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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In accordance with the same scheme, the "RegisteredRead" function can also be used, which
is particularly useful for recurring data readouts. Take into account, however, that depending
on the application it may be advisable to use a Subscription instead.
Recommendation: It is best to place registrations in the startup program of the OPC UA client,
since the registration takes up time.
Please note that you can set the maximum number of registered nodes in the properties of
the S7-1500 CPU and that the Clients have to respect this number, see General settings of the
OPC UA server (Page 222).
Subscription
The term "Subscription" is used for a function in which only those tags for which an OPC UA
client has registered at the OPC UA server are transferred. The OPC UA server only sends a
message to the OPC UA client for these registered tags (monitored Items) when a value has
changed. The monitoring of these tags makes constant sampling by the OPC UA client
(Polling)superfluous, which reduces the network load.
You have to create a Subscription to use this function. For this purpose, you specify the
"Publishing Interval" at the UA client and click the "Create" button. The publishing interval is
the time interval in which the server sends new values to the client in a notification (data
change notification).
In the following example a subscription has been created: The client receives a message with
the new values (publishing interval 50 ms) every 50 milliseconds here.