7 BACnet Communication
3-100 WindO/I-NV4 User’s Manual
● Subscribed COV (COV) function
In the COV (Change Of Value) function, the MICRO/I monitors an object for which a report request has been received
from a BACnet device. Then when Present_Value or Status_Flags has changed, the MICRO/I notifies the BACnet
device of the change.
The properties of objects that correspond to the COV function and the timing of when to provide notification of
changes are given as follows.
*1 For example, the following diagram shows when a report request is received from a BACnet device and AI-10 (Analog Input
object, instance number 10) has the status in the following table.
Object Properties Timing to Provide Notification of Change
Analog Input
Analog Output
Analog Value
Present_Value
Status_Flags
Notification of change is provided when (1) or (2) as follows.
(1)
When
Present_Value has changed to a value greater than or equal to the value
set for
COV_Increment (starting point is from when the previous COV
notification was sent)
*1
(2) When any of the bits in Status_Flags have changed
Binary Input
Binary Output
Binary Value
Present_Value
Status_Flags
Notification of change is provided when (1) or (2) as follows.
(1) When
Present_Value has changed
(2) When any of the bits in Status_Flags have changed
Notification may not be provided for changes in values that are faster than the synchronization cycle of
objects and device addresses.
AI-10 Properties Value
Present_Value 10.0
COV_Increment 3.0
BACnet device MICRO/I
Report request
COV notification
transmission
(To report requester)
Present_Value of AI-10
has changed to 15.0.
COV notification
transmission
Present_Value of AI-10
has changed to 18.5.
Present_Value of AI-10: 10.0
COV_Increment: 3.0
Present_Value of AI-10 has changed to 15.0
(Change in Present_Value (5.0) > COV_Increment (3.0))
This change corresponds to timing to provide notification of change (1).
Change in Present_Value = Present value - Value when report request
was received = 15.0 - 10.0 = 5.0
Because COV_Increment = 3.0,
change in present value ≥ COV_Increment has been satisfied.
Present_Value of AI-10 has changed to 17.5
(Change in Present_Value (2.5) > COV_Increment (3.0))
Change in Present_Value = Present value - Value when previous
COV notification was transmitted = 17.5 - 15.0 = 2.5
Change in Present_Value < COV_Increment, and the condition
for timing to provide notification of change (1) is not satisfied.
Therefore, COV notification is not sent.
Present_Value of AI-10 has changed to 18.5
(Change in Present_Value (3.5) > COV_Increment (3.0))
This change corresponds to timing to provide notification of change (1).
Change in Present_Value = Present value - Value when previous
COV notification was transmitted = 18.5 - 15.0 = 3.5
Because COV_Increment = 3.0,
Change in Present Value ≥ COV_Increment has been satisfied.
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