Change of Value (COV) Services

class ChangeOfValueServices(Capability)

This class provides the capability of managing COV subscriptions and initiating COV notifications.

do_SubscribeCOVRequest(apdu):

Parameters:apdu (SubscribeCOVRequest) – request from the network

This method processes the request by looking up the referenced object and attaching a COV detection algorithm object. Any changes the to referenced object properties (such as presentValue to statusFlags) will trigger the algorithm to run and initiate COV notifications as necessary.

add_subscription(cov)

This method adds a subscription to the internal dictionary of subscriptions indexed by the object reference. There can be multiple COV subscriptions for the same object.

cancel_subscription(cov)

This method removes a subscription from the internal dictionary of subscriptions. If all of the subscriptinos have been removed, for example they have all expired, then the detection “hook” into the object is removed.

cov_notification(cov, request)

This method is used to wrap a COV notification request in an IOCB wrapper, submitting it as an IO request. The following confirmation function will be called when it is complete.

cov_confirmation(iocb)

This method looks at the response that was given to the COV notification and dispatchs one of the following functions.

cov_ack(cov, request, response)

This method is called when the client has responded with a simple acknowledgement.

cov_error(cov, request, response)

This method is called when the client has responded with an error. Depending on the error, the COV subscription might be canceled.

cov_reject(cov, request, response)

This method is called when the client has responded with a reject. Depending on the error, the COV subscription might be canceled.

cov_abort(cov, request, response)

This method is called when the client has responded with an abort. Depending on the error, the COV subscription might be canceled.

Support Classes

class ActiveCOVSubscriptions(Property)

An instance of this property is added to the local device object. When the property is read it will return a list of COVSubscription objects.

class SubscriptionList

append(cov)

Parameters:cov (Subscription) – additional subscription

remove(cov)

Parameters:cov (Subscription) – subscription to remove

find(client_addr, proc_id, obj_id)

Parameters:

• client_addr (Address) – client address
• proc_id (int) – client process identifier
• obj_id (ObjectIdentifier) – object identifier

This method finds a matching Subscription object where all three parameters match. It is used when a subscription request arrives it is used to determine if it should be renewed or canceled.

class Subscription(OneShotTask)

Instances of this class are active subscriptions with a lifetime. When the subscription is created it “installs” itself as a task for the end of its lifetime and when the process_task function is called the subscription is canceled.

__init__(obj_ref, client_addr, proc_id, obj_id, confirmed, lifetime)

Parameters:

• obj_ref – reference to the object being monitored
• client_addr – address of the client
• proc_id – process id of the client
• obj_id – object identifier
• confirmed – issue confirmed notifications
• lifetime – subscription lifetime

cancel_subscription()

This method is called to cancel a subscription, it is called by process_task.

renew_subscription(lifetime)

Parameters:lifetime (int) – seconds until expiration

This method is called to renew a subscription.

process_task()

Call when the lifetime of the subscription has run out.

class COVDetection(DetectionAlgorithm)

This is a base class for a series of COV detection algorithms. The derived classes provide a list of the properties that are being monitored for changes and a list of properties that are reported.

execute()

This method overrides the execute function of the detection algorithm.

send_cov_notifications()

This method sends out notifications to all of the subscriptions that are associated with the algorithm.

class GenericCriteria(COVDetection)

This is the simplest detection algorithm that monitors the present value and status flags of an object.

class COVIncrementCriteria(COVDetection)

This detection algorithm is used for those objects that have a COV increment property, such as Analog Value Objects, where the change in the present value needs to exceed some delta value.

class AccessDoorCriteria(COVDetection)

This detection algorithm is used for Access Door Objects.

class AccessPointCriteria(COVDetection)

This detection algorithm is used for Access Point Objects.

class CredentialDataInputCriteria(COVDetection)

This detection algorithm is used for Credential Data Input Objects.

class LoadControlCriteria(COVDetection)

This detection algorithm is used for Load Control Objects.

class PulseConverterCriteria(COVDetection)

This detection algorithm is used for Pulse Converter Objects.