System events are messages which inform a client (e.g., Desigo CC) of specific events in an automation station, such as:
- Change in operating state of the automation station (STOP, RUN)
- Overflow of the operating hours counter built into certain I/O objects
Conceptually, system events are similar to alarms, however, they differ from alarms in some ways:
- System events have no memory, that is, they do not have a finite state machine.
- System events do not affect the status of a plant, that is, they can occur in any alarm state without influencing it.
- System events are displayed, but do not need to be acknowledged or reset.
System events are forwarded to clients using the same mechanism as alarms.
If a value of a specific process variable changes, the change is transmitted to other system components by means of Change Of Value (COV) reporting. Polling is used only in exceptional cases. COV reporting can be used to transfer value changes to several automation stations. A COV notification is issued only when the value of the process variable changes in comparison with the preset or default incremental value. There is no need to poll the process variables at regular intervals.
There are two roles:
- COV-Server: The automation station which reads the process variable and whose change of value is to be reported.
- COV-Client: The recipient of the COV notifications. This may be another automation station or a BACnet client.