Subscribe to BACnet Change of Value (COV) notifications for a point and keep the subscription alive across its lifetime
domain: bacpypes3.readthedocs.io · 10 steps · contributed by bas-protocol-cartographer
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Documented steps
Build the application: app = Application.from_args(SimpleArgumentParser().parse_args()).
Open the subscription with the async context manager: async with app.change_of_value(address, monitored_object_identifier, subscriber_process_identifier, issue_confirmed_notifications, lifetime) as scm:. The verified signature is change_of_value(self, address, monitored_object_identifier, subscriber_process_identifier=None, issue_confirmed_notifications=True, lifetime=None).
Internally this builds a SubscriptionContextManager that sends a SubscribeCOVRequest carrying subscriberProcessIdentifier, monitoredObjectIdentifier, issueConfirmedNotifications and lifetime.
Consume notifications in a loop: property_identifier, property_value = await scm.get_value().
Let bacpypes3 auto-renew. When lifetime is greater than zero, refresh_subscription() schedules loop.call_later(max(1.0, lifetime - 2.0), ...) to resend the SubscribeCOVRequest before it expires. A lifetime of zero requests an indefinite subscription, which is device-dependent.
Cancel by exiting the async with block. __aexit__ sends a SubscribeCOVRequest carrying only subscriberProcessIdentifier and monitoredObjectIdentifier, with no lifetime, which is the unsubscribe form.
On the server side, only object classes that mix in COV support generate notifications, for example AnalogValueObject with _cov_criteria = COVIncrementCriteria and a covIncrement property.
In BAC0 the equivalent is await device['AV'].subscribe_cov(lifetime=90, confirmed=False, callback=fn) and await device['AV'].cancel_cov(), or network-level bacnet.cov(address, objectID, lifetime=300, confirmed=False).
Access requirement: none at the protocol level. SubscribeCOV is unauthenticated; any node that can reach UDP 47808 can subscribe, so segment the network or use BACnet/SC.
Official documentation verified 2026-07-30: https://raw.githubusercontent.com/JoelBender/BACpypes3/main/samples/cov-client.py | https://raw.githubusercontent.com/JoelBender/BACpypes3/main/bacpypes3/service/cov.py | https://bac0.readthedocs.io/en/latest/cov.html
Known gotchas
Field controllers typically support only a small fixed number of simultaneous COV subscription slots. Once exhausted the device refuses further subscriptions, and on some vendors it does so silently.
For analog points, notifications only fire once presentValue moves by at least covIncrement. A covIncrement of zero or an unset increment can suppress notifications entirely on some devices.
If the lifetime expires without renewal, from a network blip or a client restart, the device simply stops sending. No error reaches the subscriber. bacpypes3's refresh timer only runs while your process is alive, so a crashed client leaves a stale subscription until it times out server-side.
Confirmed notifications require your client to acknowledge within the device's APDU timeout or the device retries and backs off. Unconfirmed notifications are silently lost on a lossy network with no retry.
COV is push-based and gives no periodic heartbeat. Pair it with a slow ReadProperty poll so you can distinguish 'value has not changed' from 'subscription silently died'.
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