Example of how to read a holding register in C++ using QT Modbus Serial Client 6.x

qt_modbus_read_example.cpp

 #include <QCoreApplication>
#include <QVariant>
#include <QSerialPort>
#include <QSerialPortInfo>
#include <QModbusRtuSerialClient>
#include <QModbusDevice>
#include <QModbusClient>
#include <QVariant>
#include <QSerialPort>
#include <thread>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QModbusRtuSerialClient* client = new QModbusRtuSerialClient();

    client->setConnectionParameter(QModbusDevice::SerialPortNameParameter,"COM7");
    client->setConnectionParameter(QModbusDevice::SerialBaudRateParameter,QSerialPort::Baud9600);
    client->setConnectionParameter(QModbusDevice::SerialDataBitsParameter,QSerialPort::Data8);
    client->setConnectionParameter(QModbusDevice::SerialStopBitsParameter,QSerialPort::OneStop);
    client->setConnectionParameter(QModbusDevice::SerialParityParameter,QSerialPort::NoParity);
    client->setTimeout(2000);

    if(!client->connectDevice())
        qDebug() << client->errorString();

    QModbusDataUnit readUnit;
    readUnit.setRegisterType(QModbusDataUnit::HoldingRegisters);
    readUnit.setStartAddress(0);
    readUnit.setValueCount(1);

    QModbusReply* reply = client->sendReadRequest(readUnit,2);

    QObject::connect(reply, &QModbusReply::finished, QCoreApplication::instance(),  [&]() {
        qDebug() << reply->result().values();
    });
    QObject::connect(QCoreApplication::instance(), &QCoreApplication::aboutToQuit, QCoreApplication::instance(), [&]() {
        client->disconnect();
    });
    std::this_thread::sleep_for(std::chrono::seconds(3));
    return a.exec();
}