Methods represent the function calls of Objects. They are defined in OPC 10000-3. Methods are invoked and return only after completion (successful or unsuccessful). Execution times for Methods may vary, depending on the function that they perform.
The Method Service Set defines the means to invoke Methods. A Method shall be a component of an Object. Discovery is provided through the Browse and Query Services. Clients discover the Methods supported by a Server by browsing for the owning Objects References that identify their supported Methods.
Because Methods may control some aspect of plant operations, Method invocation may depend on environmental or other conditions. This may be especially true when attempting to re-invoke a Method immediately after it has completed execution. Conditions that are required to invoke the Method might not yet have returned to the state that permits the Method to start again.