Siemens SIMATIC ET 200AL, System Manual

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OPC UA communication
9.3 Using the S7-1500 as an OPC UA server
Communication
Function Manual, 05/2021, A5E03735815-AJ
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9.3.6.3 OPC UA conditions and OPC UA alarms
The following goes further in depth about the basic concepts for OPC UA Conditions and OPC
UA Alarms based on the explanations of events in the previous sections. Again, the terms
used in the various parts of the OPC UA specification have been retained here.
Properties of Conditions
A prerequisite for understanding is the concept of "Events" in OPC UA.
In OPC UA, if an event alarm object provides status information in addition to its ability to fire
Events, we speak of Conditions. Conditions represent a state of a system or one of its
components. Basic states are "enabled" and "disabled", other state definitions are also
possible.
In turn, interested OPC UA clients are notified of state changes by means of events (Condition
Events).
An example of a Condition is state information, for example, that a device requires
maintenance.
Properties of Alarms
However, the properties of ConditionType are not sufficient to completely map the
characteristics of SIMATIC alarms in the OPC UA server.
From the ConditionType, which is derived from the BaseEventType, OPC UA defines further
derived event types such as AcknowledgeableConditionType and AlarmConditionType.
AcknowledgeableConditionType supplements the properties of ConditionType with the
"Acknowledgeable" characteristic (AckedState).
AlarmConditionType thus adds the "ActiveState" characteristic to the properties of
ConditionType and the AcknowledgeableConditionType. In SIMATIC terminology, this is an
incoming alarm. The ActiveState signals that the situation, which the Condition represents, is
currently present or has occurred.
Example: A temperature has exceeded a limit. If "ActiveState" is not set, the situation that
represents the condition no longer exists - this is usually referred to as a "normal state". In
SIMATIC terminology, this corresponds to an outgoing alarm.
In OPC UA, other statuses such as SilenceState and ShelvingState are defined, but these are
not relevant to mapping to the SIMATIC alarm system and will therefore not be described
further here.
The SimaticAlarmConditionType is derived from the AlarmConditionType and contains all
event fields to map the state and acknowledgment information of SIMATIC messages.