Qt OPC UA Overview

OPC UA is a protocol for data modelling and exchange of data in industrial applications.

An OPC UA server offers access to data that is organized in a mesh of nodes connected by references. The use of different reference types and nodes containing metadata enable a client to navigate and interpret the data without knowing their structure in advance.

Each node has a unique identifier and attributes that can be read and written. Among others, there are Variable nodes that store values and callable Method nodes with attached nodes describing parameters and return values. Notifications in case of events and monitoring of Variable nodes for value changes are offered too.

Complex objects can be created by combining nodes using references. Inheritance is also possible. OPC UA offers support for pre-made models that can be extended to fit special needs.

OPC UA is the platform-independent successor of OPC Classic intended for usage on all levels, from embedded sensors up to manufacturing execution and enterprise resource planning systems. It has a service-oriented architecture based on standardized messages for service requests and responses. There are different ways for these messages to be encoded and transported over the network. The most common way is binary encoding over TCP.

Implemented Features from the OPC UA Protocol

The following table summarizes the features of OPC UA that are supported by Qt OPC UA.

Feature
Read
Batch read
HistoryRead (Raw)
Write
Batch write
Multidimensional arrays
Browse
Data change subscriptions
Event subscriptions
Modify subscriptions / monitored Items
Method calls
Browse path resolution
GetEndpoints
FindServers
NodeManagement
RegisterNodes
UnregisterNodes

Data Types

A subset of the OPC UA data types is currently supported in Qt OPC UA.

Data type	Qt OPC UA data type
Int16, Int32, Int64	Directly used
UInt16, UInt32, UInt64	Directly used
Byte	quint8
SByte	qint8
Boolean	Directly used
Double	Directly used
Float	Directly used
String	QString
LocalizedText	QOpcUaLocalizedText
DateTime	QDateTime
ByteString	QByteArray
XmlElement	QString
NodeId	QString
GUID	QUuid
QualifiedName	QOpcUaQualifiedName
StatusCode	UaStatusCode
Range	QOpcUaRange
EUInformation	QOpcUaEUInformation
ComplexNumber	QOpcUaComplexNumber
DoubleComplexNumber	QOpcUaDoubleComplexNumber
AxisInformation	QOpcUaAxisInformation
XV	QOpcUaXValue
Argument	QOpcUaArgument
ExpandedNodeId	QOpcUaExpandedNodeId
ExtensionObject	QOpcUaExtensionObject
StructureDefinition	QOpcUaStructureDefinition
StructureField	QOpcUaStructureField
EnumDefinition	QOpcUaEnumDefinition
EnumField	QOpcUaEnumField
DiagnosticInfo	QOpcUaDiagnosticInfo
SimpleAttributeOperand	QOpcUaSimpleAttributeOperand
AttributeOperand	QOpcUaAttributeOperand
LiteralOperand	QOpcUaLiteralOperand
ElementOperand	QOpcUaElementOperand
RelativePathElement	QOpcUaRelativePathElement
ContentFilterElement	QOpcUaContentFilterElement
EventFilter	EventFilter

Classes and Ownership

Two important classes are exposed to the user: QOpcUaClient and QOpcUaNode .

Objects of both classes are owned by the user and must be deleted when they are no longer needed.

Logging Categories

The following table summarizes the logging categories that are used by Qt OPC UA.

Logging Category	Description
qt.opcua	Plugin independent messages generated by QOpcUaProvider and QOpcUaClient
qt.opcua.plugins.open62541	Messages generated by the open62541 plugin

Open62541 specific Logging Categories

The open62541 sdk logging categories depend on the log level for open62541, which is configured at compile time. The default is 300, which enables: info, warning, error and critical. To obtain less or more verbose output, the open62541 sdk needs to be rebuilt with a different configuration.

The following table shows the logging categories used by open62541:

Open62541 Logging Category	Description
qt.opcua.plugins.open62541.sdk.network	Messages generated by the open62541 network
qt.opcua.plugins.open62541.sdk.securechannel	Messages generated by the open62541 channel
qt.opcua.plugins.open62541.sdk.session	Messages generated by the open62541 session
qt.opcua.plugins.open62541.sdk.server	Messages generated by open62541 server components
qt.opcua.plugins.open62541.sdk.client	Messages generated by the open62541 client
qt.opcua.plugins.open62541.sdk.userland	Messages generated by the open62541 userland
qt.opcua.plugins.open62541.sdk.securitypolicy	Messages generated by the open62541 security policies

Licenses

The Qt OPC UA module is available under commercial licenses from The Qt Company. In addition, it is available under free software licenses. These free software licenses are GNU Lesser General Public License, version 3, or the GNU General Public License, version 2. See Qt Licensing for further details.

Qt OPC UA in Qt 6.8.0 may contain third party modules under following permissive licenses:

Linking to external libraries involves licenses from the backend providers.

Open62541

The Open62541 plugin is available under the same licenses as Qt OPC UA. The Open62541 library itself is licensed under Mozilla Public License v2.0 .

The security support is provided by the OpenSSL plugins of the open62541 library. If the open62541 plugin is built with security support and OpenSSL is not available on the target system, the open62541 plugin will fail to load.