SFC Execution at Runtime

SFC programs are executed sequentially within a target cycle according to the order defined when entering programs in the hierarchy tree.

  • A parent SFC program is executed before its children.
    • This implies that when a parent starts or stops a child, the corresponding actions in the child program are performed during the same cycle.
  • In a chart, all valid transitions are evaluated first and then actions of active steps are performed.
    • The chart is evaluated from the left to the right and from the top to the bottom.

Example

Execution order:

  • Evaluate transitions:
    • 1, 101, 2
  • Manage steps:
    • 1, 101, 201, 102
    • 3

The initial steps define the initial status of the program when it is started.

  • All top level (main) programs are started when the application starts.
  • Child programs are explicitly started from action blocks within the parent programs.

The evaluation of transitions leads to changes of active steps, according to these rules:

  • A transition is crossed if:
    • Its condition is TRUE and all steps linked to the top of the transition (before) are active.
  • When a transition is crossed:
    • All steps linked to the top of the transition (before) are deactivated.
    • All steps linked to the bottom of the transition (after) are activated.

Divergence

  • All conditions are considered as exclusive, according to a left-to-right priority order.
    • It means a transition is considered as FALSE if at least one of the transitions connected to the same divergence on its left side is TRUE.

Order of Action Block Execution

For a given cycle, if a transition is:

  • FALSE, the N-action blocks are evaluated for all active steps waiting on that transition.
  • TRUE, the P0 action blocks are evaluated for all active steps waiting on that transition, followed by the P1 and N steps for all steps waiting on that transition.
    • The steps that were waiting on that transition are then marked as active.

Example

Using this SFC:

The order of action block execution for a given cycle is:

Incoming State

Evaluating Transition

If Transition is FALSE

If Transition is TRUE

First Cycle

N/A

[1] P1

[1] N

Transition 1.

Not yet TRUE.

Transition 1

[1] N

[1] P0

[101] P1

[101] N

[201] P1

[201] N

Passed transition 1.

Transition 101 not yet TRUE.

Transition 101

[101] N

[201] N

[101] P0

[201] N

[102] P1

[102] N

Passed transitions 1 and 101

Transition 2 not yet TRUE.

Transition 2

[201] N

[102] N

[201] P0

[102] P0

[3] P1

[3] N

  • Execution of SFC in the IEC 61131-3 target is sampled according to the target cycles.
    When a transition is crossed within a cycle, these steps are activated.
    The evaluation of the chart continues in the next cycle.
    If several consecutive transitions are TRUE within a branch, only one of them is crossed within one target cycle.


  • Some runtime systems may not support exclusivity of the transitions within an divergence.
    See the OEM instructions for more information about SFC support.