OPC UA communication
9.1 What you need to know about OPC UA
Communication
132 Function Manual, 12/2017, A5E03735815-AF
The nodes are linked over references, for example, the reference "HasComponent, which
represents a hierarchical relationship between a node and its subordinate nodes. With their
references, the nodes form a network that can, for example, take the form of a tree.
A network of nodes is also called an address space. Starting from the root, all nodes can be
reached in the address space.
Nodes in the OPC UA address space are uniquely identified by a NodeId (Node ID or 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.
All the tags or methods of an S7-1500 are contained in the namespace (Namespace)
"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. The current index of the namespace therefore needs to be requested from
the server before values are read or written.
The figure below shows the result of such a query. The Siemens "UaClient" program is used
as an example; see S7-1500 Communication function manual
(https://support.industry.siemens.com/cs/ww/en/view/59192925
).
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