(QSerialPort::Baud38400);
serial->setDataBits(QSerialPort::Data8);
serial->setParity(QSerialPort::NoParity);
serial->setStopBits(QSerialPort::OneStop);
serial->setFlowControl(QSerialPort::NoFlowControl);

// open the selected com port
if(!serial->open(QIODevice::ReadWrite)){
    QMessageBox::warning(this, "COM Error", "Can't open port");
    return;
}

// write data to port
QByteArray data = "test message";
serial->write(data);

// close the selected com port
serial->close();

}
@

In addition, I've changed the .pro file line to greaterThan(QT_MAHOR_VERSION, 4) : QT += widgets serialport.
The combobox populates correctly and there are no compilation errors. The port appears to open correctly but no data is written. What am I doing wrong?

Thanks for any help.

SGaist

Hi,

How do you know there's nothing sent ?

starfire151

I hooked up the output to an oscilloscope and looked for any data transitions... Nothing showed up.

SGaist

Does it also happen if you use a standard Windows utility ?

starfire151

Hi again -

I have confirmed that the serial port works correctly using a Tera Term session to an external device connected to the same serial port I'm trying to get the Qt app to work with.

Thanks.

kuzulis

[quote]
@
// write data to port
QByteArray data = "test message";
serial->write(data);

// close the selected com port
serial->close();

@
[/quote]

This is wrong sequence. The serial port nothing to write in this case because the close() immediately does closing of device.

You should or:

@
serial->write(data);
serial->flush();
serial->close();
@

or:

@
serial->write(data);
serial->waitForBytesWritten();
serial->close();
@

or to close the device after the bytesWritten(qint64) signal received.

NOTE: All I/O in serialport is asynchronous!

starfire151

Thanks so much for responding! I tried the serial->flush() but still didn't see anything on the scope. I tried the serial->waitForBytesWritten(strlen(data)) and it worked like a charm!

I'm still learning about how the timing of everything works but this is a grat start.

Thanks, again!