The collection of Attributes associated with any given A&E Event is encapsulated within the ONEVENTSTRUCT. Therefore, the A&E COM UA Proxy populates the Attribute fields within the ONEVENTSTRUCT using corresponding values from UA Event Notifications either directly (e.g., Source, Time, Severity) or indirectly (e.g., OPC COM Event category determined by way of the UA Event type). Table D.5 lists the Attributes currently defined in the ONEVENTSTRUCT in the leftmost column. The rightmost column of Table D.5 indicates how the A&E COM UA proxy defines that Attribute.

Table D.5 – Event category attribute mapping table

Condition Event instance records are stored locally within the A&E COM UA Proxy. Each record holds ONEVENTSTRUCT data for each EventSource/Condition instance. When the Condition instance transitions to the state INACTIVE|ACKED, where AckRequired = True or simply INACTIVE, where AckRequired = False, the local Condition record is deleted. When a Condition Event is received from the UA A & C Server and a record for this Event (identified by source/Condition pair) already exists in the proxy Condition Event store, the existing record is simply updated to reflect the new state or other change to the Condition, setting the change mask accordingly and producing an OnEvent callback to any subscribing Clients. In the case where the Client application acknowledges an Event which is currently unacknowledged (AckRequired = True), the UA A & C Server Acknowledge Method associated with the Condition is called and the subsequent Event produced by the UA A & C Server indicating the transition to acknowledged will result in an update to the current state of the local Condition record as well as an OnEvent Notification to any subscribing Clients.

The A&E COM UA Proxy maintains the mapping of Attributes on an Event category basis. An Event category inherits its Attributes from the Properties defined on all supertypes in the UA Event Type hierarchy. New Attributes are added for any Properties defined on the direct UA Event type to A&E category mapping. The A&E COM UA Proxy adds two Attributes to each category: AckComment and Areas. Figure D.4 shows an example of this mapping.


Figure D.4 – Example mapping of UA Event Types to A&E categories with attributes


A&E COM UA Proxy Mapping

The following items are present for all A&E event types


UA BaseEventType Property: SourceName


UA BaseEventType Property: Time


UA BaseEventType Property: Message


See Clause D.3.3


See Clause D.3.4


UA BaseEventType Property: Severity


Calculated within A&E COM UA Proxy


Constructed within A&E COM UA Proxy

The following items are present only for A&E Condition-Related Events


UA ConditionType Property: ConditionName


UA ActiveState Property: EffectiveDisplayName


Calculated within Alarms and Events COM UA proxy


A & C AlarmConditionType Property: ActiveState

Note that events mapped as non-Condition Events and those that do not derive from AlarmConditionType are set to ACTIVE by default.


A & C ConditionType Property: EnabledState

Note, Events mapped as non-Condition Events are set to ENABLED (state bit mask = 0x1) by default.


A & C AcknowledgeableConditionType Property: AckedState

Note that A & C Events mapped as non-Condition Events or which do not derive from AcknowledgeableConditionType are set to UNACKNOWLEDGED and AckRequired = False by default.


A & C ConditionType Property: Quality

Note that Events mapped as non-Condition Events are set to OPC_QUALITY_GOOD by default.

In general, the Severity field of the StatusCode is used to map COM status codes OPC_QUALITY_BAD, OPC_QUALITY_GOOD and OPC_QUALITY_UNCERTAIN. When possible, specific status' are mapped directly. These include (UA => COM):

Bad status codes

Bad_ConfigurationError => OPC_QUALITY_CONFIG_ERROR






Uncertain status codes

Uncertain_NoCommunicationLastUsableValue => OPC_QUALITY_LAST_USABLE

Uncertain_LastUsableValue => OPC_QUALITY_LAST_USABLE

Uncertain_SensorNotAccurate => OPC_QUALITY_SENSOR_CAL

Uncertain_EngineeringUnitsExceeded => OPC_QUALITY_EGU_EXCEEDED

Uncertain_SubNormal => OPC_QUALITY_SUB_NORMAL

Good status codes



If the ACKNOWLEDGED bit (OPC_CONDITION_ACKED) is set then the Ack Required Boolean is set to False, otherwise the Ack Required Boolean is set to True. If the Event is not of type AcknowledgeableConditionType or subtype then the AckRequired Boolean is set to False.


If the Event is of type AlarmConditionType or subtype and a transition from ActiveState of False to ActiveState to True is being processed then the TransitionTime Property of ActiveState is used. If the Event is not of type AlarmConditionType or subtype then this field is set to current time.

Note: Additional logic applies to exclusive limit alarms, This value should be mapped to the LimitState.TransitionTime.


Generated by the A&E COM UA Proxy. These unique Condition Event cookies are not associated with any related identifier from the address space of the UA A & C Server.

The following is used only for A&E tracking events and for A&E condition-relate events which are acknowledgement notifications


Vendor specific Attributes – ALL

ACK Comment


All A&E Events are assumed to support the "Areas" Attribute. However, no Attribute or Property of an A & C Event is available which provides this value. Therefore, the A&E COM UA Proxy initializes the value of the Areas Attribute based on the MonitoredItem producing the Event. If the A&E COM Client has applied no area filtering to a Subscription, the corresponding A & C Subscription will contain just one MonitoredItem – that of the UA A & C Server Object. Events forwarded to the A&E COM Client on behalf of this Subscription will carry an Areas Attribute value of empty string. If the A&E COM Client has applied an area filter to a Subscription then the related UA A & C Subscription will contain one or more MonitoredItems for each notifier Node identified by the area string(s). Events forwarded to the A&E COM Client on behalf of such a Subscription will carry an areas Attribute whose value is the relative path to the notifier which produced the Event (i.e., the fully qualified area name).

Vendor specific Attributes – based on category


All the UA A & C subtype Properties that are not part of the standard set exposed by BaseEventType or ConditionType