Siemens SIMATIC S7-1500 Function Manual page 142

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 179).
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.
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 181).
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 171).
Communication
Function Manual, 11/2019, A5E03735815-AH
OPC UA communication
9.1 What you need to know about OPC UA
141