Siemens SIMATIC ET 200AL, System Manual

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Connection resources
12.2 Allocation of connection resources
Communication
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Function Manual, 05/2021, A5E03735815-AJ
When are connection resources occupied?
The time for the occupation of connection resources depends on how the connection is set
up (see section Setting up a connection (Page 33)).
•
Programmed setup of a connection :
As soon as an instruction to establish a connection is called in the user program
(TSEND_C/TRCV_C or TCON), a connection resource is occupied.
With suitable parameter assignment of the CONT parameter of the TSEND_C/TRCV_C
instructions or by calling the TDISCON instruction, the connection can be terminated
following data transfer and the connection resource is available again. When the
connection is terminated, the connection resources on the CPU/CP/CM are available again.
• Configured connections (e.g. S7 connection):
If you have configured a connection in STEP 7, the connection resource is occupied as
soon as the hardware configuration is downloaded to the CPU.
After using a configured connection for data transfer, the connection is not terminated.
The connection resource is permanently occupied. To release the connection resource
again, you need to delete the configured connection in STEP 7 and download the
modified configuration to the CPU.
• PG connection :
As soon as you have connected the PG to a CPU online in STEP 7, connection resources are
occupied.
• Web server :
As long as you have opened the Web server of the CPU in a browser, connection resources
are occupied in the CPU.
• OPC UA server
Each connection to the OPC UA server of the CPU occupies a connection resource in the
station. This connection resource is released immediately when the connection is
terminated.
• OPC UA client
Each connection that the OPC UA client of the CPU has established to an OPC UA server
occupies a connection resource in the station. When an OPC UA connection is established,
the OPC UA client temporarily occupies an additional connection resource. When an OPC
UA connection is terminated, the connection resource is not released again until after a
wait time of approx. 60 seconds in accordance with RFC 793.
Monitoring the maximum possible number of connection resources
Offline
During configuration of connections, STEP 7 monitors the occupation of the connection
resources. If the maximum possible number of connection resources is exceeded, STEP 7
signals this with a suitable warning.
Online
The CPU monitors the use of connection resources in the automation system. If you establish
more connections in the user program than those provided by the automation system, the
CPU acknowledges the instruction to establish the connection with an error.