Configuring SFC Resuming Strategies and Step Recovery Without Full Sequence Restart in PCS 7

In SIMATIC PCS 7 Sequential Function Charts (SFC Types and SFC Charts), resetting or recovering from an error on the current step without restarting the entire sequence from Step 1 is achieved by configuring the 'Resuming' operating state logic. In the SFC Properties under Operating State Logic (OSM), configure the transition from 'Held' to 'Resuming' to evaluate a recovery branch that loops back specifically to the interrupted step, or configure the TargetStep parameter to resume execution at the current active step.

Architecture of SFC Operating State Logic (OSM) Resuming Behavior

In batch processing and sequential automation running SIMATIC PCS 7, Sequential Function Charts (SFC) govern complex operational phases (e.g., heating, dosing, reacting, discharging). During active execution, field anomalies (such as a transient valve interlock trip, air supply pressure drop, or operator pause) can cause the sequence to halt.

Under standard ISA-88 state machine logic:
* An abnormal process event forces the SFC from state Running into state Holding, which completes into state Held.
* In a poorly engineered SFC, the only mechanism provided to operators is the Abort or Stop command, which forces the sequencer through Stopping / Aborting and resets the sequence back to the Initial Step (Step 1: Idle).

Restarting from Step 1 can result in massive operational losses: partially filled vessels must be scrapped, and pre-heated chemical batches are wasted.

Process engineers require a targeted recovery mechanism: resolving the field fault and resuming execution strictly at the current active step where the error occurred, without restarting preceding steps.

SFC State Transition and Resuming Strategy Matrix

Evaluate how the SFC Operating State Logic (OSM) handles resuming from the Held state:

Resuming Strategy ConfigurationResuming Transition DestinationBehavior upon Operator 'Resume' CommandTechnological SuitabilityResume at Current Step (Default)Transitions directly back to interrupted stepRe-enters active step; re-executes step initialization or continues timerTemporary process hold; resolved sensor tripResume at Defined Recovery StepAlternative branch to dedicated recovery stepExecutes purging or re-heating sequence, then returns to interrupted stepChemical phases requiring parameter re-stabilizationResume via TargetStep ParameterDynamic pointer to specific step numberOperator selects specific step index from faceplate dropdownManual supervisory intervention and batch reworkRestart from Initial Step (Prohibited for Holds)Jumps back to Step 1 (Initial Step)Wipes all phase progress; restarts batch sequenceCatastrophic batch aborts only

Safety rule: Ensure process safety interlocks permit re-entering active steps without hazardous pressure surges.

Configuring Resume at Current Step in SFC Types and Charts

To ensure the SFC resumes at the interrupted step:

Verify Operating State Logic Properties:
* Open the SFC Type or SFC Chart in SFC Editor.
* Select Edit > Properties.
* Under the Operating State Logic (OSM) tab, review the state diagram.
* Ensure that state Held connects via transition to state Resuming, and Resuming transitions back into the Running sequencer.
Configure Step Return Attribute in SFC Compendium Guidelines:
* In the SFC Editor, navigate to Options > Customize > Compile / Check.
* Verify that Return to interrupted step when resuming is selected.
* When this option is active, the runtime engine (FB300 or SFC Type instance) stores the active step pointer in an internal state buffer upon entering Holding.
* When the operator commands Resume, the sequencer bypasses prior steps and re-activates the stored step pointer.
Handle Step Timers upon Resuming:
* Double-click the target step to inspect Step Properties > Times.
* Decide whether the step run time counter (T_ACT) should continue from where it stopped or reset:
- Check Retain elapsed step time on hold if cumulative heating/mixing time must be preserved.
- Uncheck if the full step time must re-execute from 0 upon recovery.

Implementing Error Recovery Branches in SFC Phase Logic

If the process requires explicit fault clearing actions before re-entering the step:
1. Design an Alternative Resuming Branch:
* In the SFC structure, create an alternative transition branch below the active step:
* Transition A: Standard process condition met (advances to Next Step).
* Transition B: Fault detected (e.g., Motor_Trip = TRUE).
* When Transition B fires, route the sequence into an internal Step_Recover step.
2. Execute Local Reset Actions:
* In the Initialization tab of Step_Recover, pulse Fault_Reset on the motor block.
* Place an unconditional transition following Step_Recover that loops control directly back into the original step.
* This provides automatic in-step fault recovery without ever transitioning the global SFC into the Held state.

Verification

Verify current step resuming in WinCC Runtime:
1. Start the SFC sequence from the OS faceplate: Allow it to advance to Step 3 (Heating).
2. Trigger an interlock fault to trip the sequence: Confirm the SFC transitions to state Held and remains parked at Step 3.
3. Clear the physical fault in CFC.
4. Click the Resume (Fortsetzen) button on the SFC faceplate:
5. Confirm that the SFC enters Resuming and immediately reactivates Step 3 without jumping back to Step 1 or Step 2.
6. Verify that heating output commands resume cleanly.

Limitations and Operational Precautions

If an SFC step performs cumulative material dosing, resuming the step without checking scale tare values can cause double-dosing; always latch dosed weights prior to resuming.
Resuming at the current step requires operator Authorization Level 5 (Process controlling) or Level 6 (Higher process controlling).
In SIMATIC BATCH recipes, resuming must be commanded through the BATCH Control Center (BCC) to maintain electronic batch record (EBR) audit trails.

Engineer FAQ

Can an operator select a different step to resume from the WinCC faceplate?

Yes. If the SFC Type provides the 'TargetStep' parameter and the operator has supervisory authorization, they can select a specific step number from the faceplate dropdown before clicking Resume.

What happens to CFC continuous control blocks when an SFC enters Held?

In the Holding state, SFC actions typically set valves to safe hold positions or switch controllers to Manual mode. The Resuming state actions must restore controllers to Automatic mode.

What is the difference between Held and Aborted states?

Held is a temporary, recoverable operational pause. Aborted is a permanent termination that discharges or vents the unit and resets the sequence to Idle.