OPC Alarms and Events Specification: Handling real-time access to alarms and events in the industry

OPC AE is acronym of OPC Alarms and Events. It is a set of interfaces that handle real time access to alarms and events occurred in the automation industry. This is achieved by exposing alarms and events to the control process level with respect to OPC standard format.

The OPC Alarms and Events specification follows similar basics as Data Access and Historical Data Access specifications. The type of manipulated data differs from an OPC specification to another. In fact, unlike OPC DA specification that addresses real-time data and OPC HDA that addresses historical data, OPC AE manipulates alarms and events data.
The main goal of OPC AE specification is to define and provide a common communication interface for transferring real-time alarms and events data in the process control environment

A typical architecture of the OPC AE communication is as follows:
First an OPC AE Server is installed on the production level acting as a supervisor for a given manufacturer’s device in respect with OPC standard that defines the I/O operations. Therefore, all information retrieved from the device by the OPC Server via its specific API will be exposed as OPC alarms format.
Then and to allow access and acknowledgment of these alarms, one or many OPC AE Clients are installed anywhere in the manufacturer’s local network. An OPC AE Client will have the possibility to subscribe for all or for a filtered set of alarms notification and to acknowledged alarms.

Follow and to better explain the fundamentals of the OPC AE standard, I will try to answer to the most important questions that may include the following:

1. What is the purpose of the OPC AE specification?
The purpose of the OPC AE specification is to define interfaces used to notify the automation process control level by the areas of the process that require immediate attention. Therefore, it provides an industry standard that will enable communication of different components related to different providers in an heterogeneous environment.

The OPC AE specification is intended to increase the alarming solutions capabilities by the plug-n-play environment feature.

The OPC AE specification define a series of interfaces that provide the functionalities required by an OPC AE Client to be notified by the occurrence of an alarms and events and the services to query the OPC AE servers capabilities.

2. What is the difference between alarms and events?
An alarm is an abnormal condition.
An event is a detectable occurrence.
Therefore, the alarm and event meanings are not totally separate and can be related. They are not limited to safety limits of equipment, event detection or abnormal situations.

3. What is a condition?
A condition is a named state of the OPC AE Server. It can include multiple sub-conditions. Thus, a condition can be single state, or multi-state.
3.1 What is a multi-state condition?
A multi-state condition is one whose state encompasses multiple "ranges" or sub-states that are of interest
3.2 What is a single state condition?
A single state condition has only one sub-state of interest; and thus has only one sub-condition associated with it.

Condition definitions are server specific. It can be a Boolean expression or a text string message indicating an abnormal situation.

4. What is a sub-condition?
A sub-condition is a state of a condition. To better understand this, let us assume that we have a condition name C1 with this Boolean expression: 100 < CV < 1000. For this condition we can define multiple sub-conditions in order to get more information about the value generating the alarm. So we can have SB1 (100 < CV < 500) and SB2 (500 < CV < 1000)

5. What are the different types of alarms and events?
The OPC AE specifications define 3 types of events: Simple, Tracking and Condition.

6. How are event sources organized inside the OPC AE Server?
Event sources are organized as Area space and Event space.

7. What are the different operations that can be performed by an OPC AE Client?
The OPC AE Specification defines interfaces and methods to allow the OPC AE Client to:
- Query the configuration supported by the OPC AE Server including the type of event, the conditions names and sub-conditions names
- Subscribe to specified events. Thus, it can receive notifications of their occurrences.

8. How can the OPC AE Client filter on the events of interest?
This subscription is in fact characterized by a Filter. A Filter is a structure containing criteria for selecting events of interest to the client. To receive all events, the OPC Client can pass a null Filter as inputs.
Events may be selected using the following criteria: Type of event, Event categories, severity, Process areas and Event sources

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