The third of the Access Type specification parts, Part 10 – Programs has been set to Release Candidate.  The Access specs are now targeting the end of December for final Release.

I’ve been giving brief overviews of the how the Access specifications fit into the overall OPC UA picture.   (See Part 8 and Part 11 overviews).  Part 10 is entitled Programs, which is a more comprehensive covering of the functionality that was intended for the OPC Commands specification.  The Command document didn’t get beyond the Draft level, since the Working Group decided to concentrate on the OPC UA spec.  As with all the Access Type specifications, Part 10 is built on OPC UA Services and ObjectTypes that are part of the base framework.

Typically, performing an OPC Write is a one-time, ‘fire-and-forget’ type of action.  You attempt to update the value of an item, and get some level of feedback on whether or not it worked.  There are many cases, when a more complex system is needed.  An application may want to request an action, get feedback, go to an intermediate state, request next action , etc.  In a nutshell, OPC UA provides a normalizing mechanism for the semantic description, invocation of, and result reporting of these types of functions.  The key to this is the Program Finite State Machine which provides well defined states, transitions, and cause and effect behavior descriptions.  The bulk of the specification deals with describing the ObjectTypes and outlining the states, transitions, methods and events associated with Programs.

To date, this sort of functionality has always been part of specialized OPC clients.  Part 10 now creates a standardized, and extensible way of performing more deterministic operations, and moves the functionality into the Server.  A long looked for, and powerful feature for OPC applications.  

As always Members can get the full details of the specifications from the downloads section