Recovering Stuck SFC Instances and Clearing Aborting States After CFC Chart Deletion in PCS 7

When an SFC chart or SFC Type instance is deleted from a CFC chart while currently active in an 'Aborting', 'Holding', or faulted state, the S7-400 CPU retains the instance data block and running state machine in its active work memory. Because the technological chart no longer exists offline, the operator faceplate is severed, preventing manual abort or reset commands. To clear the stuck state and return the controller to IDLE without an uncommanded CPU stop: 1. Locate the orphaned instance DB number in the S7 Program blocks folder; 2. Create a temporary Variable Table (VAT) or test CFC chart to pulse the SFC command input (forcing a manual Reset); or 3. Temporarily recreate a dummy SFC instance with the identical DB number, transition it to IDLE in Manual mode, and then cleanly delete it.

Executive Summary

In continuous batch processing and automated sequence engineering with SIMATIC PCS 7, Sequential Function Charts (SFCs) govern multi-step unit operations. An operational crisis occurs when an engineer deletes an SFC chart or an SFC Type block from a CFC sheet while the sequence is in the middle of executing an abort sequence (Aborting) or holding state (Held). While the CFC chart disappears from the offline engineering system, the compiled instance data block and active state machine remain alive in the CPU's online RAM. The operator faceplate on WinCC loses communication, reporting a disconnected tag, and the sequence becomes orphaned, permanently holding equipment interlocks or blocking unit re-allocation. This article provides a comprehensive engineering recovery methodology to reset orphaned SFC instances back to IDLE.

Architecture and Fundamental Mechanics

SFC Runtime State Machine, Instance DB Persistence, and Orphaned Execution Mechanics:

SFC State Machine Architecture (ISA-88 / NAMUR NE 33):
An SFC is governed by an internal finite state machine: Idle -> Starting -> Running -> Completing -> Completed -> Aborting -> Aborted.
When an SFC enters Aborting, it executes dedicated abort sequence steps. If an abort step is waiting on a field condition (e.g. valve closed confirmation) that never arrives, the state machine stalls indefinitely in Aborting.
Under standard operation, an operator switches to Manual mode (T or O - Transition or Operator), clicks the transition confirmation button (purple 'O'), and forces the sequence into Aborted, followed by pressing Reset to reach Idle.
The Orphaned Deletion Problem:
If an engineer deletes the CFC chart offline before the state machine reaches Idle, the offline link is broken.
If an online delta download is attempted, the CPU may reject the download because the active block cannot be cleanly deleted while its state machine is non-idle.
The CPU work memory continues executing the instance DB in cyclic organization blocks, continuously asserting actuator interlocks and output control words (BA_STATE, QIDLE = 0).
Internal Command Interface and Control Words:
The SFC runtime engine evaluates an internal control word (COMMAND / MAN_EXEC) mapped into its instance DB.
Specific command bits force state transitions regardless of process interlocks:
- ABORT_OP / Command 16#0003: Forces immediate transition to Aborted.
- RESET_OP / Command 16#0004: Forces transition from Aborted / Completed back to Idle.
Writing directly to these command offsets in the instance DB via a Variable Table (VAT) allows engineers to force the orphaned state machine back to Idle.

Step-by-Step Implementation Procedure

Identify the Orphaned Instance DB Number:
* In SIMATIC Manager, open the S7 Program -> Blocks folder.
* Switch view to Details (View -> Details).
* Look for Data Blocks whose author is SFC or whose comment references the deleted SFC name.
* Alternatively, check the CPU Diagnostic Buffer or Symbol Table to identify the DB number (e.g. DB142).
Force Reset via STEP 7 Variable Table (VAT):
* In SIMATIC Manager, right-click the Blocks folder -> Insert New Object -> Variable Table (name it VAT_SFC_Reset).
* Open VAT_SFC_Reset.
* In the address column, enter the SFC command input address in the instance DB (e.g. DB142.DBW10 or the specific manual command bit).
* Enter the status word address (e.g. DB142.DBW4 - BA_STATE).
* Click PLC -> Monitor to observe the live state.
* In the Modify Value column, write the Reset command value (or force the manual mode bit MAN_ACT = 1 and trigger RESET = 1).
* Execute PLC -> Modify Variable; verify that BA_STATE transitions immediately to 1 (Idle).
Alternative Recovery Method: Recreate Dummy SFC Instance:
* In SIMATIC Manager Plant View, create a new temporary CFC chart.
* Insert the original SFC Type (or an SFC Chart).
* In the block properties, manually assign the instance DB number to match the orphaned DB (e.g. DB142).
* Compile changes and open the SFC in online Test Mode (Ctrl+F).
* Switch mode to Manual (MANUAL).
* Under step control, select T or O (Transition or Operator confirmation).
* Manually confirm each stuck step until the sequence reaches Aborted or Completed.
* Click the Reset button on the SFC toolbar; confirm the state transitions cleanly to IDLE.
Cleanly Delete the Resolved SFC Instance:
* Once in IDLE, close the SFC online window.
* Delete the dummy SFC instance from the CFC chart.
* Execute Chart -> Compile -> Changes only and download changes to the CPU in RUN mode.
* Verify that the instance DB is now cleanly purged from CPU memory.

Verification

In SIMATIC Manager, open VAT_SFC_Reset and monitor the target DB;
Confirm that BA_STATE indicates Idle (16#0001) or that the DB has been deleted from online memory.
Verify that equipment previously locked by the SFC is released and can be controlled in manual mode.
Check the CPU Diagnostic Buffer; confirm zero block calling errors or cycle timing anomalies.

Limitations and Operational Precautions

Directly forcing memory offsets in active instance DBs requires extreme care; writing to an incorrect DB address can overwrite critical system pointers, causing an immediate CPU crash. Recreating the dummy SFC instance is the safest, officially supported method to gracefully walk a state machine back to IDLE.

Engineer FAQ

Why can't I delete an SFC chart while it is running or aborting?

Because the CPU operating system prevents deletion of active blocks that are executing within cyclical interrupt OBs.

What happens if an SFC is deleted offline while active online?

The CPU work memory retains the running instance DB, orphaning the state machine and leaving actuator locks permanently active.

What is the 'T or O' mode in an SFC?

'Transition or Operator' mode allows the operator to manually confirm and force transition steps without satisfying physical process conditions.

How do I force an SFC state machine to IDLE?

Transition the state machine to Aborted or Completed, then pulse the Reset command input to reach IDLE.

Can I clear orphaned DBs by stopping the CPU?

Stopping the CPU and performing a warm restart does NOT clear retentive DBs; only a memory reset (MRES) or full download clears them.

Technical References

SFC

Sequential Function Chart

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Recovering Stuck SFC Instances and Clearing Aborting States After CFC Chart Deletion in PCS 7