Learn what Felgo offers to help your business succeed. Start your free evaluation today! Felgo for Your Business

QModbusPdu Class

QModbusPdu is a abstract container class containing the function code and payload that is stored inside a Modbus ADU. More...

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

QModbusRequest and QModbusResponse

Public Types

enum ExceptionCode { IllegalFunction, IllegalDataAddress, IllegalDataValue, ServerDeviceFailure, Acknowledge, …, ExtendedException }
enum FunctionCode { Invalid, ReadCoils, ReadDiscreteInputs, ReadHoldingRegisters, ReadInputRegisters, …, UndefinedFunctionCode }

Public Functions

virtual ~QModbusPdu()
QByteArray data() const
qint16 dataSize() const
void decodeData(Args &&... data) const
void encodeData(Args... data)
QModbusPdu::ExceptionCode exceptionCode() const
QModbusPdu::FunctionCode functionCode() const
bool isException() const
bool isValid() const
void setData(const QByteArray &data)
virtual void setFunctionCode(QModbusPdu::FunctionCode code)
qint16 size() const

Static Public Members

const quint8 ExceptionByte

Protected Functions

QModbusPdu(const QModbusPdu &other)
QModbusPdu(QModbusPdu::FunctionCode code, const QByteArray &data)
QModbusPdu & operator=(const QModbusPdu &other)
QDebug operator<<(QDebug debug, const QModbusPdu &pdu)
QDataStream & operator<<(QDataStream &stream, const QModbusPdu &pdu)

Detailed Description

The class provides access to the raw Modbus protocol packets as defined by the Modbus Application Protocol Specification 1.1b.

Member Type Documentation

enum QModbusPdu::ExceptionCode

This enum describes all the possible error conditions as defined by Modbus Exception Codes. They are set by the server after checking the appropriate error conditions in the reply to a request and must be decoded by the client to operate on the exception code.

Constant Value Description
QModbusPdu::IllegalFunction 0x01 Function code is not supported by device.
QModbusPdu::IllegalDataAddress 0x02 The received data address in the query is not an allowable address for the Modbus server.
QModbusPdu::IllegalDataValue 0x03 The contained value in the request data field is not an allowable value for the Modbus server.
QModbusPdu::ServerDeviceFailure 0x04 An irrecoverable error occurred while the server was attempting to perform the requested action.
QModbusPdu::Acknowledge 0x05 Specialized use in conjunction with programming commands.
QModbusPdu::ServerDeviceBusy 0x06 The server is engaged in processing a long duration program command.
QModbusPdu::NegativeAcknowledge 0x07 The server cannot perform the program function received in the query. This code is returned for an unsuccessful programming request. The client should request diagnostic or error information from the server.
QModbusPdu::MemoryParityError 0x08 Indicates that the extended file area failed to pass a consistency check. Used in conjunction with function codes 20 and 21. The exception code does not refer to any parity settings of the transmission line but only to the servers' internal memory of file records.
QModbusPdu::GatewayPathUnavailable 0x0A Indicates that the gateway was unable to allocate an internal communication path from the input port to the output port for processing the request.
QModbusPdu::GatewayTargetDeviceFailedToRespond 0x0B Indicates that no response was obtained from the target device behind a gateway. Usually this means the target device is not online on the network.
QModbusPdu::ExtendedException 0xFF This is an extended exception as per Modbus specification. Generally this code is used to describe an exception that is otherwise further described.

enum QModbusPdu::FunctionCode

Defines the function code and the implicit type of action required by the server. Not all Modbus devices can handle the same set of function codes.

Constant Value Description
QModbusPdu::Invalid 0x00 Set by the default constructor, do not use.
QModbusPdu::ReadCoils 0x01 Requests the status of one or more coils from a device.
QModbusPdu::ReadDiscreteInputs 0x02 Requests the status of one or more input registers from a device.
QModbusPdu::ReadHoldingRegisters 0x03 Requests the status of one or more holding register values from a device.
QModbusPdu::ReadInputRegisters 0x04 Requests the status of one or more input register values from a device.
QModbusPdu::WriteSingleCoil 0x05 Requests to write a single coil on a device.
QModbusPdu::WriteSingleRegister 0x06 Requests to write a single holding register on a device.
QModbusPdu::ReadExceptionStatus 0x07 Requests the status of the eight Exception Status outputs on a device.
QModbusPdu::Diagnostics 0x08 Used to provide a series of tests for checking the client server communication system, or checking internal
QModbusPdu::GetCommEventCounter 0x0B Requests a status word and an event count from the device's communication event counter.
QModbusPdu::GetCommEventLog 0x0C Requests a status word, event count, message count, and a field of event bytes from a device.
QModbusPdu::WriteMultipleCoils 0x0F Requests to write one or more coils on a device.
QModbusPdu::WriteMultipleRegisters 0x10 Requests to write one or more holding registers on a device.
QModbusPdu::ReportServerId 0x11 Requests the description of the type, the current status, and other information specific to a device.
QModbusPdu::ReadFileRecord 0x14 Requests a file record read.
QModbusPdu::WriteFileRecord 0x15 Requests a file record write.
QModbusPdu::MaskWriteRegister 0x16 Requests to modify the contents of a specified holding register using a combination of an AND or OR mask, and the register's current contents.
QModbusPdu::ReadWriteMultipleRegisters 0x17 Requests the status of one or more holding register and at the same time to write one or more holding registers on a device.
QModbusPdu::ReadFifoQueue 0x18 Requests to read the contents of a First-In-First-Out (FIFO) queue of register in a remote device.
QModbusPdu::EncapsulatedInterfaceTransport 0x2B Please refer to Annex A of the Modbus specification.
QModbusPdu::UndefinedFunctionCode 0x100 Do not use.

Member Function Documentation

[protected] QModbusPdu::QModbusPdu(const QModbusPdu &other)

Constructs a QModbusPdu that is a copy of other.

[protected] QModbusPdu::QModbusPdu(QModbusPdu::FunctionCode code, const QByteArray &data)

Constructs a QModbusPdu with function code set to code and payload set to data. The data is expected to be stored in big-endian byte order already.


Constructs an invalid QModbusPdu.

[protected] QModbusPdu &QModbusPdu::operator=(const QModbusPdu &other)

Makes a copy of the other and assigns it to this QModbusPdu object.

[virtual] QModbusPdu::~QModbusPdu()

Destroys a QModbusPdu.

QByteArray QModbusPdu::data() const

Returns the PDU's payload, excluding the function code. The payload is stored in big-endian byte order.

See also setData().

qint16 QModbusPdu::dataSize() const

Returns the PDU's data size, excluding the function code.

template <typename Args> void QModbusPdu::decodeData(Args &&... data) const

Converts the payload into host endianness and reads it into data. Data can be a variable length argument list.

QModbusResponsePdu response(QModbusPdu::ReportServerId);
response.encodeData(quint8(0x02), quint8(0x01), quint8(0xff));
quint8 count, id, run;
response.decodeData(&count, &id, &run);

Note: Usage is limited quint8 and quint16 only. This is because QDataStream stream operators will not only append raw data, but also e.g. size, count, etc. for complex types.

template <typename Args> void QModbusPdu::encodeData(Args... data)

Sets the payload to data. The data is converted and stored in big-endian byte order.

QModbusRequestPdu request(QModbusPdu::ReadCoils);
// starting address and quantity of coils
request.encodeData(quint16(0x0c), quint16(0x0a));

Note: Usage is limited quint8 and quint16 only. This is because QDataStream stream operators will not only append raw data, but also e.g. size, count, etc. for complex types.

QModbusPdu::ExceptionCode QModbusPdu::exceptionCode() const

Returns the response's exception code.

QModbusPdu::FunctionCode QModbusPdu::functionCode() const

Returns the PDU's function code.

See also setFunctionCode().

bool QModbusPdu::isException() const

Returns true if the PDU contains an exception code; otherwise false.

bool QModbusPdu::isValid() const

Returns true if the PDU is valid; otherwise false.

A PDU is considered valid if the message code is in the range of 1 to 255 decimal and the PDU's compound size (function code + data) does not exceed 253 bytes. A default constructed PDU is invalid.

void QModbusPdu::setData(const QByteArray &data)

Sets the PDU's function payload to data. The data is expected to be stored in big-endian byte order already.

See also data().

[virtual] void QModbusPdu::setFunctionCode(QModbusPdu::FunctionCode code)

Sets the PDU's function code to code.

See also functionCode().

qint16 QModbusPdu::size() const

Returns the PDU's full size, including function code and data size.

Member Variable Documentation

const quint8 QModbusPdu::ExceptionByte

The variable is initialized to 0x80.

Exceptions are reported in a defined packet format. A function code is returned to the requesting client equal to the original function code, except with its most significant bit set. This is equivalent to adding 0x80 to the value of the original function code.

This field may be used to mask the exception bit in the function field of a raw Modbus packet.

Related Non-Members

QDebug operator<<(QDebug debug, const QModbusPdu &pdu)

Writes the Modbus pdu to the debug stream.

QDataStream &operator<<(QDataStream &stream, const QModbusPdu &pdu)

Writes a pdu to the stream and returns a reference to the stream.

Qt_Technology_Partner_RGB_475 Qt_Service_Partner_RGB_475_padded