User Manual

Events and COV reporting
COV reporting
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134 | 351 CM110664en_07
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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