SFC Execution Behavior

The SFC execution behavior consists of three stages: initial situation (start), code execution, and end. Each virtual machine cycle consists of determining all clearable transitions and executing their active steps. Execution ends upon reaching unclearable transitions or the end of the control chart.

Within the execution cycle, the dynamic behaviors of the SFC language are:

Initial situation

The Initial Situation is characterized by the initial steps which are, by definition, in the active state at the beginning of the operation. At least one initial step must be present in each SFC program.

Clearing of a transition

A transition has three properties: enabled/disabled, active/inactive, and clearable/non-clearable. A transition is enabled when all immediately preceding steps linked to its corresponding transition symbol are active, otherwise, the transition is disabled. A transition is active if its condition is True.

A transition is clearable if it is enabled and active at the same time. When a transition is clearable, the steps immediately preceding it become inactive and those immediately following it become active. When transitions follow a divergence, multiple transitions may become clearable. However, the only transition that is cleared is the one having the highest priority

Changing of state of active steps

The clearing of a transition simultaneously leads to the active state of the immediately following steps and to the inactive state of the immediately preceding steps. The code within a step is only executed if the step is active.

Simultaneous clearing of transitions

All transitions (of all SFC programs) that can be cleared (enabled and active), are simultaneously cleared.

However, for transitions following divergences, the only transition that is cleared is the one having the highest priority among those that are enabled and active.

End

The End is characterized by reaching the end of clearable transitions or the end of the control chart.