Chapter 11 - Modeling Event-Based Behavior with State Machines

Abstract

This chapter describes how to use state machines to model the behavior of blocks as they respond to internal and external events. Typically, state machines are used in SysML to describe the state-dependent behavior of a block throughout its life cycle in terms of its states and the transitions between them. A state machine for a block may be started when it initiates power up, transition through multiple states in response to different stimuli, and terminate when it completes power down. In each state, the block may perform different sets of actions. The state machine defines how the block's behavior changes as it transitions through different states. State machines in SysML can be used to describe a wide range of state-related behavior, from the behavior of a simple lamp switch to the complex modes of an advanced aircraft. State machines are normally owned by blocks and executed within the context of an instance of that block. The behavior of a state machine is specified by a set of regions, each of which defines its own set of states. The states in any one region are exclusive; that is, when the region is active, exactly one of its substates is active. A region normally has an initial pseudostate, which is the place the region starts executing when it first becomes active.