User Manual

Events and COV reporting
Sources and causes of COVs
8
CM110664en_07 127 | 353
System events cannot be acknowledged or reset. A Confirmed Event
Notification message is sent to all alarm recipients. The Notify_Type data field
in the message defines that the event is a system event and 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.
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.
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 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.
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
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 .
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
S
i
m
p
l
e
A
c
k
t
t
t
S
i
m
p
l
e
A
c
k
Comparison with the
alarm strategy
Event texts
I/O function block
Interface variables
COV increment
Subscription