Siemens SIMATIC ET 200AL, System Manual

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OPC UA communication
9.1 What you need to know about OPC UA
Communication
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Function Manual, 05/2021, A5E03735815-AJ
Preventing server overload
You can set the OPC UA server of the S7-1500 CPU by means of the "Minimum publishing
interval" so that it does not serve extremely short send intervals requested by the client, see
Settings of the server for subscriptions (Page 224).
Example : A client wants to be operated at a publishing interval of 50 ms as detailed above.
Such a short publishing interval would, however, result in a high network and server load.
You should therefore set 1000 ms as the "Minimum publishing interval" for the server. Clients
whose subscription requires shorter publishing intervals are "slowed" to 1000 ms and the
server is protected from overload.
Sampling and transmission (Sampling & Publishing) within the scope of a subscription are
communication processes which, like other communication processes (TCP/UDP/Web server
communication...), are processed by the CPU with priority 15. OBs with higher priority
interrupt the communication. If you set the sampling and transmission intervals too short,
this setting causes a high communication load. Therefore, select intervals as large as possible,
which are still sufficient for the application.
For information about the consistency of tags, refer to Consistency of CPU tags (Page 213).
Monitoring of PLC tags
When the Subscription has been created, you inform the server which tags are to be
monitored with it. In the following example, the "Voltage" tag was added to the subscription.
The "Voltage" tag contains the value of a voltage that is detected by an S7-1500 CPU.
The sampling interval ("Sampling Interval") contains a negative value (-1). This determines
that the default setting of the OPC UA server is used for the sampling interval. The default
setting is defined by the transmission interval ("Publishing Interval") of the subscription. If you
want to set the smallest possible sampling interval, select the value "0".
In this example, the length of the queue is set to "1": Only one value is read from the CPU at
an interval of 50 milliseconds and subsequently sent to the OPC UA client when the value has
changed.
The "Deadband" parameter in this example is "0.1": Changes in value have to amount to 0.1
Volt; only then does the sender send the new value to the client. The server does not send
smaller changes in value. You can use this parameter, for example, to disable signal noise:
Slight changes in a process variable which do not have a real meaning.