Siemens SIMATIC ET 200AL, System Manual

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OPC UA communication
9.1 What you need to know about OPC UA
Communication
Function Manual, 05/2021, A5E03735815-AJ
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9.1.5 Addressing nodes
Nodes are the basic elements of OPC UA, they are comparable with objects from object-
oriented programming. Nodes are used, for example, for user data (tags) or other metadata.
Nodes are used to model an OPC UA address space that also contains a type model with type
definitions.
Node ID (NodeId)
Nodes in the OPC UA address space are uniquely identified by a NodeId (Node Identifier).
The NodeId consists of an identifier, identifier type and a namespace index. Namespaces are
used to avoid naming conflicts.
The OPC Foundation has defined a wide range of nodes that provide information about the
given OPC UA server. These nodes can be found in the namespace of the OPC Foundation and
have the index 0.
The OPC Foundation also defines data types and tag types.
Namespace (Namespace)
In addition to the above-described namespace of the OPC Foundation, the namespace for
accessing CPU data is of interest: All the tags or methods of an S7-1500 OPC UA server are
contained in the namespace (Namespace) of the standard server interface
"http://www.siemens.com/simatic-s7-opcua".
By default this namespace has the Index 3. The index may change later if additional
namespaces are inserted into the server or if existing ones are deleted. It is therefore
necessary for an OPC UA client to request the current index of the namespace (e.g.
"http://www.siemens.com/simatic-s7-opcua") from the server before reading or writing its
values.
The following figure shows an example of the result of such a request.