User Manual

Events and COV reporting
COV reporting
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CM110664en_07 129 | 353
receipt of the value with a SimpleAck acknowledgement. The COV connection
between the COV server and COV client is now established, and
ConfirmedCOVNotifications are sent whenever a trigger for the subscribed COV
occurs.
The BACnet service SubscribeCOV includes a time limit for the COV
connection. However, the COV client re-registers with the COV server before
this limit expires, thus ensuring that the connection is maintained. A COV
connection ends when the subscription period expires and is not renewed, or
when the COV client can no longer be contacted, causing the COV server to
stop sending notifications.
In addition to the SubscribeCOV service, a SubscribeCOV Property service is
implemented, e.g., for the operation of plant graphics in Desigo CC. This
enables the system to respond with appropriate speed to changes in the high or
low limit.
COV reporting between automation stations
COV connections between automation stations are used to implement pre-
engineered references, that is, for the exchange of process values between
individual plant parts on different automation stations. In this case the receiver
is an input function block of the relevant data type (Analog, Binary, Multistate).
The input function block contains the technical designation of the required COV
source in its input/output address parameter [IOAddr]. These COV connections
must be permanently live. The COV mechanism enables a dropped COV
connection to be re-established.
When an automation station connects, the BACnet service WhoHas searches
the entire network for the object referred to in the COV client. The automation
station concerned responds to the COV client with the BACnet service IHave. If
the COV client cannot find the COV server, it repeats the WhoHas request after
the time period defined in the BACnet Device Info Object Property Name
resolution interval [NamRI] until the COV server is found.
The COV client registers for a limited period as a COV client with the COV
server using the BACnet service SubscribeCOV. The server sends a SimpleAck
acknowledgement. The value is then sent to the COV client for the first time by
the COV server, using the BACnet service ConfirmedCOVNotification. The COV
client acknowledges receipt of the value with a SimpleAck acknowledgement.
The COV connection between the COV server and the COV client is established
from this point on. According to the global property COV renewal interval CovRI
of the BACnet Device Info Object, the COVsubscription is renewed. The lifetime
used for SubcribeCOV is twice the COV renewal interval CovRI. The COV
connection ends when the subscription period expires and is not renewed, or
when the COV client can no longer be contacted, causing the COV server to
stop sending notifications.
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