Implementing Step Sequences in SIMATIC PCS 7: SFC Licensing vs Pure CFC State Machine Logic

In SIMATIC PCS 7 V8.1, programming step-by-step sequential logic (Step 1 -> Step 2 -> Step 3) does not require purchasing an additional runtime license if you utilize the Sequential Function Chart (SFC) Editor in the Automation Station. The basic PCS 7 AS Engineering license includes full SFC creation capabilities. If your Operator Station (OS) lacks the optional 'SFC Visualization' runtime license, simply uncheck 'SFC Visualization' during the OS compilation; the sequence executes natively in the controller while avoiding OS licensing costs. Alternatively, sequential logic can be implemented in pure CFC using an integer step sequencer block programmed in SCL or using basic comparator (CMP_I) and multiplexer (SEL_I) blocks.

Executive Summary

Automation engineers designing batch, startup, or sequential phase procedures in SIMATIC PCS 7 often require structured state machines to transition equipment through distinct operational steps (e.g. Filling -> Heating -> Agitating -> Discharging). While the Sequential Function Chart (SFC) Editor is the premier, vendor-recommended environment for sequential control, engineers occasionally seek to implement sequences in pure Continuous Function Chart (CFC) logic under the mistaken belief that SFC execution requires expensive runtime licenses on both AS and OS. In reality, AS-level SFC execution is included in the base AS Engineering package, and disabling OS SFC Visualization during OS compilation eliminates OS runtime licensing requirements. This technical guide explains the licensing mechanics and details how to construct clean sequential logic in both SFC and CFC.

SFC Architecture in PCS 7: AS Execution vs OS Visualization

The SIMATIC PCS 7 SFC subsystem comprises two decoupled components:

AS Controller Execution: The SFC Editor translates graphical state charts into compiled machine code (OBs, FBs, DBs) executing cyclically in the S7-400 CPU. This capability is permanently included in the standard PCS 7 Engineering license.
WinCC OS SFC Visualization: An optional software package that generates native WinCC faceplates (@PG_SFC.pdl), allowing operators to monitor step advancement graphically on SCADA screens. This feature requires an OS SFC Visualization license.

The SFC Visualization Licensing Boundary

When compiling an OS in SIMATIC Manager:

If the OS compilation wizard checkbox SFC Visualization is checked, WinCC generates the specialized SFC runtime tags and demands an SFC Visualization license in Automation License Manager.
If SFC Visualization is unchecked, the OS compiler generates standard process tags. The SFC chart executes normally in the AS, and standard APL block icons (MonDiL, OpDi01) can display the current step number without requiring an SFC license.

Alternative Step Sequencing via Pure CFC and SCL State Machines

If an engineering specification strictly prohibits SFC, a deterministic state machine can be constructed in CFC or SCL:

Integer Step Counter (Step_No): Uses an integer tag (10, 20, 30...) to track the active step.
Transition Conditions: Evaluates process interlocks and timers using standard AND/OR logic to increment Step_No.
Action Assignment: Compares Step_No using CMP_I blocks to activate field outputs (e.g. Step_No == 20 enables pump).

Step Sequencer Implementation Method Comparison

The following table compares the sequencing methodologies available in SIMATIC PCS 7:

Sequencing MethodEngineering ToolLicensing RequirementAS CPU OverheadOperator Visualization QualityStandard SFC ChartPCS 7 SFC EditorBase AS Engineering (No OS license needed if uncompiled)Optimized S7-400 SFC runtimeStandard text or full SFC faceplate.SFC with OS VisualizationSFC + WinCCRequires 'SFC Visualization' OS licenseOptimized S7-400 SFC runtimeFull interactive graphical flow chart.Pure CFC State MachineCFC (CMP_I / SEL_I)Standard CFC License (No add-on)Low to Moderate (Multiple blocks)Step number displayed on analog/digital icon.Custom SCL Sequencer FBSCL Source BlockStandard STEP 7 SCL CompilerMinimal (Single compact FB)Custom faceplate or integer tag readout.SIMATIC BATCHBatchCC / BATCH ServerDedicated SIMATIC BATCH PO licensesHigher (Unit/EPH blocks)Full ISA-88 recipe execution and EBR.

Step-by-Step State Machine Implementation in CFC

Method 1: Using Native SFC without OS Visualization License

In SIMATIC Manager, insert an SFC Chart directly into your S7 Program.
Program the sequence of steps, transitions, and actions using standard SFC drag-and-drop elements.
In the final step of the chart, set a completion bit or loop back to Step 1.
Compile the CFC and SFC charts (Chart > Compile > Charts as Program).
In SIMATIC Manager, right-click the OS project and choose Compile.
In the OS Compilation Wizard, UNCHECK the checkbox labeled SFC Visualization.
Download to the AS and OS. The sequence operates stably with zero OS license checks.

Method 2: Building an SCL Step Sequencer Block

Open the S7 SCL editor and create a new function block FB_StepSequencer.
Define input conditions, a timer variable, and an integer output CurrentStep.
Use a CASE CurrentStep OF construct to manage step transitions and actions.
Compile the SCL block and place it on a CFC chart, connecting step outputs to valve and motor commands.

Verification

To verify sequential execution without licensing errors:

AS Execution Test: Switch the CPU to RUN mode. Force the start permissive. Verify that the sequence transitions from Step 1 to Step 2 upon completion of transition criteria.
License Manager Check: Open Automation License Manager (ALM) on the OS workstation. Verify that no 'Missing License Key' warnings appear for SFC Visualization.
WinCC Screen Readout: In WinCC Runtime, observe the process value displaying the active step number. Confirm it updates sequentially (e.g. 10 -> 20 -> 30).

Limitations and Operational Precautions

Manual Hold/Abort Complexity: Building manual pause, resume, and emergency abort logic in pure CFC requires extensive interlock programming; native SFC provides standardized operating state logic (OSL) automatically.
Step Overlap Protection: In custom CFC state machines, always ensure transitions are strictly mutually exclusive to prevent two steps from activating simultaneously.
Recipe Management: Pure CFC state machines are unsuitable for multi-recipe plants; plants requiring recipe flexibility should adopt SIMATIC BATCH.

Engineer FAQ

Does using SFC in the AS consume Process Object (PO) licenses?

An SFC Chart itself does not consume AS Process Object licenses; PO licenses are consumed by technological blocks (e.g. MotL, VlvL) that the SFC controls.

Can an operator change step timers in WinCC if SFC Visualization is disabled?

Yes, by connecting the step timer parameters to an OpAnL or MonAnL block in CFC, the operator can modify setpoints via standard faceplates.

What happens if SFC Visualization is enabled during compile but no license exists?

WinCC Runtime displays a periodic license missing popup banner ('SFC Visualization license not found'), though runtime process control continues.

Technical References

Related Tags

https://industrialautomation.net/search?query=simatic-pcs7
https://industrialautomation.net/search?query=sfc
https://industrialautomation.net/search?query=cfc-editor
https://industrialautomation.net/search?query=licensing

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Implementing Step Sequences in SIMATIC PCS 7: SFC Licensing vs Pure CFC State Machine Logic