Model Execution: Concepts and Terms : Simulation Scope : External Elements

External Elements

External elements are inputs and outputs to the system. Some of the elements in the simulated scope are marked as external. These are the elements that according to the specification may change outside of the simulated scope. Elements that flow from activities outside the simulated scope into activities inside the simulated scope are external.

It is recommended that you modify only the external elements, since these appear to be environment-driven. Modifying non-external elements is allowed to provide corrections to the behavior of the model, or to complete under-specified portions of the model.

The following figure illustrates a portion of the Activity-chart for a garage door opener. Activities DOOR and REMOTE_CONTROL correspond to the two system components.

Assume that the primary interest is in the garage door subsystem. The scope is set to the activity DOOR and contains the Statechart D_CONTROL. With DOOR as the scope, the REMOTE_CONTROL becomes part of the environment and, therefore, event OPEN_DOOR and event CLOSE_DOOR are external in this simulation execution. You must generate the events OPEN_DOOR and CLOSE_DOOR.

If you later want to execute the model on the entire system, the scope is defined as the OPENING_SYSTEM activity. In this case, the events OPEN_DOOR and CLOSE_DOOR are no longer external, but the events OPEN_BUTTON and CLOSE_BUTTON are now external.