----- 数据读取 -----


03 功能码 
/*--------------------widget.h --------------------*/
void readHoldingRegisters(quint8 slaveAddress, quint16 startAddress, quint16 count, int id)
 
/*--------------------widget.cpp --------------------*/
//读取保持寄存器（03 功能码）
void Widget::readHoldingRegisters(quint8 slaveAddress, quint16 startAddress, quint16 count, int id)
{
    //检查连接
    if (RTU_Device->state() != QModbusDevice::ConnectedState){
        qDebug() << "Modbus 设备未连接";
        return;
    }
 
    //创建读取请求
    QModbusDataUnit readUnit(QModbusDataUnit::HoldingRegisters, startAddress, count);
 
    //发送请求
    if (auto *reply = RTU_Device->sendReadRequest(readUnit, slaveAddress)){
        if (!reply->isFinished()){
            connect(reply, &QModbusReply::finished, this, [reply, this, id](){
                if (reply->error() == QModbusDevice::NoError){
                    const QModbusDataUnit unit = reply->result();
                    for (int i = 0; i < unit.valueCount(); i++){
                        qDebug() << "寄存器" << unit.startAddress() + i << "的值：" << unit.value(i);
                        switch (id) {
                        case 1:
                        {
                            qDebug() <<"第一个寄存器 = " << unit.value(0);
                            break;
                        }
                        default:
                            break;
                        }
                    }
                } else {
                    qDebug() << "读取错误：" << reply->errorString();
                }
                reply->deleteLater();  //释放资源
            });
        } else {
            reply->deleteLater();   //处理已完成的响应
        }
    } else {
        qDebug() << "读取请求失败";
    }
}


----- 数据写入 -----


06 功能码 
/*--------------------widget.h --------------------*/
void writeSingleRegister(quint8 slaveAddress, quint16 startAddress, int data)
 
/*--------------------widget.cpp --------------------*/
//写入保持寄存器（06 功能码）
void Widget::writeSingleRegister(quint8 slaveAddress, quint16 startAddress, int data)
{
    //检查连接
    if (data < 0 || data > 65535) {
        qDebug() << "错误：写入值超出范围（0~65535），实际值:" << data;
        return;
    }
 
    //检查写入值 data 是否超出 保持寄存器 范围（0 ~ 65535）
    if (RTU_Device->state() != QModbusDevice::ConnectedState) {
        qDebug() << "Modbus 设备未连接";
        return;
    }
 
    //安全转换为 quint16
    quint16 valueToWrite = static_cast<quint16>(data);
 
    //创建写入请求（写入 1 个寄存器）
    QModbusDataUnit writeUnit(QModbusDataUnit::HoldingRegisters, startAddress, 1);
 
    //设置第0个寄存器（即 startAddress 对应的那个）的值为 valueToWrite
    writeUnit.setValue(0, valueToWrite);
 
    //发送写请求到指定从站地址
    QModbusReply *reply = RTU_Device->sendWriteRequest(writeUnit, slaveAddress);
 
    if (!reply) {
        qDebug() << "写入失败：无法创建写请求";
        return;
    }
 
    if (!reply->isFinished()) {
        //异步：等待 Modbus 设备回复
        QObject::connect(reply, &QModbusReply::finished, this, [this, reply]() {
            if (reply->error() == QModbusDevice::NoError) {
                qDebug() << "写入成功！";
            } else {
                qDebug() << "写入失败，错误:" << reply->errorString();
            }
            reply->deleteLater();  //须释放内存
        });
    } else {
        //如果回复已经完成（极少数同步情况）
        if (reply->error() == QModbusDevice::NoError) {
            qDebug() << "写入成功！";
        } else {
            qDebug() << "写入失败，错误:" << reply->errorString();
        }
        reply->deleteLater();  //须释放内存
    }
}