BACnet PTP Serial Driver Manual
Page 20 of 34
Appendix A.6. COV and Intrinsic Reporting
The COV (Change of Value) and Intrinsic Reporting services are two distinct ways in which point values
can be reported to a client workstation as they change, i.e. in an event-driven opposed to a polling
method. This can increase performance dramatically compared to polling method alone. It also reduces
network traffic significantly.
The services are suited to different purposes:
•
COV is suited to value updates. On analog points the sensitivity can be set using the COV_Increment
property. Only changes larger than the COV_Increment value will be reported.
•
Intrinsic Reporting is used for alarming. It is implemented via Notification_Class objects, which can
receive subscriptions from client workstations that add themselves to the RecipientList property of a
Notification_Class
object.
Notifications
are
done
using
ConfirmedEventNotification
or
UnconfirmedEventNotification. Intrinsic Reporting also allows for alarms to be acknowledged (using
the AcknowledgeAlarm service) and for all subscribed client workstations to be notified of alarm
acknowledgements (using EventNotifications of type ACK_NOTIFICATION).
Appendix A.6.1. Notes on COV Configuration
•
No special configuration entries are needed to enable COVs. The service is enabled by default for all
protocols except MSTP. Most client workstations will automatically subscribe to all points once they
discover that the FieldServer supports COV services. This only applies to BACnet Objects - Property
subscribes are not supported by the FieldServer. COV only works for BACnet Map Descriptors with
length set to 1. If length is not specified, then it defaults to 1, so this is only a problem where length
has been specified as greater than 1.
•
The Node_Option parameter can be configured to enable or disable COV. An example configuration
is presented in
•
For analog Server Map Descriptors, the user may optionally configure a COV_Increment value to
adjust the reporting threshold. If it is not set the COV_Increment defaults to zero.
•
Change of Value (COV) Notifications are generated for all data objects for which a remote client has
issued a SubscribeCOV-Request. COV subscription is on a per-point basis. The SubscribeCOV-
Request regulates whether Notifications are Confirmed or Unconfirmed. The remote client may also
write the COV_Increment property in order to control the deadband for changes in analog values. The
COV_Increment property can be initialized via the configuration file by setting the COV_Increment
Map Descriptor Property. The value set by the configuration is an initial value that is loaded on
startup. It would be replaced by any new value written by the Client.
Appendix A.6.2. Notes on Intrinsic Reporting Configuration:
•
Intrinsic Reporting is managed by Notification Class objects. At least one Notification Class object
must be configured for Intrinsic Reporting to work.
•
Each Data_Object that is to be monitored by Intrinsic Reporting must be linked to a Notification Class
object via the Notification_Class Map Descriptor Property. For analog points alarm limits must be set
up, and for binary points, the Input_Alarm_State must be set up.
•
The Notification_Class object contains properties that allow a client workstation to modify rules
governing event reporting, such as Event_Type, days of week, start and end times etc.
•
The RecipientLists are non-volatile, and subscriptions must be renewed on system restart.
