Resolving Conversion Error During SFC Type Changes Download in PCS 7

A 'Conversion error' during an AS delta download after modifying an SFC Type interface (such as adding or deleting setpoints) occurs because running SFC instances cannot have their instance DB structures reconfigured online while active. When an SFC Type interface changes, the S7-400 CPU must perform an online DB restructuring. For this download in RUN to succeed, all instances of the modified SFC Type must be in the 'Ready', 'Completed', or 'Aborted' state and disabled (DIS_I = 1 or execution deactivated). Deactivate all active instances, execute 'Update Block Types' in the SFC editor, compile changes, and download.

Mechanism of SFC Type Interface Modification and DB Restructuring

In SIMATIC PCS 7, an SFC Type functions as an object-oriented template that defines state machines, step logic, transitions, and setpoint characteristics. When an engineer modifies the interface of an SFC Type—such as adding a setpoint parameter, removing a control value, or altering characteristic data types—the engineering system regenerates the underlying Function Block (FB) interface and modifies the internal memory layout of every associated instance Data Block (DB).

When downloading changes to the controller in RUN:

The CFC download manager prepares new instance DB definitions in the CPU's load memory.
The CPU operating system attempts to convert existing instance DBs in work memory to the new layout while preserving live process parameters.
If any SFC instance is actively running, holding execution tokens in active steps, or awaiting transition clearance, the operating system cannot safely remap internal execution pointers without causing undefined CPU states.
The online DB converter aborts the process and reports Conversion error, halting the delta download.

Preconditions for Downloading SFC Type Changes in RUN

To successfully download changes to modified SFC Types without a CPU STOP, four strict preconditions must be met:

Instance Inactivity: Every SFC instance derived from the modified SFC Type must be in a non-executing state (Ready, Completed, Aborted, or Stopped). Active states (Run, Holding, Pausing) prohibit conversion.
Instance Deactivation: Execution must be explicitly disabled at the block level by setting DIS_I = 1 or disabling the chart runtime group in CFC.
SFC Type Synchronization: The modified SFC Type must be updated across all chart folders using Options -> Update Block Types... before compiling changes.
Adequate CPU Work Memory Reserves: The CPU requires sufficient contiguous work memory to hold old and new instance DBs concurrently during the conversion transaction.

Step-by-Step Procedure to Resolve the Conversion Error

Follow this sequence to resolve the conversion error and complete the delta download:

Identify Affected SFC Instances: In SIMATIC Manager, open the Master Data Library or chart folder, select the modified SFC Type, and use the Chart Reference Data editor (Options -> Chart Reference Data -> SFC Type Instances) to list all instances.
Bring Active Instances to Idle or Complete:
- On the WinCC OS faceplate or SIMATIC BATCH control center, allow active recipes or sequences to complete, or abort them cleanly.
- Verify that instance status displays Ready (Bereit) or Completed (Beendet).
Disable Execution (DIS_I):
- Set the disable input DIS_I = 1 on the SFC instance block in CFC, or disable the SFC runtime group in the CFC run sequence.
Update Block Types in SFC Editor:
- Open the SFC Editor.
- Select Options -> Update Block Types....
- Ensure all charts using the modified SFC Type are selected and updated.
Compile CFC Changes:
- In CFC, select Chart -> Compile -> Charts as Program....
- Select Scope: Changes only and compile.
Execute Delta Download in RUN:
- Select PLC -> Download -> Scope: Changes only.
- Confirm the prompt acknowledging that instances are disabled.
- The CPU converts the instance DBs smoothly without error.
Re-enable Instances: Reset DIS_I = 0 and resume sequence operation.

SFC Instance Operating States and Download Permissibility

The following table details which SFC operating states permit online DB conversion during delta download:

SFC Operating StateInternal State CodeOnline DB Conversion Allowed?Action Required Before Download ChangesReady (Bereit)State 1Yes (if disabled)Set DIS_I = 1 or disable runtime groupRunning (Läuft)State 3No (Conversion Error)Allow phase to finish, stop, or abort sequenceHolding (Anhalten)State 5No (Conversion Error)Resume and complete, or abort sequenceHeld (Angehalten)State 6No (Conversion Error)Abort or complete sequenceCompleted (Beendet)State 11Yes (if disabled)Set DIS_I = 1 or reset to ReadyAborted (Abgebrochen)State 13Yes (if disabled)Set DIS_I = 1 or reset to ReadyStopped (Gestoppt)State 15Yes (if disabled)Set DIS_I = 1

[!IMPORTANT]
Operational Precaution: Never attempt to force a full AS download in STOP mode on a running production plant to bypass a > conversion error. Doing so halts all control loops, de-energizes digital outputs, and trips active process equipment.

Verification

Confirm that the SFC Type interface modification was successfully applied:

Successful Delta Download Log: Confirm that the CFC download log concludes with: 'Download changes executed successfully in RUN mode' without conversion or memory warnings.
Online SFC Instance Testing: Open the modified SFC instance chart and switch to Test -> Test Mode. Verify that newly added setpoint values, actual values, and transition logic evaluate accurately.
OS Faceplate Synchronization: Open the SFC faceplate on WinCC Runtime. Verify that new setpoints appear in the operator view and can be set within configured high/low limits.

Limitations and Operational Precautions

Adhere to the following guidelines during SFC engineering:

Batch Recipe Interruption: If SIMATIC BATCH is executing a control recipe that references the modified SFC Type, modifying the interface online will invalidate the recipe instance. Always complete active batches before modifying SFC Types.
Avoid Deleting Active Parameters: When modifying interfaces on operational systems, prefer adding new setpoints over deleting or reordering existing parameters to maintain backward compatibility.
CPU 410-5H vs Legacy S7-400: Modern CPU 410-5H controllers handle dynamic DB conversions with enhanced memory buffering; however, the requirement that instances be disabled remains mandatory across all S7-400 architectures.

Engineer FAQ

Why does PCS 7 require SFC instances to be disabled for a download in RUN?

Modifying the SFC Type changes the memory layout of its instance DB. If the CPU is actively executing steps inside that DB, converting memory on-the-fly would cause execution pointers to jump to invalid addresses, risking CPU crash.

How do I disable an SFC instance in CFC without modifying the chart?

You can disable the runtime group containing the SFC instance via the CFC Run Sequence editor (Options -> Edit Run Sequence), or write 1 to the DIS_I input via an online variable watch table.

Can I download SFC Type changes if only one instance out of ten is running?

No. If even a single instance of that SFC Type is actively running in the controller, the DB converter fails for that block family and aborts the entire download.

Does modifying SFC step actions (without changing inputs/outputs) require disabling instances?

No. If you only modify internal step logic or transition conditions without altering the SFC Type interface (inputs, outputs, setpoints, characteristics), the DB structure remains unchanged and delta download is permitted while running.