RTU Serial port isn't opening

Tague Carlyon

Hello! I can't figure out why my QSerialPort object isn't opening. I am on Linux.

It merely outputs "QIODevice::read (QSerialPort): device not open" in the console whenever I try to read the readAll() function.
I need to connect to an RTU modbus device.

.cpp file:

#include <QSerialPort>
#include <QtCore>
#include <iostream>

int main(int argc, char** argv)
{
QByteArray temp;
char data[8];

qint64 size = 8;

QSerialPort *solarCharger = new QSerialPort;

solarCharger->setPortName("/dev/ttyUSB0");	
solarCharger->setDataBits(QSerialPort::Data8);
solarCharger->setStopBits(QSerialPort::OneStop);
solarCharger->setBaudRate(QSerialPort::Baud9600);
solarCharger->setParity(QSerialPort::NoParity);
solarCharger->setFlowControl(QSerialPort::NoFlowControl);

std::cout << solarCharger->isOpen() << std::endl;

solarCharger->open(QIODevice::ReadOnly);

std::cout << solarCharger->isOpen() << std::endl;

temp = solarCharger->readAll();

std::cout << temp[0] << std::endl;

delete solarCharger;
solarCharger = nullptr;

return 0;

}

.pro file:

TEMPLATE = app
TARGET = Qt
INCLUDEPATH += .

DEFINES += QT_DEPRECATED_WARNINGS

SOURCES += simple.cpp

QT += serialport core

ChrisW67

@Tague-Carlyon Does your open() call succeed?

Connect something to the QSerialPort::errorOccurred() signal to see what sort of error you have.

You never execute the application event loop so it is unlikely that there will ever be data to read.

Tague Carlyon

@ChrisW67 When I had access to the RTU port on the MPPT solar charger, I don't think the open() call succeeded. I will update you tomorrow with more information when I will have access to the MPPT solar charger again. In the meantime, I apologize but I don't fully understand how to connect something to the QSerialPort::errorOccurred(). I understand the QObject::connect() function loosely but not enough to do what you are asking from scratch.

ChrisW67

QSerialPort is, like most Qt I/O, designed to be used asynchronously. Setting it up that way and connecting the errorOccurred() signal to a slot that can handle it goes something like this:

#include <QCoreApplication>
#include <QDebug>
#include <QSerialPort>
#include <QTimer>

class SerialReceiver: public QObject
{
    Q_OBJECT
public:
    explicit SerialReceiver(QObject *p = nullptr)
        : QObject(p)
        , m_solarCharger(nullptr)
    {
        m_solarCharger = new QSerialPort(this);
        connect(m_solarCharger, &QSerialPort::readyRead, this, &SerialReceiver::handleReadyRead);
        connect(m_solarCharger, &QSerialPort::errorOccurred, this, &SerialReceiver::handleErrorOccurred);

        m_solarCharger->setPortName("/dev/ttyUSB0");
        m_solarCharger->setDataBits(QSerialPort::Data8);
        m_solarCharger->setStopBits(QSerialPort::OneStop);
        m_solarCharger->setBaudRate(QSerialPort::Baud9600);
        m_solarCharger->setParity(QSerialPort::NoParity);
        m_solarCharger->setFlowControl(QSerialPort::NoFlowControl);

        if (!m_solarCharger->open(QSerialPort::ReadOnly)) {
            qDebug() << Q_FUNC_INFO << "Failed to open serial port";
        }
    }
    ~SerialReceiver() { }

private slots:
    void handleReadyRead() {
        QByteArray temp = m_solarCharger->readAll();
        qDebug() << temp.size() << "bytes read";
    }

    void handleErrorOccurred(QSerialPort::SerialPortError error) {
        if (error != QSerialPort::NoError) {
            qDebug() << Q_FUNC_INFO << error;
        }
    }

private:
    QSerialPort *m_solarCharger    ;

};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    SerialReceiver receiver;
    // arrange for the program to end itself after 15 seconds
    QTimer::singleShot(15000, &a, &QCoreApplication::quit);
    return a.exec();
}

#include "main.moc"

J.Hilk

@Tague-Carlyon

the event loop of QCoreApplications needs to be running, otherwise this won't work at all.