Resolving SFC Runtime Errors and Manual Operator Demands After CPU Restart in SIMATIC PCS 7 V8.2

In SIMATIC PCS 7 V8.2, when Sequential Function Charts (SFC) halt with a runtime error and issue an operator intervention demand (Bedienanforderung) following a CPU restart, the issue is caused by the SFC startup configuration conflicting with unfulfilled step conditions. In the SFC Properties dialog under 'Operating parameters for CPU restart', selecting 'Retain SFC state' causes the sequencer to freeze in place upon CPU restart to prevent uncontrolled plant movement, requiring conscious operator confirmation to resume. To restore automatic sequencing, configure 'Autostart' or re-initialize the sequencer, verify that step execution timers (MAX_TIME) have not expired, and ensure step transitions are fulfilled.

SFC State Machine Architecture and CPU Restart Behavior

Sequential Function Charts (SFC) in SIMATIC PCS 7 govern complex batch recipes, phase sequences, and plant state logic. The execution engine operates as a standardized finite state machine conforming to NAMUR NE33 and ISA-88 recommendations. Valid states include Idle, Starting, Run, Completing, Completed, Holding, Held, Resuming, Error, and Aborting.

When an S7-400 CPU transitions from STOP to RUN (Warm Restart via OB100), the SFC runtime coordinator (SFC_RINT / SFC_CTRL) evaluates the restart characteristics configured in the chart properties to determine how active sequencers should resume.

Operating Parameters for CPU Restart: Re-initialize vs Retain State vs Abort

In the SFC Editor, under Chart -> Properties -> Operating Parameters for CPU Restart, three fundamental modes are selectable:

Re-initialize SFC (SFC neu initialisieren): The sequencer is reset to its initial step (Initial Step) in the Idle state. Any in-progress sequence is abandoned.
Retain SFC State (SFC-Zustand beibehalten): The sequencer preserves its pre-stop state, active step, and elapsed timers. When the CPU restarts, the sequencer re-enters the exact step that was active prior to the stop.
Abort SFC (SFC abbrechen): The chart enters the Aborting state and executes the abort sequence before transitioning to Aborted.

Why 'Retain SFC State' Triggers Operator Demands (Bedienanforderung)

When engineers configure 'Retain SFC state', they often expect the sequence to resume execution autonomously without human touch. However, Siemens deliberately designs the runtime engine with safety interlocks:

Uncontrolled Motion Hazard: If a CPU stopped due to power loss or maintenance, automatic machinery restarting without warning presents a severe personnel and equipment hazard.
The Operator Demand Lock: Consequently, the runtime engine transitions the chart into a held or waiting state and raises an Operator Demand (Bedienanforderung) on the WinCC SFC Visualization faceplate. The sequencer will not advance until an authorized operator explicitly clicks Resume (Fortsetzen) or Start.

Step Timeouts (MAX_TIME) and Process Interlock Re-evaluation

A secondary driver of SFC runtime errors on startup relates to timing:

Every SFC step has configurable parameters: MIN_TIME (minimum run time) and MAX_TIME (maximum step duration).
If a CPU is stopped for several minutes while a step is active, the system clock advances.
Upon restart, if the accumulated real-time step duration exceeds MAX_TIME, the step flags a Step Error, transitioning the sequencer into the Error state and raising an alarm.

Comparative Matrix: SFC Restart Modes and Operational Consequences

The following table compares behavioral outcomes across SFC restart modes:

Restart Parameter SettingPost-Restart SFC StateOperator Demand Raised?Active Step ExecutionRecommended ApplicationRetain SFC StateWaiting / Held in active stepYes (Operator must confirm Resume)Step active; waiting for operator clickHazardous batch processes, chemical reactorsRe-initialize SFCIdle (Initial step)NoReset to start stepContinuous conveyor lines, utility sequencesAbort SFCAborting -> AbortedNo (Requires explicit restart)Executes abort logicHigh-safety units requiring clean flushAutostart EnabledRun (Starts automatically)NoSequence starts on CPU RUNNon-critical auxiliary routines, air handling

Step-by-Step Procedure to Configure Automated SFC Startup

To configure predictable startup behavior according to plant requirements:

Open the SFC chart in the SFC Editor.
Click Chart -> Properties.
In the Operating Parameters for CPU Restart section:
- If the sequence must reset safely, select Re-initialize SFC.
- If the sequence must continue from where it stopped, select Retain SFC state.
In the Start Options tab, if autonomous resumption without operator click is strictly required by the technological process, check Autostart.
Audit Step Timers: Open step properties for long-running steps. Verify MAX_TIME is set to 0s (disabled) or sufficiently large to accommodate planned CPU restarts.
Central Resume Interconnection (Optional): If multiple SFCs must resume simultaneously, interconnect an external control pulse to the chart input RESUME (ENRESUM = 1) to clear operator demands programmatically.
Save, compile, and download the SFC charts to the CPU.

Verification

Validate restart behavior using S7-PLCSIM or physical CPU:

Start Sequence: Start the SFC in automatic mode and allow it to reach Step 3.
Simulate CPU Stop: Switch the CPU to STOP for 15 seconds, then return to RUN.
Faceplate Observation: Open WinCC SFC Visualization. Confirm whether the SFC displays Operator Demand or resumes execution.
Error Log Audit: Confirm zero runtime errors or step timeout alarms in the WinCC Alarm Logging journal.

Limitations and Operational Precautions

Observe the following safeguards:

Safety Compliance: Never configure 'Autostart' or automated 'Resume' on physical equipment where unexpected motion violates machinery safety directives (ISO 13849 / IEC 62061).
Field Device Alignment: Always verify that field valves and motors remained in their expected physical states during the CPU outage before resuming complex chemical batches.
Commercial Accuracy Disclaimer: Software order numbers (MLFBs), package structures, and hardware capacity prerequisites vary across PCS 7 versions and hardware platforms. System integrators must confirm exact software entitlements and licensing requirements against the official Siemens Catalog CA 01 and obtain an authorized Siemens distributor quotation prior to purchasing.

Engineer FAQ

Can I resume all SFCs in a plant simultaneously with a single button?

Yes. By interconnecting the RESUME input pin of each SFC to a central command block (such as OpTrig or SFC coordinator), a single operator click can resume all sequencers.

What is the difference between warm restart and cold restart for SFCs?

A warm restart (OB100) evaluates the configured restart options; a cold restart (OB102) always re-initializes all SFCs to their initial step regardless of configuration.