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 received, the transmitting device tries to send the information again (three times).
For system limits, see chapter System Configuration.
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
To ensure that the COV service is maintained over a long period, a maximum time period without COV reporting can be set in the BACnet Device Info Object via the BACnet property COV Resubscription Interval [CovRI]. The client must subscribe with SubscribeCOV again before [CovRI] expires.
COV clients and COV servers
The local PXM10 operator unit is not a BACnet client and cannot, therefore, be used as a COV client.
See PXM10 operator unit: User's guide (CM110397).
COV reporting between COV client and COV server
BACnet clients use the COV mechanism for continuous monitoring of process variables without putting an excessive load on the bus through continuous polling. They subscribe to the objects that they are monitoring. These COV connections must be maintained as long as the object is being monitored.
The BACnet client subscribes to the COV server as a COV client using the BACnet service SubscribeCOV. The server sends a SimpleAck acknowledgement. Immediately after the acknowledgement, the COV server transmits an initial ConfirmedCOVNotification. The COV client acknowledges 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.