EventsFromOPC-UA

OPC UA includes a system for event-handling, similar to how it is done in the CDP Framework. OPC UA also has a special event class called condition events that can carry information about the state of the object that the event is associated with. The condition events will be sent out when some monitored condition changes. Similar to the CDP alarms, the OPC UA condition events have a special event sub-class called alarm events that can be used to indicate that the object is in a state that requires attention.

Read more about the OPC UA events from the OPC UA reference documentation, at events specification.

EventsFromOPC-UA is a CDP node that can be added to the OPC-UA-Client in order to receive and forward OPC UA events to the CDP system as CDP events.

To start forwarding all OPC UA events to CDP, add an instance of the EventsFromOPC-UA block to the OPC-UA-Client. With the default configuration, the block will start to listen to all the OPC UA events from the OPC UA server, convert and forward them to the CDP system as CDP events. In addition, the block is also able to forward the CDP alarm acknowledge actions to all OPC UA event senders to acknowledge and/or confirm the OPC UA alarm event.

EventsFromOPC-UA has the following configuration properties:

EventsFromOPC-UA state can be observed at runtime via these read-only properties:

You can also add multiple EventsFromOPC-UA blocks to OPC-UA-Client with different configurations. Multiple EventsFromOPC-UA blocks with altered FilterWhere or EventConverterScript can be useful when different types of OPC UA events should be converted using very different conversion logic.

Event Filter Query Language

OPC UA event filtering is an OPC UA core mechanism that enables OPC UA clients to receive only the specific events that are of interest, rather than being flooded with every event generated by the server. That way, by applying tailored filter criteria, an EventsFromOPC-UA block can subscribe only to the events that meet defined conditions - such as a minimum severity level or events originating from specific sources - which helps reduce unnecessary network traffic and processing overhead.

The EventsFromOPC-UA block leverages a query language similar in structure to SQL to define the event filter. The query language consists of two parts: the SELECT part and the WHERE part.

SELECT Part of the Query

The SELECT part of the query is used to specify which event attributes need to be fetched from the server. The SELECT part can be altered via the FilterSelect property. When the FilterSelect is left empty, a minimal set of event attributes that are needed for the block functioning is always queried.

The SELECT part is a comma-separated list of event attribute RelativePaths that are to be fetched from the server.

RelativePath is built up from one or more path elements, each starting with a forward slash (“/”). Some example event attribute ReplativePath's are /EventId, e {/SourceName}, /Severity, /e {/Message}, and /e {/AckedState/Id}.

Read more about the RelativePath format from the OPC UA reference documentation, at RelativePath specification.

WHERE Part of the Query

The WHERE part of the query is used to define the criteria that the events must meet in order to be sent to the client. The WHERE part can be altered via the FilterWhere property. When the FilterWhere is left empty, all events from the OPC UA server will be retrieved.

The WHERE part is a logical expression that can be built from the event attributes that are fetched from the server. The logical expression can be built using the event attributes tested using the following OPC UA filter operators:

The WHERE and SELECT parts can be verified using the online OPC UA EventFilter Query Language test tool.

To test out the FilterSelect and FilterWhere statements, combine them into one query statement using SELECT and WHERE parts, like this:

SELECT {FilterSelect property content} WHERE {FilterWhere property content}

Copy this combined query sentence to the Query Input pane and then press the Submit button. When the statements in the query are all valid then a JSON describing the OPC UA SELECT and WHERE clause metadata will be displayed in the Result pane. On the other hand, when something in the query is invalid then an error message will be displayed describing the problem.