Diagnosing Frozen Online Data Refresh and State Inconsistencies in PCS 7 SFC Sequences

When an online Sequential Function Chart (SFC) in SIMATIC PCS 7 shows status 'RUN' but fails to dynamically refresh step states or displays completed blocks, verify four core execution parameters: (1) Ensure Test Mode (Testbetrieb) is actively enabled in the SFC Editor via Debug -> Test Mode, and verify the refresh cycle (e.g., 500 ms) is not paused; (2) In the CFC Run Sequence (Ctrl+F11), confirm that the SFC runtime group is set to 'Active' in its scheduled cyclic interrupt OB and has not been disabled via EN = 0; (3) Verify the operating mode—if Step Mode or Manual Mode is active, transitions will not advance automatically; and (4) In CPU Properties -> Cycle/Clock Memory, verify that 'Scan cycle load from communication' is allocated sufficient bandwidth (minimum 20%) to prevent PG communication timeouts.

Executive Summary

Sequential Function Charts (SFC) in SIMATIC PCS 7 provide high-level, recipe-driven phase control for batch reactors, clean-in-place (CIP) circuits, and sequential machinery. During commissioning, automation engineers frequently encounter a perplexing state where an SFC is opened online: the top status banner indicates 'RUN', but step indicators, transitions, and active action timers appear completely frozen or show initial 'completed' states from a prior run. No live color transitions occur, leaving engineers unable to determine whether the controller logic is genuinely executing or deadlocked. This issue stems from disabled SFC Test Mode, deactivated runtime groups, single-step operational holds, or S7-400 CPU communication throttling.

Architecture and Fundamental Mechanics

Online monitoring of SFC state machines in SIMATIC PCS 7 relies on a dedicated background diagnostic protocol between the ES and AS:

The Test Mode Architecture (Debug -> Test Mode): Opening an SFC chart online does not automatically initiate real-time polling of every internal variable. To protect CPU cycle times, PCS 7 requires explicit activation of Test Mode. When Test Mode is active, the programming station establishes a cyclic diagnostic job with the CPU, requesting the status of active step flags, transition evaluations, and run timers at the configured sampling rate (typically 500 ms to 1000 ms).
Runtime Group Enablement and EN Pins: When an SFC chart compiles, CFC creates an SFC runtime group within a cyclic interrupt OB (typically OB35 or OB32). If an upstream interlock, startup routine, or maintenance technician set the SFC block's EN pin to 0, or unchecked the 'Active' property of the runtime group in the Run Sequence editor, the AS operating system skips the SFC scan entirely. The SFC instance data block retains its last written status flags (such as 'RUN' and 'Completed' steps), creating a frozen image.
Operating Mode Conflicts (Auto vs. Manual vs. Step): An SFC sequence can execute in Automatic, Manual, or Step-by-Step mode. If an operator placed the SFC into Step Mode via the WinCC faceplate or SFC visualization, the sequencer halts at the current active step, waiting for an explicit manual 'Advance' command before evaluating subsequent transition criteria.
CPU Communication Load Constraints: In S7-400 controllers, the parameter 'Scan cycle load from communication' in CPU Hardware Properties sets the maximum percentage of scan time the CPU may spend servicing PG/PC diagnostic requests and WinCC polling. If this is set too low (e.g., 10%) on a heavily loaded plant bus, the CPU drops diagnostic telegrams to the SFC Editor, causing the online view to freeze.

Step-by-Step Implementation Procedure

Execute the following structured sequence to restore dynamic online SFC monitoring:

Activate and Configure SFC Test Mode:
- In the SFC Editor, select Debug -> Test Mode (or click the Test Mode glasses icon).
- If already active, toggle Test Mode off and back on.
- Select Debug -> Settings -> verify that the 'Cycle time' is set to 500 ms or 1000 ms.
- Verify that monitoring is not paused (Debug -> Resume).
Verify Operating Mode and Hold States:
- In the SFC menu bar or standard view header, inspect the operating mode.
- If set to Manual or Step, switch the mode to Automatic.
- Check the execution state: if the SFC is in Holding or Held, inspect transition conditions or issue a Resume command.
Inspect Runtime Group and EN Pin in CFC:
- Open the CFC chart containing the SFC instance block.
- Check the EN input pin: confirm it evaluates to 1 (True).
- Press Ctrl+F11 to open the CFC Run Sequence.
- Locate the SFC runtime group in OB35 (or applicable OB).
- Right-click the runtime group, select Object Properties, and verify that the Active checkbox is checked.
Adjust CPU Communication Bandwidth:
- In SIMATIC Manager, open HW Config for the S7-400 station.
- Double-click the CPU to open Properties.
- Navigate to the Cycle/Clock Memory tab.
- Locate Scan cycle load from communication (%). If configured at 10% or 15%, increase it to 20% or 25%.
- Save, compile, and download HW Config to the controller.

Verification

Verify dynamic sequence monitoring and state execution:

Active Step Transition Monitoring: Observe the SFC online. Confirm that active steps illuminate in blue/green and that elapsed action timers increment dynamically in real time.
Transition Satisfaction Check: Double-click a running transition. Confirm that boolean criteria dynamically evaluate to green (satisfied) or red (unsatisfied) in the transition detail window.
OS Synchronization: Open the SFC faceplate on the WinCC OS. Confirm that the phase state displayed in WinCC matches the active step in the SFC Editor.

Limitations and Operational Precautions

Avoid setting 'Scan cycle load from communication' higher than 30% on heavily loaded controllers, as excessive communication allocation can lengthen the OB1 cyclic scan time and delay time-critical process interlocks. Additionally, remember that online SFC monitoring places an active communication demand on the CPU; close online SFC windows when commissioning tests are complete.

Engineer FAQ

Why does my SFC say RUN but no steps are advancing?

The SFC is either held at a transition whose conditions are unsatisfied, the sequencer was placed into Step-by-Step mode, or the SFC runtime group was deactivated in the CFC Run Sequence.

What is the difference between Test Mode and Process Mode in SFC?

Test Mode polls all step and transition variables cyclically for detailed engineering diagnostics. Process Mode reduces communication overhead by updating only major state changes.

Can an SFC be edited while online in Test Mode?

No. To modify steps or transitions, you must exit Test Mode, make the edits offline, compile changes, and download to the AS.

Why does the SFC show steps from a previous batch as completed?

If the SFC finished its sequence and was not reset (READY_TO_START or idle reset), the instance data block retains the completed flags until a new start pulse is commanded.