COV in BAC0

BACnet supports a change of value (COV) mechanism that allow to subscribe to a device point to get notified when the value of this point changes.

In BAC0, you can subscribe to a COV from a point directly (awaitable)

# subscribe for 90 seconds (renew as needed)
await device['AV'].subscribe_cov(lifetime=90)

or from the network itself (starts a background task immediately)

bacnet.cov(address, objectID, lifetime=300, confirmed=False)

Note

objectID is a tuple created with the object type as a string and the instance. For example analog input 1 would be : (“analogInput”, 1)

Minimal async example

This mirrors the pattern in tests/manual_test_cov.py, trimmed to essentials

async with BAC0.start() as bacnet:
    # Connect to a remote device and get a point
    dev = await BAC0.device("192.168.1.50:47808", 1234, bacnet, poll=0)
    av = dev["AV"]

    # Subscribe to COV notifications for 90 seconds
    await av.subscribe_cov(lifetime=90)

Confirmed COV

If the device to which you want to subscribe a COV supports it, it is possible to use a confirmed COV. In this case, the device will wait for a confirmation that you received the notification. The default in BAC0 is unconfirmed (confirmed=False).

To require confirmation, pass confirmed=True to subscribe_cov or bacnet.cov.

Lifetime

COV subscription duration is controlled by the lifetime argument (seconds). By default, this is set to 900 seconds. Renew as needed to keep the subscription alive.

Callback

It can be required to call a function when a COV notification is received. This is done by providing the function as a callback to the subscription

# For device['point'].subscribe_cov the callback receives keyword args:
#   property_identifier, property_value
def my_point_callback(property_identifier, property_value, **_):
    print(f"{property_identifier}: {property_value}")

await device['AV'].subscribe_cov(callback=my_point_callback)

# For bacnet.cov(address, objectID) the callback also receives
# property_identifier and property_value (runs in background)
bacnet.cov('3:10', ("analogValue", 1), callback=my_point_callback)

Canceling a subscription

Point-level: await device[‘AV’].cancel_cov()

Network-level: use bacnet.cov_tasks to locate the process identifier and cancel it

ids = list(bacnet.cov_tasks.keys())
if ids:
    bacnet.cancel_cov(ids[0])

Note

Point-level subscribe_cov is awaited to establish the subscription. The network-level bacnet.cov(…) helper starts a background task immediately and does not return a task id; inspect bacnet.cov_tasks to manage running subscriptions.