Modbus 自定義命令

範例展示如何處理自定義 Modbus 功能代碼。

The example acts as both Modbus client and server in a single application. The connection between them is established via Modbus TCP. It is used to send and receive custom Modbus requests and adjusts its internal states based on the request and response.

The main purpose of the example is to provide some demo code on how to implement a Modbus client or Modbus server handling custom Modbus function codes.

用戶定義 Modbus 功能代碼

Modbus 協議支持功能代碼範圍 1 - 127 (0x01 - 0x7F HEX) . Most of the function codes are well defined and publicly documented. However, there are two ranges that can be used for user-defined functions. Those are 65 - 72 (0x41 - 48 HEX) and 100 - 110 (0x64 - 0x6E HEX) . The user can select function codes from these ranges and handle them in some custom way.

此應用程序使用功能代碼 65 (0x41 HEX) 以實現 CustomRead 命令和功能代碼 66 (0x42 HEX) 以實現 CustomWrite command. In this example the custom commands are used to simply read and write the 保持寄存器 .

發送自定義 Modbus 命令

自定義 Modbus 命令的發送是使用 QModbusClient::sendRawRequest () method. This method requires to generate a QModbusRequest object with the desired function code and a list of arguments which will be encoded into a QByteArray :

    QModbusDataUnit unit {
        QModbusDataUnit::HoldingRegisters,
        ui->startAddress->value(),
        qMin(ui->numberOfRegisters->currentText().toUShort(),
            quint16(10 - ui->startAddress->value())) // do not go beyond 10 entries
    };
    for (qsizetype i = 0, total = unit.valueCount(); i < total; ++i)
        unit.setValue(i, m_model->m_registers[i + unit.startAddress()]);
    const quint8 byteCount = quint8(unit.valueCount() * 2);
    QModbusRequest writeRequest {
        QModbusPdu::FunctionCode(ModbusClient::CustomWrite),
        quint16(unit.startAddress()),
        quint16(unit.valueCount()), byteCount, unit.values()
    };

The QModbusClient::sendRawRequest () 方法返迴 QModbusReply object which can be used to check for errors as usual:

    if (auto *reply = m_client.sendRawRequest(writeRequest, ui->serverAddress->value())) {
        if (!reply->isFinished()) {
            connect(reply, &QModbusReply::finished, this, [this, reply]() {
                if (reply->error() == QModbusDevice::ProtocolError) {
                    statusBar()->showMessage(tr("Write response error: %1 (Modbus exception: 0x%2)")
                        .arg(reply->errorString()).arg(reply->rawResult().exceptionCode(), -1, 16),
                        5000);
                } else if (reply->error() != QModbusDevice::NoError) {
                    statusBar()->showMessage(tr("Write response error: %1 (code: 0x%2)").
                        arg(reply->errorString()).arg(reply->error(), -1, 16), 5000);
                }
                reply->deleteLater();
                }
            );
        }
    } else {
        statusBar()->showMessage(tr("Write error: ") + m_client.errorString(), 5000);
    }

自定義 Modbus 服務器

自定義服務器派生自 QModbusTcpServer 類。它覆寫 QModbusServer::processPrivateRequest () 方法。

class ModbusServer : public QModbusTcpServer
{
    Q_OBJECT
    Q_DISABLE_COPY_MOVE(ModbusServer)
public:
    ModbusServer(QObject *parent = nullptr);
private:
    QModbusResponse processPrivateRequest(const QModbusPdu &request) override;
};

基服務器類調用 processPrivateRequest () 方法，當收到帶有自定義功能代碼的命令時。

自定義實現處理 CustomRead 命令通過生成 QModbusResponse 采用請求寄存器的值：

    if (ModbusClient::CustomRead == request.functionCode()) {
        quint16 startAddress, count;
        request.decodeData(&startAddress, &count);
        QModbusDataUnit unit(QModbusDataUnit::HoldingRegisters, startAddress, count);
        if (!data(&unit)) {
            return QModbusExceptionResponse(request.functionCode(),
                QModbusExceptionResponse::IllegalDataAddress);
        }
        return QModbusResponse(request.functionCode(), startAddress, quint8(count * 2), unit.values());
    }

處理 CustomWrite command includes extracting the new values from the received QModbusPdu , doing the actual value update, and returning a QModbusResponse with the registers that were actually updated:

    if (ModbusClient::CustomWrite == request.functionCode()) {
        quint8 byteCount;
        quint16 startAddress, numberOfRegisters;
        request.decodeData(&startAddress, &numberOfRegisters, &byteCount);
        if (byteCount % 2 != 0) {
            return QModbusExceptionResponse(request.functionCode(),
                QModbusExceptionResponse::IllegalDataValue);
        }
        const QByteArray pduData = request.data().remove(0, WriteHeaderSize);
        QDataStream stream(pduData);
        QList<quint16> values;
        for (int i = 0; i < numberOfRegisters; i++) {
            quint16 tmp;
            stream >> tmp;
            values.append(tmp);
        }
        if (!writeData({QModbusDataUnit::HoldingRegisters, startAddress, values})) {
            return QModbusExceptionResponse(request.functionCode(),
                QModbusExceptionResponse::ServerDeviceFailure);
        }
        return QModbusResponse(request.functionCode(), startAddress, numberOfRegisters);
    }

自定義 Modbus 客戶端

自定義客戶端派生自 QModbusTcpClient 類。它覆寫 QModbusClient::processPrivateResponse () 方法。

class ModbusClient : public QModbusTcpClient
{
    Q_OBJECT
    Q_DISABLE_COPY_MOVE(ModbusClient)
public:
    ModbusClient(QObject *parent = nullptr);
    static constexpr QModbusPdu::FunctionCode CustomRead {QModbusPdu::FunctionCode(0x41)};
    static constexpr QModbusPdu::FunctionCode CustomWrite {QModbusPdu::FunctionCode(0x42)};
private:
    bool processPrivateResponse(const QModbusResponse &response, QModbusDataUnit *data) override;
};

基客戶端類調用 processPrivateResponse () method to process the server responses with custom function codes.

The custom implementation handles the responses with CustomRead and CustomWrite 函數代碼：

bool ModbusClient::processPrivateResponse(const QModbusResponse &response, QModbusDataUnit *data)
{
    if (!response.isValid())
        return QModbusClient::processPrivateResponse(response, data);
    if (CustomRead == response.functionCode())
        return collateBytes(response, data);
    if (CustomWrite == response.functionCode())
        return collateMultipleValues(response, data);
    return QModbusClient::processPrivateResponse(response, data);
}

The CustomRead response is handled by decoding the provided QModbusPdu and extracting the values for requested registers:

static bool collateBytes(const QModbusPdu &response, QModbusDataUnit *data)
{
    if (response.dataSize() < MinimumReadResponseSize)
        return false;
    quint16 address; quint8 byteCount;
    response.decodeData(&address, &byteCount);
    if (byteCount % 2 != 0)
        return false;
    if (data) {
        QDataStream stream(response.data().remove(0, 3));
        QList<quint16> values;
        const quint8 itemCount = byteCount / 2;
        for (int i = 0; i < itemCount; i++) {
            quint16 tmp;
            stream >> tmp;
            values.append(tmp);
        }
        *data = {QModbusDataUnit::HoldingRegisters, address, values};
    }
    return true;
}

The CustomWrite response is handled by simply validating the response parameters:

static bool collateMultipleValues(const QModbusPdu &response, QModbusDataUnit *data)
{
    if (response.dataSize() != WriteResponseSize)
        return false;
    quint16 address, count;
    response.decodeData(&address, &count);
    if (count < 1 || count > 10)
        return false;
    if (data)
        *data = {QModbusDataUnit::HoldingRegisters, address, count};
    return true;
}

運行範例

要運行範例從 Qt Creator ，打開歡迎模式，然後選擇範例從範例。更多信息，見 Qt Creator：教程：構建並運行 .

This example cannot be used in conjunction with other applications. Once the example is started, it can only exchange custom Modbus commands within the application itself. All interactions between the client and server use the Modbus TCP protocol.

範例工程 @ code.qt.io