The OPC Alarms and Events specification defines an API for communicating real time event data from control devices. The OPC AE specification event model can be simplified and viewed as a stream of event or message records with a fixed record structure. Each event record can be viewed as a row in database table with pre-defined columns. The screen capture below shows sample OPC AE event streams. Each event contains information such as the source of the event, the time it occurred in the device, a message describing the event, the type of event, its severity etc. For a more advanced description of the OPC AE architecture the OPC Foundation specifications on the subject are recommended.
As with the OPC DA specification, implementations of the OPC AE specification are called OPC servers. Another more specific way to reference the implementations are as OPC AE servers. An OPC AE server is required to be available for the OPC MetaBridge application to read events from a control device.
The fixed structure of OPC AE events does make it easier to consume the event information, but a side effect of this is that the AE data model won't necessarily map directly to a particular control device's event data. OPC AE server vendors have a data mapping decision to make in those scenarios. A common approach that is taken is that the Message column within an AE event then contains a Vendor specific format and definition for a device event. Because of this, Deployment Engineers should be aware that just because an OPC AE server is available for a control device it does not imply that all of their event data integration requirements have been fulfilled. Each OPC AE server scenario has to be evaluated on a case by case basis.