Recovering SFC Types Blocked or Inconsistent After AS Controller Warm Restart

In SIMATIC PCS 7 V9.0, when an SFC type or instance remains blocked or displays an inconsistent state (indicated by a green arrow symbol and held step) following an AS controller warm restart, the behavior is determined by the SFC Operating State Logic (OSL) and the 'Retain SFC state on PLC restart' configuration. If this retention option is active and the CPU stopped during an uncompleted block calculation or step transition, the SFC enters an 'Inconsistent' state upon restart to prevent unintended equipment motion. To recover, switch the SFC instance from Automatic to Manual (MAN) mode in the SFC faceplate, execute a manual Step Advance or Reset command to return the sequencer to IDLE, and verify that safety interlocks and Mode signals (AutModLi / ModLiOp) are healthy before returning to Automatic.

Executive Summary

Following planned maintenance or transient power outages, automation controllers execute a warm restart (transitioning from STOP to RUN). In plants utilizing PCS 7 V9.0 Sequential Function Charts (SFC types and instances), operating teams frequently find that critical sequence phases refuse to advance, remaining frozen at the active step with a green indicator arrow or displaying an 'Inconsistent state' warning on WinCC faceplates. Standard automatic sequence triggers fail to restart the sequencer. This technical article analyzes SFC Operating State Logic during restart transitions, explains why retentive sequencers lock into inconsistent states, and provides the step-by-step operator and engineering recovery procedures.

Architecture and Fundamental Mechanics

The SFC Operating State Logic (OSL) in SIMATIC PCS 7 governs sequencer behavior across controller restarts:
1. Warm Restart Evaluation Modes:
- When an AS controller enters RUN, OB100 (Warm Restart) executes initialization routines for all runtime blocks.
- In SFC properties, the engineer configures the restart behavior under 'Operating parameters':
a. Reset SFC on PLC Restart: The sequencer aborts active steps, resets all internal variables, and returns directly to the Ready / Idle state.
b. Retain SFC State on PLC Restart: The sequencer preserves its active step, step elapsed times, and variable states, attempting to resume operation.
2. Inconsistent vs Consistent Restart States:
- Consistent State: The CPU was stopped when all active step actions had completed execution and no intermediate state transitions were pending. Upon restart, the sequencer resumes automatically.
- Inconsistent State (Green Arrow Indicator): The CPU stopped mid-execution—for example, during a mathematical calculation, an unacknowledged interlock trip, or an incomplete transition evaluation. The SFC cannot guarantee process safety and flags itself as 'Inconsistent'.
3. Safety Locking in Inconsistent State:
- To prevent uncommanded valve openings or motor starts, the SFC firmware locks automatic sequencer transitions.
- The SFC remains frozen until an authorized operator intervenes manually.

Step-by-Step Implementation Procedure

Identify the Blocked SFC Instance in WinCC:
a. On the OS Client screen, locate the affected SFC block icon displaying the green arrow or warning banner.
b. Click the block icon to open the SFC Faceplate.
c. Inspect the operating state field: observe whether the status indicates 'Held', 'Inconsistent', or 'Stopped'.
Switch SFC Instance from Automatic to Manual Mode:
a. In the SFC faceplate, locate the mode selector (Auto / Manual).
b. If ModLiOp = 1 (linked to external logic), verify whether the interlock logic allows manual control.
c. Click the Manual (MAN) button to take manual command of the sequencer.
Acknowledge and Clear Inconsistent State:
a. In the SFC faceplate command bar, inspect active command buttons.
b. If the sequencer allows resuming, click Resume (RESUME).
c. If the step remains locked due to an unfulfilled transition, click Reset (RESET) to force the sequencer back to the IDLE / READY state.
d. If step-by-step progression is required, enable 'Step Control Mode' to manually pulse past the blocked transition.
Verify Field Equipment Interlocks and Signal Health:
a. Open the CFC chart containing the SFC instance.
b. Inspect technological interlock inputs (Intlock, LockAct, EnStart).
c. Confirm that all field feedback signals (limit switches, pressure switches) are healthy (Signal Quality = 16#80).
Return Sequencer to Automatic Execution:
a. In the SFC faceplate, click Auto (AUTO) to return control to the supervisory batch manager or higher-level sequence.
b. Verify that the sequencer starts its normal progression without generating inconsistent state warnings.
Adjust Engineering Restart Parameters (If Automated Reset is Preferred):
a. In SIMATIC Manager, open the SFC Type in the SFC Editor.
b. Select Type -> Properties -> Operating Parameters.
c. If plant safety permits returning to Idle upon power restoration, uncheck 'Retain SFC state on PLC restart'.
d. Save, compile, and download the updated SFC type.

Verification

In the SFC faceplate in WinCC: confirm that the operating state displays 'Idle' or 'Running' without error icons.
Perform a test restart in SIMIT or offline simulation: verify the SFC transitions smoothly according to the configured restart mode.
Verify in the WinCC Alarm Logging banner that no 'SFC Inconsistent State' alarms remain active.

Limitations and Operational Precautions

Switching an SFC to Manual and forcing a Step Advance or Reset directly affects field machinery. Always verify physical equipment positions and valve alignments before manually advancing a blocked sequencer.

Engineer FAQ

Why does an SFC show an inconsistent state after a PLC warm restart?

The CPU stopped mid-execution during step processing or transition evaluation, causing the SFC to lock automatically to prevent unsafe equipment motion.