Settings Of The Server For Subscriptions - Siemens SIMATIC ET 200AL System Manual

OPC UA communication
9.3 Using the S7-1500 as an OPC UA server
9.3.2.4

Settings of the server for subscriptions

Subscription instead of cyclic queries
An alternative to cyclic queries for a PLC tag (polling) is to supervise the value. Use a
Subscription: The server informs the client if the value of PLC tags changes. See "The
OPC UA client (Page 142)".
One server usually monitors a large number of PLC values. At regular intervals, the server
therefore sends the client messages (notifications) containing the new values of the PLC
tags.
How frequently does the server send notifications?
When a Subscription is set up, the OPC UA client specifies the intervals at which it wants to
be sent the new values in the event of changes. To limit the communication load from
OPC UA, you set a minimum notification interval that must be respected. For this purpose,
use the parameters for the minimum publishing interval and the minimum sampling interval.
Minimum publishing interval
With "Minimum publishing interval", you set the time intervals at which the server sends a
message to the client with the new values in the event of changes.
250 ms is used as the "Minimum sampling interval" in the figure below. The value 200 ms is
entered as the "Minimum publishing interval".
In the example, following a value change the OPC UA server will send a new message every
200 ms if the OPC UA client requests an update.
If the OPC UA client requests an update every 1000 ms, for example, the OPC UA server
will only send a notification with the new values once every 1000 ms (one second).
If the client requests an update every 100 ms, the server will still only send a message every
200 ms (minimum publishing interval).
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Function Manual, 12/2017, A5E03735815-AF
Communication