CFC vs SFC in SIMATIC PCS 7 - Continuous Process Control vs Sequential Batch Architecture

In SIMATIC PCS 7, Continuous Function Charts (CFC) and Sequential Function Charts (SFC) represent complementary control paradigms that operate in tandem. CFC implements continuous, cyclical process control (such as analog signal conditioning, PID regulation, motor interlocks, and I/O driver processing) executing cyclically in timed interrupt organization blocks (OB30 to OB38). SFC implements state-based, sequential batch and phase control (such as recipe steps, startup/shutdown procedures, and CIP sequences) that dynamically manipulates CFC block inputs via step-and-transition logic.

CFC and SFC Paradigms in SIMATIC PCS 7

In process control engineering, industrial automation tasks divide fundamentally into continuous and sequential domains:
* Continuous Control (CFC): Continuous processes require constant, deterministic evaluation of analog variables, closed-loop PID control, discrete safety interlocks, and equipment protection. A motor block in CFC runs 24/7, continuously sampling feedback contacts and enforcing safety trips.
* Sequential Control (SFC): Production processes (such as brewing, batch chemical reactions, boiler startup, or automated cleaning-in-place) progress through discrete operational states: Filling -> Heating -> Agitating -> Reacting -> Discharging. SFC manages this chronological sequence by executing step actions and evaluating transition conditions.

CFC Architecture - Cyclic Execution and Technological Interconnection

CFC is a graphical programming language compliant with IEC 61131-3:
* Block-Oriented: Function blocks (FBs) from the Advanced Process Library (APL) are placed on chart sheets and interconnected by drawing graphical signal lines.
* Cyclic Interrupt OB Assignment: Unlike traditional PLC ladder logic executed sequentially in OB1, PCS 7 CFC blocks are scheduled into cyclic interrupt organization blocks (e.g. OB35 at 100 ms, OB32 at 1000 ms), ensuring deterministic scan rates for PID loops.
* Signal Conditioning: CFC handles physical I/O mapping via channel driver blocks (Pcs7AnIn, Pcs7DiIn), raw-to-engineering unit conversion, alarm threshold monitoring, and fail-safe passivation.

SFC Architecture - State-Based Sequential Control

SFC is a state-transition modeling language conforming to IEC 61131-3:
* Steps and Transitions: An SFC chart consists of Steps (containing initialization, cyclic, and terminating actions) and Transitions (boolean logical conditions evaluated to advance to the next step).
* SFC Types vs SFC Charts:
* SFC Chart: A standalone sequential chart configured once in the chart folder.
* SFC Type: A reusable, object-oriented sequence template instantiated across multiple technological units (e.g. identical fermenters or reactors).
* Target Control: SFC does not read physical I/O directly; instead, it coordinates the technological blocks instantiated in CFC charts.

Coordinated Interaction Between CFC and SFC

The integrated execution flow between CFC and SFC follows a strict master-slave hierarchy:

[SFC Sequential State Machine]
   │
   ├─► Step 1: "Preheat Vessel"
   │     • Action: Set TIC101.SP_LiOp := TRUE
   │     • Action: Write TIC101.SP_Li := 85.0 (°C)
   │     • Action: Command Agitator MotL.StartAut := TRUE
   │
   ├─► Transition 1: "Temperature Achieved?"
   │     • Condition: (TIC101.PV_Out >= 84.5) AND (MotL.Running = TRUE)
   │
   └─► Step 2: "Dose Catalyst" -> (Advances sequence...)
         │
         ▼
[CFC Continuous Technological Engine (OB35 - 100ms)]
   │  • TIC101 (PIDConL): Continuously calculates manipulated variable (MV)
   │  • MotL (Motor): Continuously enforces interlocks (Permit, Intlock, Protect)
   │  • Channel Drivers: Continuously sample analog 4-20mA inputs and fieldbus I/O

Architectural and Functional Comparison Matrix

Compare the core architectural attributes of CFC and SFC within SIMATIC PCS 7:

Dimension / FeatureContinuous Function Chart (CFC)Sequential Function Chart (SFC)Control PhilosophyContinuous, regulatory, interlock-basedSequential, phase-based, recipe-drivenExecution DomainCyclic Interrupt OBs (OB30 - OB38)Cyclic Interrupt OBs (typically OB32 / OB35)Primary Building BlocksFBs, FCs, APL Technological BlocksSteps, Transitions, Jump statementsI/O AccessDirectly mapped to process I/O via driversReads/writes parameters of CFC blocksTypical ApplicationsPID loops, motor/valve logic, signal scalingBatch phases, equipment sequences, CIPWinCC Faceplate SupportStandard technological block icons / faceplatesStandard SFC Visualization ControlBatch IntegrationControlled by SIMATIC BATCH via SFCNative interface for SIMATIC BATCH recipes

Verification

To confirm proper coordination between CFC and SFC charts:
1. In SIMATIC Manager, open both the CFC chart (e.g. TIC101) and the SFC chart (e.g. Reactor_Seq).
2. Switch both CFC and SFC into Test Mode online.
3. Start the SFC sequence; verify that the active step transitions from Idle to Preheat.
4. In the CFC chart, observe the PID block SP_Li parameter; verify that the setpoint updates dynamically to the value assigned by the SFC step action.
5. In WinCC Runtime, open the SFC Visualization window; verify that the sequence displays the active step and elapsed step time in sync with the AS controller.

Limitations and Operational Precautions

Never connect an SFC step action directly to raw hardware output addresses (PQW / PAW). All field actuations must pass through CFC technological blocks (such as VlvL or MotL) to ensure software interlocks, protection trips, and manual operator overrides remain strictly enforced. Direct hardware overrides from SFC bypass safety interlocks, creating severe process hazards.

Engineer FAQ

Do all PCS 7 projects require SFC charts?

No. Plants consisting purely of continuous processes (such as continuous distillation, water treatment, or oil refining) can be engineered entirely in CFC. SFC is implemented when sequential, recipe, or state-based control is required.

Can an operator pause or abort an SFC sequence from WinCC?

Yes. SIMATIC PCS 7 provides the standard SFC Visualization control, allowing operators to start, hold, resume, abort, and manually step through sequences from the SCADA screen.

Why are PCS 7 blocks executed in cyclic interrupt OBs instead of OB1?

Cyclic interrupts (such as OB35 at 100 ms) guarantee exact deterministic execution intervals. Executing PID controllers in OB1 causes calculation errors because OB1 cycle time varies based on program load.

What is the difference between an SFC Chart and an SFC Type?

An SFC Chart is an individual chart instance. An SFC Type is an object-oriented template defined in the Master Data Library that can be instantiated across multiple technological units and integrated directly with SIMATIC BATCH.