Step in SFC

Steps are basic elements of SFC networks. One or more action blocks can be associated to a step.

The state of a step can either be active or inactive. Active means that the step is actually processed by the controller. The action associated to the step is executed. If a step becomes inactive, the action is executed once again. A step without associated action blocks is waiting for a succeeding condition to become true.

Steps are associated with its preceding and succeeding transitions via direct links which are represented as lines. The name of the step is indicated in the rectangle which symbolizes a step in the free graphic editor.

Note
Step names must be unique within all SFC charts, i.e., between all SFC code worksheets of the SFC POU where they are used.

Several types of steps can be used.

Type Representation Description

Initial step An SFC network requires one initial step which is the first step to be executed when the POU is called. The first step you have inserted is automatically an initial step. There is only one initial step allowed in the SFC network.
When a normal step or end step is converted to an initial step, the previously defined initial step becomes a normal step.

Normal step Normal steps are the steps which are executed by the program in the order in the network.

End step An end step is required to finish a closed SFC network (and to go back to the calling POU, if necessary). End steps cannot be used within simultaneous branches.
When an end step is created, all objects connected after the step in its line are deleted (the editor ensures that a valid SFC network remains when deleting the succeeding objects). See also the following description of the start step.

Start step If the creation of an end step leads to only one step in the SFC network, i.e., the step has no predecessor, a so-called start step is created.

Type	Representation	Description
Initial step		An SFC network requires one initial step which is the first step to be executed when the POU is called. The first step you have inserted is automatically an initial step. There is only one initial step allowed in the SFC network. When a normal step or end step is converted to an initial step, the previously defined initial step becomes a normal step.
Normal step		Normal steps are the steps which are executed by the program in the order in the network.
End step		An end step is required to finish a closed SFC network (and to go back to the calling POU, if necessary). End steps cannot be used within simultaneous branches. When an end step is created, all objects connected after the step in its line are deleted (the editor ensures that a valid SFC network remains when deleting the succeeding objects). See also the following description of the start step.
Start step		If the creation of an end step leads to only one step in the SFC network, i.e., the step has no predecessor, a so-called start step is created.

Note
While the SFC chart is in halt mode, a particular step can be activated/deactivated via the SFC function block parameters STEP_ID and ACTIVATE_STEP/DEACTIVATE_STEP. See section "Input parameters" in topic "SFC Parameters, Operating Modes and Data Types" for details.

Interlocks can be added to steps.