Integrated automation facilities manage their operations through the exchange of data and the coordinated invocation of system Functions as illustrated in Figure 1. Services are required to perform the data exchanges and to invoke the Functions that constitute system operation. These Functions may be invoked through Human Machine Interfaces, cell controllers, or other supervisory control and data acquisition type systems. OPC UA defines Methods and Programs as an interoperable way to advertise, discover, and request these Functions. They provide a normalizing mechanism for the semantic description, invocation, and result reporting of these Functions. Together Methods and Programs complement the other OPC UA Services and ObjectTypes to facilitate the operation of an automation environment using a client-server hierarchy.
Figure 1 – Automation facility control
Methods and Programs model Functions typically have different scopes, behaviours, lifetimes, and complexities in OPC Servers and the underlying systems. These Functionsare not normally characterized by the reading or writing of data which is accomplished with the OPC UA Attribute service set.
Methods represent basic Functions in the Server that can be invoked by a Client. Programs, by contrast, model more complex and stateful functionality in the system. For example, a method call may be used to perform a calculation or reset a counter. A Program is used to run and control a batch process, execute a machine tool part program, or manage a domain download. Methods and their invocation mechanism are described in OPC 10000-3 and OPC 10000-4.