Chapter 14 Sequential Function Chart 14.1.2 Principle of a Sequential Function Chart A sequence in a sequential function chart is depicted as a series of steps shown as rectangular boxes connected by vertical lines. Each step represents a particular state of the system being controlled. A transition is associated with a condition which, when true, causes the step before the transition to be deactivated and the step that follows the transition to be activated. Steps in a sequential function chart can be active or inactive. Actions are only executed for active steps.
If a step is inactive, the transition follows it will not affect the execution of the program, and the actions associated with the step will not be executed. In a sequential function chart in ISPSoft, the steps must alternate with the transitions. If there are two continuous steps or transitions in a sequential function chart, the program can not be compiled. The example above is the structure of a simple sequential function chart. ISPSoft also provides other flexible programming functions, such as the use of divergent and convergent paths, the use of modular actions and transitions, and the qualification of actions. These programming functions will be introduced in the sections below. 14-3