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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