QSerialPort Class

Provides functions to access serial ports. More...

Header:	#include <QSerialPort>
qmake:	QT += serialport
Since:	Qt 5.1
Inherits:	QIODevice

This class was introduced in Qt 5.1.

• List of all members, including inherited members
• Obsolete members

Note: All functions in this class are reentrant.

Public Types

enum	BaudRate { Baud1200, Baud2400, Baud4800, Baud9600, Baud19200, …, UnknownBaud }
enum	DataBits { Data5, Data6, Data7, Data8, UnknownDataBits }
enum	Direction { Input, Output, AllDirections }
flags	Directions
enum	FlowControl { NoFlowControl, HardwareControl, SoftwareControl, UnknownFlowControl }
enum	Parity { NoParity, EvenParity, OddParity, SpaceParity, MarkParity, UnknownParity }
enum	PinoutSignal { NoSignal, TransmittedDataSignal, ReceivedDataSignal, DataTerminalReadySignal, DataCarrierDetectSignal, …, SecondaryReceivedDataSignal }
flags	PinoutSignals
enum	SerialPortError { NoError, DeviceNotFoundError, PermissionError, OpenError, NotOpenError, …, UnknownError }
enum	StopBits { OneStop, OneAndHalfStop, TwoStop, UnknownStopBits }

Properties

baudRate : qint32 breakEnabled : bool dataBits : DataBits dataTerminalReady : bool error : SerialPortError

flowControl : FlowControl parity : Parity requestToSend : bool stopBits : StopBits

Public Functions

	QSerialPort(const QSerialPortInfo &serialPortInfo, QObject *parent = nullptr)
	QSerialPort(const QString &name, QObject *parent = nullptr)
	QSerialPort(QObject *parent = nullptr)
virtual	~QSerialPort()
qint32	baudRate(QSerialPort::Directions directions = AllDirections) const
bool	clear(QSerialPort::Directions directions = AllDirections)
void	clearError()
QSerialPort::DataBits	dataBits() const
QSerialPort::SerialPortError	error() const
QSerialPort::FlowControl	flowControl() const
bool	flush()
QSerialPort::Handle	handle() const
bool	isBreakEnabled() const
bool	isDataTerminalReady()
bool	isRequestToSend()
QSerialPort::Parity	parity() const
QSerialPort::PinoutSignals	pinoutSignals()
QString	portName() const
qint64	readBufferSize() const
bool	sendBreak(int duration = 0)
bool	setBaudRate(qint32 baudRate, QSerialPort::Directions directions = AllDirections)
bool	setBreakEnabled(bool set = true)
bool	setDataBits(QSerialPort::DataBits dataBits)
bool	setDataTerminalReady(bool set)
bool	setFlowControl(QSerialPort::FlowControl flowControl)
bool	setParity(QSerialPort::Parity parity)
void	setPort(const QSerialPortInfo &serialPortInfo)
void	setPortName(const QString &name)
void	setReadBufferSize(qint64 size)
bool	setRequestToSend(bool set)
bool	setStopBits(QSerialPort::StopBits stopBits)
QSerialPort::StopBits	stopBits() const

Reimplemented Public Functions

virtual bool	atEnd() const override
virtual qint64	bytesAvailable() const override
virtual qint64	bytesToWrite() const override
virtual bool	canReadLine() const override
virtual void	close() override
virtual bool	isSequential() const override
virtual bool	open(QIODevice::OpenMode mode) override
virtual bool	waitForBytesWritten(int msecs = 30000) override
virtual bool	waitForReadyRead(int msecs = 30000) override

Signals

void	baudRateChanged(qint32 baudRate, QSerialPort::Directions directions)
void	breakEnabledChanged(bool set)
void	dataBitsChanged(QSerialPort::DataBits dataBits)
void	dataTerminalReadyChanged(bool set)
void	errorOccurred(QSerialPort::SerialPortError error)
void	flowControlChanged(QSerialPort::FlowControl flow)
void	parityChanged(QSerialPort::Parity parity)
void	requestToSendChanged(bool set)
void	stopBitsChanged(QSerialPort::StopBits stopBits)

Reimplemented Protected Functions

virtual qint64	readData(char *data, qint64 maxSize) override
virtual qint64	readLineData(char *data, qint64 maxSize) override
virtual qint64	writeData(const char *data, qint64 maxSize) override

Detailed Description

You can get information about the available serial ports using the QSerialPortInfo helper class, which allows an enumeration of all the serial ports in the system. This is useful to obtain the correct name of the serial port you want to use. You can pass an object of the helper class as an argument to the setPort() or setPortName() methods to assign the desired serial device.

After setting the port, you can open it in read-only (r/o), write-only (w/o), or read-write (r/w) mode using the open() method.

Note: The serial port is always opened with exclusive access (that is, no other process or thread can access an already opened serial port).

Use the close() method to close the port and cancel the I/O operations.

Having successfully opened, QSerialPort tries to determine the current configuration of the port and initializes itself. You can reconfigure the port to the desired setting using the setBaudRate(), setDataBits(), setParity(), setStopBits(), and setFlowControl() methods.

There are a couple of properties to work with the pinout signals namely: QSerialPort::dataTerminalReady, QSerialPort::requestToSend. It is also possible to use the pinoutSignals() method to query the current pinout signals set.

Once you know that the ports are ready to read or write, you can use the read() or write() methods. Alternatively the readLine() and readAll() convenience methods can also be invoked. If not all the data is read at once, the remaining data will be available for later as new incoming data is appended to the QSerialPort's internal read buffer. You can limit the size of the read buffer using setReadBufferSize().

QSerialPort provides a set of functions that suspend the calling thread until certain signals are emitted. These functions can be used to implement blocking serial ports:

• waitForReadyRead() blocks calls until new data is available for reading.
• waitForBytesWritten() blocks calls until one payload of data has been written to the serial port.

See the following example:

int numRead = 0, numReadTotal = 0;
char buffer[50];

for (;;) {
    numRead  = serial.read(buffer, 50);

    // Do whatever with the array

    numReadTotal += numRead;
    if (numRead == 0 && !serial.waitForReadyRead())
        break;
}

If waitForReadyRead() returns false, the connection has been closed or an error has occurred.

If an error occurs at any point in time, QSerialPort will emit the errorOccurred() signal. You can also call error() to find the type of error that occurred last.

Programming with a blocking serial port is radically different from programming with a non-blocking serial port. A blocking serial port does not require an event loop and typically leads to simpler code. However, in a GUI application, blocking serial port should only be used in non-GUI threads, to avoid freezing the user interface.

For more details about these approaches, refer to the example applications.

The QSerialPort class can also be used with QTextStream and QDataStream's stream operators (operator<<() and operator>>()). There is one issue to be aware of, though: make sure that enough data is available before attempting to read by using the operator>>() overloaded operator.

See also QSerialPortInfo.

Member Type Documentation

enum QSerialPort::BaudRate

This enum describes the baud rate which the communication device operates with.

Note: Only the most common standard baud rates are listed in this enum.

Constant	Value	Description
QSerialPort::Baud1200	1200	1200 baud.
QSerialPort::Baud2400	2400	2400 baud.
QSerialPort::Baud4800	4800	4800 baud.
QSerialPort::Baud9600	9600	9600 baud.
QSerialPort::Baud19200	19200	19200 baud.
QSerialPort::Baud38400	38400	38400 baud.
QSerialPort::Baud57600	57600	57600 baud.
QSerialPort::Baud115200	115200	115200 baud.
QSerialPort::UnknownBaud	-1	Unknown baud. This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

See also QSerialPort::baudRate.

enum QSerialPort::DataBits

This enum describes the number of data bits used.

Constant	Value	Description
QSerialPort::Data5	5	The number of data bits in each character is 5. It is used for Baudot code. It generally only makes sense with older equipment such as teleprinters.
QSerialPort::Data6	6	The number of data bits in each character is 6. It is rarely used.
QSerialPort::Data7	7	The number of data bits in each character is 7. It is used for true ASCII. It generally only makes sense with older equipment such as teleprinters.
QSerialPort::Data8	8	The number of data bits in each character is 8. It is used for most kinds of data, as this size matches the size of a byte. It is almost universally used in newer applications.
QSerialPort::UnknownDataBits	-1	Unknown number of bits. This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

See also QSerialPort::dataBits.

enum QSerialPort::Directionflags QSerialPort::Directions

This enum describes the possible directions of the data transmission.

Note: This enumeration is used for setting the baud rate of the device separately for each direction on some operating systems (for example, POSIX-like).

Constant	Value	Description
QSerialPort::Input	1	Input direction.
QSerialPort::Output	2	Output direction.
QSerialPort::AllDirections	Input | Output	Simultaneously in two directions.

The Directions type is a typedef for QFlags<Direction>. It stores an OR combination of Direction values.

enum QSerialPort::FlowControl

This enum describes the flow control used.

Constant	Value	Description
QSerialPort::NoFlowControl	0	No flow control.
QSerialPort::HardwareControl	1	Hardware flow control (RTS/CTS).
QSerialPort::SoftwareControl	2	Software flow control (XON/XOFF).
QSerialPort::UnknownFlowControl	-1	Unknown flow control. This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

See also QSerialPort::flowControl.

enum QSerialPort::Parity

This enum describes the parity scheme used.

Constant	Value	Description
QSerialPort::NoParity	0	No parity bit it sent. This is the most common parity setting. Error detection is handled by the communication protocol.
QSerialPort::EvenParity	2	The number of 1 bits in each character, including the parity bit, is always even.
QSerialPort::OddParity	3	The number of 1 bits in each character, including the parity bit, is always odd. It ensures that at least one state transition occurs in each character.
QSerialPort::SpaceParity	4	Space parity. The parity bit is sent in the space signal condition. It does not provide error detection information.
QSerialPort::MarkParity	5	Mark parity. The parity bit is always set to the mark signal condition (logical 1). It does not provide error detection information.
QSerialPort::UnknownParity	-1	Unknown parity. This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

See also QSerialPort::parity.

enum QSerialPort::PinoutSignalflags QSerialPort::PinoutSignals

This enum describes the possible RS-232 pinout signals.

Constant	Value	Description
QSerialPort::NoSignal	0x00	No line active
QSerialPort::TransmittedDataSignal	0x01	TxD (Transmitted Data). This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.
QSerialPort::ReceivedDataSignal	0x02	RxD (Received Data). This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.
QSerialPort::DataTerminalReadySignal	0x04	DTR (Data Terminal Ready).
QSerialPort::DataCarrierDetectSignal	0x08	DCD (Data Carrier Detect).
QSerialPort::DataSetReadySignal	0x10	DSR (Data Set Ready).
QSerialPort::RingIndicatorSignal	0x20	RNG (Ring Indicator).
QSerialPort::RequestToSendSignal	0x40	RTS (Request To Send).
QSerialPort::ClearToSendSignal	0x80	CTS (Clear To Send).
QSerialPort::SecondaryTransmittedDataSignal	0x100	STD (Secondary Transmitted Data).
QSerialPort::SecondaryReceivedDataSignal	0x200	SRD (Secondary Received Data).

The PinoutSignals type is a typedef for QFlags<PinoutSignal>. It stores an OR combination of PinoutSignal values.

See also pinoutSignals(), QSerialPort::dataTerminalReady, and QSerialPort::requestToSend.

enum QSerialPort::SerialPortError

This enum describes the errors that may be contained by the QSerialPort::error property.

Constant	Value	Description
QSerialPort::NoError	0	No error occurred.
QSerialPort::DeviceNotFoundError	1	An error occurred while attempting to open an non-existing device.
QSerialPort::PermissionError	2	An error occurred while attempting to open an already opened device by another process or a user not having enough permission and credentials to open.
QSerialPort::OpenError	3	An error occurred while attempting to open an already opened device in this object.
QSerialPort::NotOpenError	13	This error occurs when an operation is executed that can only be successfully performed if the device is open. This value was introduced in QtSerialPort 5.2.
QSerialPort::ParityError	4	Parity error detected by the hardware while reading data. This value is obsolete. We strongly advise against using it in new code.
QSerialPort::FramingError	5	Framing error detected by the hardware while reading data. This value is obsolete. We strongly advise against using it in new code.
QSerialPort::BreakConditionError	6	Break condition detected by the hardware on the input line. This value is obsolete. We strongly advise against using it in new code.
QSerialPort::WriteError	7	An I/O error occurred while writing the data.
QSerialPort::ReadError	8	An I/O error occurred while reading the data.
QSerialPort::ResourceError	9	An I/O error occurred when a resource becomes unavailable, e.g. when the device is unexpectedly removed from the system.
QSerialPort::UnsupportedOperationError	10	The requested device operation is not supported or prohibited by the running operating system.
QSerialPort::TimeoutError	12	A timeout error occurred. This value was introduced in QtSerialPort 5.2.
QSerialPort::UnknownError	11	An unidentified error occurred.

See also QSerialPort::error.

enum QSerialPort::StopBits

This enum describes the number of stop bits used.

Constant	Value	Description
QSerialPort::OneStop	1	1 stop bit.
QSerialPort::OneAndHalfStop	3	1.5 stop bits. This is only for the Windows platform.
QSerialPort::TwoStop	2	2 stop bits.
QSerialPort::UnknownStopBits	-1	Unknown number of stop bits. This value is obsolete. It is provided to keep old source code working. We strongly advise against using it in new code.

See also QSerialPort::stopBits.

Member Function Documentation

QSerialPort::QSerialPort(const QSerialPortInfo &serialPortInfo, QObject *parent = nullptr)

Constructs a new serial port object with the given parent to represent the serial port with the specified helper class serialPortInfo.

QSerialPort::QSerialPort(const QString &name, QObject *parent = nullptr)

Constructs a new serial port object with the given parent to represent the serial port with the specified name.

The name should have a specific format; see the setPort() method.

QSerialPort::QSerialPort(QObject *parent = nullptr)

Constructs a new serial port object with the given parent.

[signal] void QSerialPort::baudRateChanged(qint32 baudRate, QSerialPort::Directions directions)

This signal is emitted after the baud rate has been changed. The new baud rate is passed as baudRate and directions as directions.

Note: Notifier signal for property baudRate.

See also QSerialPort::baudRate.

[signal] void QSerialPort::dataBitsChanged(QSerialPort::DataBits dataBits)

This signal is emitted after the data bits in a frame has been changed. The new data bits in a frame is passed as dataBits.

Note: Notifier signal for property dataBits.

See also QSerialPort::dataBits.

[signal] void QSerialPort::dataTerminalReadyChanged(bool set)

This signal is emitted after the state (high or low) of the line signal DTR has been changed. The new the state (high or low) of the line signal DTR is passed as set.

Note: Notifier signal for property dataTerminalReady.

See also QSerialPort::dataTerminalReady.

[signal] void QSerialPort::errorOccurred(QSerialPort::SerialPortError error)

This signal is emitted when an error occurs in the serial port. The specified error describes the type of error that occurred.

This function was introduced in Qt 5.8.

See also QSerialPort::error.

[signal] void QSerialPort::flowControlChanged(QSerialPort::FlowControl flow)

This signal is emitted after the flow control mode has been changed. The new flow control mode is passed as flow.

Note: Notifier signal for property flowControl.

See also QSerialPort::flowControl.

[signal] void QSerialPort::parityChanged(QSerialPort::Parity parity)

This signal is emitted after the parity checking mode has been changed. The new parity checking mode is passed as parity.

Note: Notifier signal for property parity.

See also QSerialPort::parity.

[signal] void QSerialPort::requestToSendChanged(bool set)

This signal is emitted after the state (high or low) of the line signal RTS has been changed. The new the state (high or low) of the line signal RTS is passed as set.

Note: Notifier signal for property requestToSend.

See also QSerialPort::requestToSend.

[signal] void QSerialPort::stopBitsChanged(QSerialPort::StopBits stopBits)

This signal is emitted after the number of stop bits in a frame has been changed. The new number of stop bits in a frame is passed as stopBits.

Note: Notifier signal for property stopBits.

See also QSerialPort::stopBits.

[virtual] QSerialPort::~QSerialPort()

Closes the serial port, if necessary, and then destroys object.

[override virtual] bool QSerialPort::atEnd() const

Reimplements: QIODevice::atEnd() const.

Returns true if no more data is currently available for reading; otherwise returns false.

This function is most commonly used when reading data from the serial port in a loop. For example:

// This slot is connected to QSerialPort::readyRead()
void QSerialPortClass::readyReadSlot()
{
    while (!port.atEnd()) {
        QByteArray data = port.read(100);
        ....
    }
}

See also bytesAvailable() and readyRead().

[override virtual] qint64 QSerialPort::bytesAvailable() const

Reimplements: QIODevice::bytesAvailable() const.

Returns the number of incoming bytes that are waiting to be read.

See also bytesToWrite() and read().

[override virtual] qint64 QSerialPort::bytesToWrite() const

Reimplements: QIODevice::bytesToWrite() const.

Returns the number of bytes that are waiting to be written. The bytes are written when control goes back to the event loop or when flush() is called.

See also bytesAvailable() and flush().

[override virtual] bool QSerialPort::canReadLine() const

Reimplements: QIODevice::canReadLine() const.

Returns true if a line of data can be read from the serial port; otherwise returns false.

See also readLine().

bool QSerialPort::clear(QSerialPort::Directions directions = AllDirections)

Discards all characters from the output or input buffer, depending on given directions directions. This includes clearing the internal class buffers and the UART (driver) buffers. Also terminate pending read or write operations. If successful, returns true; otherwise returns false.

Note: The serial port has to be open before trying to clear any buffered data; otherwise returns false and sets the NotOpenError error code.

[override virtual] void QSerialPort::close()

Reimplements: QIODevice::close().

Note: The serial port has to be open before trying to close it; otherwise sets the NotOpenError error code.

See also QIODevice::close().

bool QSerialPort::flush()

This function writes as much as possible from the internal write buffer to the underlying serial port without blocking. If any data was written, this function returns true; otherwise returns false.

Call this function for sending the buffered data immediately to the serial port. The number of bytes successfully written depends on the operating system. In most cases, this function does not need to be called, because the QSerialPort class will start sending data automatically once control is returned to the event loop. In the absence of an event loop, call waitForBytesWritten() instead.

Note: The serial port has to be open before trying to flush any buffered data; otherwise returns false and sets the NotOpenError error code.

See also write() and waitForBytesWritten().

QSerialPort::Handle QSerialPort::handle() const

If the platform is supported and the serial port is open, returns the native serial port handle; otherwise returns -1.

Warning: This function is for expert use only; use it at your own risk. Furthermore, this function carries no compatibility promise between minor Qt releases.

This function was introduced in Qt 5.2.

[override virtual] bool QSerialPort::isSequential() const

Reimplements: QIODevice::isSequential() const.

Always returns true. The serial port is a sequential device.

[override virtual] bool QSerialPort::open(QIODevice::OpenMode mode)

Reimplements: QIODevice::open(QIODevice::OpenMode mode).

Opens the serial port using OpenMode mode, and then returns true if successful; otherwise returns false and sets an error code which can be obtained by calling the error() method.

Note: The method returns false if opening the port is successful, but could not set any of the port settings successfully. In that case, the port is closed automatically not to leave the port around with incorrect settings.

Warning: The mode has to be QIODevice::ReadOnly, QIODevice::WriteOnly, or QIODevice::ReadWrite. Other modes are unsupported.

See also QIODevice::OpenMode and setPort().

QSerialPort::PinoutSignals QSerialPort::pinoutSignals()

Returns the state of the line signals in a bitmap format.

From this result, it is possible to allocate the state of the desired signal by applying a mask "AND", where the mask is the desired enumeration value from QSerialPort::PinoutSignals.

Note: This method performs a system call, thus ensuring that the line signal states are returned properly. This is necessary when the underlying operating systems cannot provide proper notifications about the changes.

Note: The serial port has to be open before trying to get the pinout signals; otherwise returns NoSignal and sets the NotOpenError error code.

See also QSerialPort::dataTerminalReady and QSerialPort::requestToSend.

QString QSerialPort::portName() const

Returns the name set by setPort() or passed to the QSerialPort constructor. This name is short, i.e. it is extracted and converted from the internal variable system location of the device. The conversion algorithm is platform specific:

Platform	Brief Description
Windows	Removes the prefix "\\.\" or "//./" from the system location and returns the remainder of the string.
Unix, BSD	Removes the prefix "/dev/" from the system location and returns the remainder of the string.

See also setPortName(), setPort(), and QSerialPortInfo::portName().

qint64 QSerialPort::readBufferSize() const

Returns the size of the internal read buffer. This limits the amount of data that the client can receive before calling the read() or readAll() methods.

A read buffer size of 0 (the default) means that the buffer has no size limit, ensuring that no data is lost.

See also setReadBufferSize() and read().

[override virtual protected] qint64 QSerialPort::readData(char *data, qint64 maxSize)

Reimplements: QIODevice::readData(char *data, qint64 maxSize).

[override virtual protected] qint64 QSerialPort::readLineData(char *data, qint64 maxSize)

Reimplements: QIODevice::readLineData(char *data, qint64 maxSize).

bool QSerialPort::sendBreak(int duration = 0)

Sends a continuous stream of zero bits during a specified period of time duration in msec if the terminal is using asynchronous serial data. If successful, returns true; otherwise returns false.

If the duration is zero then zero bits are transmitted by at least 0.25 seconds, but no more than 0.5 seconds.

If the duration is non zero then zero bits are transmitted within a certain period of time depending on the implementation.

Note: The serial port has to be open before trying to send a break duration; otherwise returns false and sets the NotOpenError error code.

See also setBreakEnabled().

void QSerialPort::setPort(const QSerialPortInfo &serialPortInfo)

Sets the port stored in the serial port info instance serialPortInfo.

See also portName() and QSerialPortInfo.

void QSerialPort::setPortName(const QString &name)

Sets the name of the serial port.

The name of the serial port can be passed as either a short name or the long system location if necessary.

See also portName() and QSerialPortInfo.

void QSerialPort::setReadBufferSize(qint64 size)

Sets the size of QSerialPort's internal read buffer to be size bytes.

If the buffer size is limited to a certain size, QSerialPort will not buffer more than this size of data. The special case of a buffer size of 0 means that the read buffer is unlimited and all incoming data is buffered. This is the default.

This option is useful if the data is only read at certain points in time (for instance in a real-time streaming application) or if the serial port should be protected against receiving too much data, which may eventually cause the application to run out of memory.

See also readBufferSize() and read().

[override virtual] bool QSerialPort::waitForBytesWritten(int msecs = 30000)

Reimplements: QIODevice::waitForBytesWritten(int msecs).

This function blocks until at least one byte has been written to the serial port and the bytesWritten() signal has been emitted. The function will timeout after msecs milliseconds; the default timeout is 30000 milliseconds. If msecs is -1, this function will not time out.

The function returns true if the bytesWritten() signal is emitted; otherwise it returns false (if an error occurred or the operation timed out).

[override virtual] bool QSerialPort::waitForReadyRead(int msecs = 30000)

Reimplements: QIODevice::waitForReadyRead(int msecs).

This function blocks until new data is available for reading and the readyRead() signal has been emitted. The function will timeout after msecs milliseconds; the default timeout is 30000 milliseconds. If msecs is -1, this function will not time out.

The function returns true if the readyRead() signal is emitted and there is new data available for reading; otherwise it returns false (if an error occurred or the operation timed out).

See also waitForBytesWritten().

[override virtual protected] qint64 QSerialPort::writeData(const char *data, qint64 maxSize)

Reimplements: QIODevice::writeData(const char *data, qint64 maxSize).