The standard OPC UA consists of individual specifications. Each specification describes a partial function and specifies which server and client interfaces must be implemented for this function in order to support it. OPC servers and clients do not have to support all specifications. Depending on the application, often only individual specifications are programmed. When using an OPC server and clients, it is therefore important to consider which specifications are required and which are implemented by the server and client.
OPC UA consists of these specifications:
2. Security Model
3. Address Space Model
5. Information Model
8. Data Access
9. Alarms and Conditions
11. Historical Access
For operational use it is not necessary to know the specifications in detail. The most important for the project business are the following:
The Data Access specification describes the classic exchange of current data. The OPC Classic Standard already specifies that the data exchange is data point-oriented. A value can be read and written for each data point. A data point value is described by the actual value, the time stamp at which the value was current and by the quality, which describes whether the value is valid or whether, for example, the connection to the controller was interrupted and the value is therefore not valid. This specification alone makes it possible to obtain and process data independently of the underlying system.
In the current standard, the possibilities have been extended with complex data types (structures) and functions in order to meet the new requirements.
Using the Historical Access specification, it is possible not only to read data with the current value, but also to query historical values. An OPC server that implements this specification must have an internal data memory in order to provide the values of the data points for possible historical accesses. A client that reads historical data points via Historical Access also transfers the desired time span to the server in addition to the data point information.
Alarms and Conditions
The Alarms and Conditions specification defines a standardized model for alarm messages and alarm logic as part of OPC UA. For OPC client applications this simplifies the task of generating alarms from data point values because the logic can be implemented by the OPC server and not by the manufacturer of the client software.