User Manual

Events and COV reporting
Sources and causes of COVs
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not an alarm. Each alarm recipient that receives the Confirmed Event Notification is required to respond
with a SimpleAck. If the SimpleAck is not received, the same mechanism comes into operation as for
alarms.
Event texts
Each system event has a message text assigned to it. For the system events related to the operating hours
counter, a user-specific text can be set up in Xworks Plus (XWP). Predefined system texts are available for
the other system events.
8.3 Sources and causes of COVs
Process variables which can be mapped to standard BACnet objects are COV-capable.
I/O function block
Function blocks for Analog, Binary and Multistate Inputs, Outputs and Values are mapped directly to the
associated BACnet object types. They are COV-capable and can establish COV connections with all COV
clients.
Interface variables
Interface variables of compounds and function blocks whose data type is Analog, Binary, Multistate,
Integer, Duration and DateTime are COV-capable and can be mapped to simplified BACnet value objects
for operation and monitoring.
A COV is initiated when the value of the process variable [PrVal] of the BACnet object which represents it
changes. A COV is also initiated when a status flag [StaFlg] (InAlarm, Fault, Overridden or Out of service)
changes, e.g., when a sensor open circuit (fault) occurs or when an I/O value is overwritten manually.
COV increment
For analog objects, a COV is not initiated for every minor change of [PrVal], but only when the value
changes by an amount greater than a predefined increment. This increment is saved in the COV increment
[COV] of the analog object, and can be defined in Xworks Plus (XWP) during engineering.
8.4 COV reporting
Subscription
Each COV client must subscribe to every process variable from which it requires COV notifications. Each
COV-capable object transmits COV notifications only to those COV clients which have subscribed to COV
notifications. The subscription process is carried out using the BACnet service SubscribeCOV, transmitted
by the COV client to the COV server. This message contains all the information that the COV server needs
to send the COV notifications to the correct destination. It also includes a time period which determines the
validity period of the subscription. The time period may be infinite.
For system limits, see chapter
System Configuration .
COV notifications
The COV server reports every COV individually to each COV client which has subscribed to it. The BACnet
service ConfirmedCOVNotification is used for this purpose. It contains the values of [PrVal] and [StaFlg].
The service is a Confirmed Service, which means that the COV client must acknowledge the notification
(SimpleAck). This ensures that when a COV client ceases to be available, this will be recognized by the
COV server. If no SimpleAck message is received, the transmitting device tries to send the information
again (three times).
For system limits, see chapter
System Configuration .
Connection terminated
If a COV client cannot be contacted, the COV server ceases to send COV notifications to that client. The
transmission of COV notifications to a COV client is resumed when the COV client re-subscribes.
Checking the connection
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