OPC Unified Architecture, Basics

OPC Unified Architecture (in short OPC UA) is the most recent OPC specification from the OPC Foundation and differs significantly from its predecessors (DA, A&E, HDA). OPC UA forms a new standard, which is completely based on a service-oriented architecture.

The main factor for a new OPC architecture is the cancellation of the component port by Microsoft. The previous binding of OPC to COM/DCOM has been replaced by an own communication stack.

The OPC UA Client supports the OPC UA standards, Data Access and  Alarms & Conditions.

The OPC UA Server supports the OPC UA standards, Data Access and Alarms & Conditions.

The advantages when using OPC UA are:

• Up-to-date specification (divided into 11 specifications).

• Combines OPC DA, A&E, HDA and Commands to one new standard.

• Platform-independence, therefore available on Linux for the first time.

• Redundancy support.

• Scalability (embedded controllers up to and including mainframes).

• Rapidity (in comparison to web services).

• Security (security concept as fixed-implemented part).

• Service-oriented architecture (SOA, request <-> response).

• Configurable timeouts for every service.

• Data exchange via messages.

• Messages with huge data volume can be divided into several messages.

• Servers support profiles (DA and/or A&E).

• Client is able to ask the server for supported profiles.

• Heartbeat for connection supervision in both directions. That means the server as well as the client recognize interruptions.

• Buffering of data and acknowledgements of transmitted data. Connection interrupts do not lead to data lose anymore. Lost data can be required.

• Server memorizes data that has been sent (configurable time).

Caution

When using OPC UA the known restrictions and notes have to be considered!

Architecture and Connection Establishment (connection flow)

OPC UA server architecture

The server accepts connections to several clients at the same time.

OPC UA client architecture

Connection establishment an flow

While driver start the servers, to which the connection shall be established, are read out from the configuration file. During the client start-up the first connection attempt to all active servers is started at once. In case of a failure a new connection attempt is started after awaiting the time that was set in the Reconnect Timer parameter in the Client configuration panel.

As soon as the connection to the server is established Browsing for Items is possible. The selection of the node from which data is received or to which data is sent, respectively, is possible before browsing.

After Definition of the peripheral address the driver creates the declared subscription and adds to the subscription its Monitored Items. Each subscription holds a number of its Monitored Items. From that time it is possible to receive data from the Monitored Items.

The general query can be triggered on all servers, on one selective server or on one certain subscription (see Data Exchange).

The client is able to connect to several servers at the same time (only UA Native Binary is supported).

Configuration in WinCC OA

OPC UA server

All data that shall be accessible over OPC UA has to be defined within three data point groups. The assignment of the data point elements = "items" takes place via the data point groups OPCUARead (readable), OPCUAWrite (readable and writable) and OPCUAAlarm (alarms). For further information see Access to WinCC OA data.

Static settings for the WinCC OA OPC UA server are carried out in the WinCC OA configuration file in the [opcuasrv] section, if the entry shall apply to all OPC UA servers, or in the [opcuasrv_x] section, if the entry shall apply to one certain WinCC OA OPC UA server (x indicated the manager number) - see possible config entries for the OPC UA server. Information for the WinCC OA OPC UA server is stored in the internal data point type _OPCUAPvssServer (see Internal data points of the OPC UA server).

OPC UA client

Configuration settings for the WinCC OA OPC UA client are carried out in the panel for configuration of the servers, where the parameters for each client-server connection can independent be set. The parameters are stored in the internal connection data point of the type _OPCUAServer.

The configuration of a subscription is carried out in the panel for configuration of a subscription.

Static settings for the WinCC OA OPC UA client are specified in the section [opcua] or [opcua_x] of the config file (see possible config entries for the OPC UA client).

Chapter overview