前言

modbus在上下位机数据交互时被广泛使用，因此写了这篇笔记和大家一起学习。
【Qt Modbus通信】libmodbus实现modbus的主机功能/从机功能 源码分享
之前使用libmodbus实现了modbus的主从功能，但发现主机查询从机的从机ID不能大于200+，因此参考QT5的modbusDEMO重新写了一份基于QModbus实现的modbus主机功能。

参考文献

程序执行效果

QT官方DEMO
modbus主机 运行效果

源码下载

https://gitee.com/jiang_bin_yu/QSerialBus-modbus-master/tree/master/

程序源码

下面我将官方DEMO中的关键代码移植出来，实现了modbus主机程序
一、项目配置
QT += core gui serialport serialbus
二、寻找可用串口

void MainWindow::freshSerialPortCombox()
{
    foreach (const QSerialPortInfo &info,QSerialPortInfo::availablePorts())
    {
        QSerialPort tempSer;
        tempSer.setPort(info);
        if(tempSer.open(QIODevice::ReadWrite))
        {
            ui->comboBox_serialName->addItem(tempSer.portName());
            tempSer.close();
        }
    }
}

三、初始modbus

void MainWindow::MainWindow::InitModbus()
{
    //获取modbus raw 数据帧
    connect(SaveLog::Instance(),&SaveLog::sigModbusData,this,&MainWindow::onModbusRawData);
    if (modbusDevice) {
        modbusDevice->disconnectDevice();
        delete modbusDevice;
        modbusDevice = nullptr;
    }

    modbusDevice = new QModbusRtuSerialMaster(this);
    connect(modbusDevice, &QModbusClient::errorOccurred, [this](QModbusDevice::Error) {
        qDebug() << "modbus Error:" << modbusDevice->errorString();
    });
}

四、串口连接

void MainWindow::connectModbus()
{
    disconnnectModbus();
    if (!modbusDevice)
        return;
    modbusDevice->setConnectionParameter(QModbusDevice::SerialPortNameParameter,ui->comboBox_serialName->currentText());
    modbusDevice->setConnectionParameter(QModbusDevice::SerialParityParameter,parity);
    modbusDevice->setConnectionParameter(QModbusDevice::SerialBaudRateParameter,baud);
    modbusDevice->setConnectionParameter(QModbusDevice::SerialDataBitsParameter,dataBit);
    modbusDevice->setConnectionParameter(QModbusDevice::SerialStopBitsParameter,stopBit);

    modbusDevice->setTimeout(1000);
    modbusDevice->setNumberOfRetries(0);
    //连接失败
    if(modbusDevice->connectDevice())
    {
        ui->pushButton_connect->setText("断开连接");
        qDebug() << "连接成功";
    }
    else
    {
        ui->pushButton_connect->setText("建立连接");
        qDebug() << "连接失败";
        QMessageBox::information(NULL,  "Title",  "串口打开失败");
    }
}

五、写入寄存器
a.writeUnit:写入寄存器 slaveId 写入的从机ID startAddress写入的起始地址 values写入的值

void MainWindow::writeUnit(int slaveId, int startAddress, QList<quint16> values)
{
    if (!modbusDevice)
    {
        QMessageBox::information(NULL,  "Title",  "请先连接设备");
        return;
    }
    connectModbus();
    QModbusDataUnit writeUnit = QModbusDataUnit(QModbusDataUnit::HoldingRegisters,startAddress, values.size());
    for(int i=0; i<values.size(); i++)
    {
        writeUnit.setValue(i, values.at(i));
    }

    //serverEdit 发生给slave的ID
    if (auto *reply = modbusDevice->sendWriteRequest(writeUnit,slaveId)) {
        if (!reply->isFinished()) {
            connect(reply, &QModbusReply::finished, this, [this, reply]() {
                if (reply->error() == QModbusDevice::ProtocolError) {
                    qDebug() << QString("Write response error: %1 (Mobus exception: 0x%2)")
                                .arg(reply->errorString()).arg(reply->rawResult().exceptionCode(), -1, 16);
                } else if (reply->error() != QModbusDevice::NoError) {
                    qDebug() << QString("Write response error: %1 (code: 0x%2)").
                                arg(reply->errorString()).arg(reply->error(), -1, 16);
                }
                reply->deleteLater();
            });
        } else {
            reply->deleteLater();
        }
    } else {
        qDebug() << QString(("Write error: ") + modbusDevice->errorString());
    }
}

b.writeUnit的调用方法

void MainWindow::on_pushButton_send_clicked()
{
    QList<quint16> values;
    for(quint16 i=0; i<READNUM; i++)
    {
        values.append(i);
    }
    writeUnit(0x01,0,values);	//修改从机0x01，0~50寄存器的值
}

五、读取寄存器
a.readUnit:读取寄存器 slaveId 读取的从机ID startAddress读取的起始地址 readNum读取的数量

void MainWindow::readUnit(int slaveId, int startAddress, int readNum)
{
    if (!modbusDevice)
    {
        QMessageBox::information(NULL,  "Title",  "请先连接设备");
        return;
    }
    connectModbus();
    QMutexLocker lock(&m_modbusMutex);
    if (auto *reply = modbusDevice->sendReadRequest(QModbusDataUnit(QModbusDataUnit::HoldingRegisters, startAddress, readNum), slaveId)) {
        if (!reply->isFinished())
            connect(reply, &QModbusReply::finished, this, &MainWindow::onReadReady);
        else
            delete reply;
    } else {
        qDebug() << "Read error: " << modbusDevice->errorString();
    }
}

b.接受modbus返回数据

void MainWindow::onReadReady()
{
    auto reply = qobject_cast<QModbusReply *>(sender());
    if (!reply)
        return;

    if (reply->error() == QModbusDevice::NoError) {
        const QModbusDataUnit unit = reply->result();
        if(unit.valueCount() == READNUM)
                for (uint i = 0; i < unit.valueCount(); i++) {
                const QString entry = tr("Address: %1, Value: %2").arg(unit.startAddress() + i)
                        .arg(QString::number(unit.value(i),
                                             unit.registerType() <= QModbusDataUnit::Coils ? 10 : 16));
                ui->textBrowser->append(entry);
            }
    } else if (reply->error() == QModbusDevice::ProtocolError) {
        qDebug() << QString("Read response error: %1 (Mobus exception: 0x%2)").
                    arg(reply->errorString()).
                    arg(reply->rawResult().exceptionCode(), -1, 16);
    } else {
        qDebug() << QString("Read response error: %1 (code: 0x%2)").
                    arg(reply->errorString()).
                    arg(reply->error(), -1, 16);
    }
    reply->deleteLater();
}

六、如果你想获得你发送和接受的modbus数据帧，可以参考下我下面这个愚蠢的办法
a.设置日志过滤器，开启关于qt.modbus*的日志打印

    QLoggingCategory::setFilterRules(QStringLiteral("qt.modbus* = true"));

开启后你的调试栏讲会打印modbus相关的内容
b.如果你想获取这些内容并发送给你的界面，你需要开启日志重定向，即装载新的日志钩子（详细请参考上面码云提供的源码 代码在：QModbusDemo/savelog.cpp）

//日志重定向
#if (QT_VERSION <= QT_VERSION_CHECK(5,0,0))
void Log(QtMsgType type, const char *msg)
#else
void Log(QtMsgType type, const QMessageLogContext &, const QString &msg)
#endif
{
    //加锁,防止多线程中qdebug太频繁导致崩溃
    static QMutex mutex;
    QMutexLocker locker(&mutex);
    QString content;

    //这里可以根据不同的类型加上不同的头部用于区分
    switch (type) {
    case QtDebugMsg:
        content = QString("%1").arg(msg);
        break;

    case QtWarningMsg:
        content = QString("QtWarningMsg: %1").arg(msg);
        break;

    case QtCriticalMsg:
        content = QString("QtCriticalMsg: %1").arg(msg);
        break;

    case QtFatalMsg:
        content = QString("QtFatalMsg: %1").arg(msg);
        break;
    default: break;
    }

    SaveLog::Instance()->save(content);
}
//安装日志钩子,输出调试信息到文件,便于调试
void SaveLog::start()
{
#if (QT_VERSION <= QT_VERSION_CHECK(5,0,0))
    qInstallMsgHandler(Log);
#else
    qInstallMessageHandler(Log);
#endif
}

c.在日志重定向中，拦截(RTU client) Sent Serial ADU:和(RTU client) Received ADU:关键字的日志，并将其内容截取发送给mainwindow

void SaveLog::save(const QString &content)
{
    if(1)
    {
        QString msg = static_cast<QString>(content);
        std::cout << msg.toLocal8Bit().data() << std::endl;
        //方法改进:之前每次输出日志都打开文件,改成只有当日期改变时才新建和打开文件
        if(content.contains("(RTU client) Sent Serial ADU:"))
        {
            //(RTU client) Sent Serial ADU: 0xfe0300000029901b
            //std::cout << "SendMsg" << msg.toUtf8().data() << std::endl;
            msg = msg.remove(msg.indexOf("(RTU client) Sent Serial ADU:"),32);

            msg = msg.left(msg.size());
            msg = msg.toUpper();
            int n = msg.length();
            while(n-2 > 0)
            {
                n = n - 2;
                msg.insert(n," ");
            }
            emit sigModbusData(msg,0);
            //std::cout << "SendMsg:  " << msg.toLocal8Bit().data() << std::endl;
        }
        else if(content.contains("(RTU client) Received ADU:"))
        {
            //std::cout << "RecvMsg" << msg.toLocal8Bit().data() << std::endl;
            msg = msg.remove(msg.indexOf("(RTU client) Received ADU:"),28);
            msg = msg.left(msg.size()-1);
            msg = msg.toUpper();
            int n = msg.length();
            while(n-2 > 0)
            {
                n = n - 2;
                msg.insert(n," ");
            }
            emit sigModbusData(msg,1);
            //std::cout << "Received:  " << msg.toLocal8Bit().data() << std::endl;
        }
        else if(!content.contains("(RTU client)"))
        {
            if(this->fileName.isEmpty())
            {
                QString fileName = QString("%1/Log/%2_log_%3.txt").arg(path).arg(name).arg(QDATETIME);
                if (this->fileName != fileName) {
                    this->fileName = fileName;
                    if (file->isOpen()) {
                        file->close();
                    }

                    file->setFileName(fileName);
                    file->open(QIODevice::WriteOnly | QIODevice::Append | QFile::Text);
                }
            }
            QTextStream logStream(file);
            logStream << QString("%1--Debug     %2").arg(QDATETIME_zzz).arg(msg) << "\n";
        }
    }
}

d.绑定信号与槽，将接受到的modbus数据帧显示在textBrowser上

//获取modbus raw 数据帧
    connect(SaveLog::Instance(),&SaveLog::sigModbusData,this,&MainWindow::onModbusRawData);
//获取modbus数据帧 0发送的数据 1接受的数据
void MainWindow::onModbusRawData(QString data, int type)
{
    QString color;
    QString text;
    if(type == 0)   //发送
    {
        text = "发送数据帧:";
        color = "#F37257";
    }
    else if(type == 1)  //接受
    {
        text = "接受数据帧:";
        color = "#1777D7";
    }
    // 设置文字（样式+内容）
    QString  str = QString("<font color=\"%1\">" +QDateTime::currentDateTime().toString("yyyy-MM-dd hh:mm:ss ")+ text + data + "</font>").arg(color);
    ui->textBrowser->append(str);
}

程序效果如下图所示