QModbusClient的案列

首先连接到服务端：

QModbusTcpClient *client = new QModbusTcpClient();
client->setConnectionParameter(QModbusDevice::NetworkAddressParameter, "192.168.100.1");
client->setConnectionParameter(QModbusDevice::NetworkPortParameter, 8008);
client->connectDevice();

这样就连接上服务端了，下面可以发送请求了

发送读或写请求到服务端

写请求：

QModbusDataUnit writeUnit(QModbusDataUnit::HoldingRegisters, 40003, 1); // write 1 value in address 40003
writeUnit.setValue(0, 0x253); 
//这里先建好QModbusDataUnit

if (auto *reply = client->sendWriteRequest(writeUnit, 1))
//发送写请求
{
    if (!reply->isFinished())
    {
        connect(reply, &QModbusReply::finished, this, [this, reply]() 
        {
            if (reply->error() != QModbusDevice::NoError)               
                    // error in reply

                reply->deleteLater();
            });
    }
    else
    {
        if (reply->error() != QModbusDevice::NoError)           
            // error in reply

        // broadcast replies return immediately
        reply->deleteLater();
    }
}
else
{
    // error in request
}

读请求：

QModbusDataUnit readUnit(QModbusDataUnit::InputRegisters, 40006, 1); // just read input register 40006 

if (auto *reply = client->sendReadRequest(readUnit, 255)) // client id 255
{
    if (!reply->isFinished())
    {
        // connect the finished signal of the request to your read slot
        connect(reply, &QModbusReply::finished, this, &YourClass::readReady);
    }
    else
    {
        delete reply; // broadcast replies return immediately
    }
}
else
{
    // request error
}
YourClass::readReady
{
    QModbusReply *reply = qobject_cast<QModbusReply *>(sender());
    if (!reply)
        return;

    if (reply->error() == QModbusDevice::NoError)
    {
        const QModbusDataUnit unit = reply->result();
        int startAddress = unit.startAddress(); // the start address, here 40006
        int value = unit.value(0); // value of the start address + 0
        ... 

    }
    else
    {
        // reply error
    }

    reply->deleteLater(); // delete the reply 
}