/Qt 5.13

QModbusDevice Class

The QModbusDevice class is the base class for Modbus classes, QModbusServer and QModbusClient. More...

Header: #include <QModbusDevice>
qmake: QT += serialbus
Since: Qt 5.8
Inherits: QObject
Inherited By:

QModbusClient and QModbusServer

This class was introduced in Qt 5.8.

Public Types

enum ConnectionParameter { SerialPortNameParameter, SerialParityParameter, SerialBaudRateParameter, SerialDataBitsParameter, ..., UserParameter }
enum Error { NoError, ReadError, WriteError, ConnectionError, ..., UnknownError }
enum State { UnconnectedState, ConnectingState, ConnectedState, ClosingState }

Public Functions

QModbusDevice(QObject *parent = nullptr)
virtual ~QModbusDevice()
bool connectDevice()
QVariant connectionParameter(int parameter) const
void disconnectDevice()
QModbusDevice::Error error() const
QString errorString() const
void setConnectionParameter(int parameter, const QVariant &value)
QModbusDevice::State state() const


void errorOccurred(QModbusDevice::Error error)
void stateChanged(QModbusDevice::State state)

Protected Functions

virtual void close() = 0
virtual bool open() = 0
void setError(const QString &errorText, QModbusDevice::Error error)
void setState(QModbusDevice::State newState)

Detailed Description

Member Type Documentation

enum QModbusDevice::ConnectionParameter

This enum describes the possible values that can be set for a Modbus device connection.

The general purpose value (and the associated types) are:

Constant Value Description
QModbusDevice::SerialPortNameParameter 0 This parameter holds the serial port used for device communication, e.g. COM1. QString
QModbusDevice::SerialParityParameter 1 This parameter holds the parity checking mode. QSerialPort::Parity
QModbusDevice::SerialBaudRateParameter 2 This parameter holds the data baud rate for the communication. QSerialPort::BaudRate
QModbusDevice::SerialDataBitsParameter 3 This parameter holds the data bits in a frame. QSerialPort::DataBits
QModbusDevice::SerialStopBitsParameter 4 This parameter holds the number of stop bits in a frame. QSerialPort::StopBits
QModbusDevice::NetworkPortParameter 5 This parameter holds the network port. int
QModbusDevice::NetworkAddressParameter 6 This parameter holds the host address for network communication. QString

User options:

Constant Value Description
QModbusDevice::UserParameter 0x100 The first parameter that can be used for user-specific purposes. QVariant

enum QModbusDevice::Error

This enum describes all the possible error conditions.

Constant Value Description
QModbusDevice::NoError 0 No errors have occurred.
QModbusDevice::ReadError 1 An error occurred during a read operation.
QModbusDevice::WriteError 2 An error occurred during a write operation.
QModbusDevice::ConnectionError 3 An error occurred when attempting to open the backend.
QModbusDevice::ConfigurationError 4 An error occurred when attempting to set a configuration parameter.
QModbusDevice::TimeoutError 5 A timeout occurred during I/O. An I/O operation did not finish within a given time frame.
QModbusDevice::ProtocolError 6 A Modbus specific protocol error occurred.
QModbusDevice::ReplyAbortedError 7 The reply was aborted due to a disconnection of the device.
QModbusDevice::UnknownError 8 An unknown error occurred.

enum QModbusDevice::State

This enum describes all possible device states.

Constant Value Description
QModbusDevice::UnconnectedState 0 The device is disconnected.
QModbusDevice::ConnectingState 1 The device is being connected.
QModbusDevice::ConnectedState 2 The device is connected to the Modbus network.
QModbusDevice::ClosingState 3 The device is being closed.

Member Function Documentation

QModbusDevice::QModbusDevice(QObject *parent = nullptr)

Constructs a Modbus device with the specified parent.

[signal] void QModbusDevice::errorOccurred(QModbusDevice::Error error)

This signal is emitted when an error of the type, error, occurs.

[signal] void QModbusDevice::stateChanged(QModbusDevice::State state)

This signal is emitted every time the state of the device changes. The new state is represented by state.

See also setState() and state().

[virtual] QModbusDevice::~QModbusDevice()

Destroys the QModbusDevice instance

[pure virtual protected] void QModbusDevice::close()

This function is responsible for closing the Modbus connection. The implementation must ensure that the instance's state() is set to QModbusDevice::UnconnectedState.

See also disconnectDevice().

bool QModbusDevice::connectDevice()

Connects the device to the Modbus network. Returns true if the connection process was successfully initiated; otherwise false. Final connection success confirmation requires the state() changing to QModbusDevice::ConnectedState.

This function calls open() as part of its implementation.

See also open().

QVariant QModbusDevice::connectionParameter(int parameter) const

Returns the value associated with the given connection parameter. The returned value can be empty.

By default the QModbusDevice is initialized with some common values. The serial port settings are even parity, a baud rate of 19200 bits per second, eight data bits and one stop bit. The network settings for the host address is set to local host and port to 502.

Note: For a serial connection to succeed, the SerialPortNameParameter needs to be set to a valid communication port. The information about valid serial ports can be obtained from QSerialPortInfo.

Note: If the device is already connected, the settings are taken into account after reconnecting the device.

See also setConnectionParameter() and ConnectionParameter.

void QModbusDevice::disconnectDevice()

Disconnects the device.

This function calls close() as part of its implementation.

QModbusDevice::Error QModbusDevice::error() const

Returns the error state of the device.

See also setError() and QModbusDevice::Error.

QString QModbusDevice::errorString() const

Returns descriptive error text for the device error.

See also QModbusDevice::Error.

[pure virtual protected] bool QModbusDevice::open()

This function is called by connectDevice(). Subclasses must provide an implementation that returns true on successful Modbus connection or connection initiation; otherwise returns false.

The implementation must ensure that the instance's state() is set to QModbusDevice::ConnectingState or QModbusDevice::ConnectedState upon success; otherwise QModbusDevice::UnconnectedState. Typically, QModbusDevice::ConnectingState is used when the connection process reports back asynchronously and QModbusDevice::ConnectedState in case of synchronous connect behavior.

See also connectDevice().

void QModbusDevice::setConnectionParameter(int parameter, const QVariant &value)

Sets the value of parameter to value. If the parameter already exists, the previous value is overwritten. A active or running connection is not affected by such parameter changes.

See also ConnectionParameter and connectionParameter().

[protected] void QModbusDevice::setError(const QString &errorText, QModbusDevice::Error error)

Sets the error state of the device. ModBus device implementations must use this function in case of an error to set the error type and a descriptive errorText.

See also error() and QModbusDevice::Error.

[protected] void QModbusDevice::setState(QModbusDevice::State newState)

Sets the state of the device to newState. Modbus device implementations must use this function to update the device state.

See also state().

QModbusDevice::State QModbusDevice::state() const

Returns the current state of the device.

See also setState() and stateChanged().

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