C135 PCS 7 (PCS7)
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How to Automatically Trigger Cascaded Start Sequences in PCS 7 SFC Types

How to Automatically Trigger Cascaded Start Sequences in PCS 7 SFC Types

To trigger the automated start of a secondary Sequential Function Chart (SFC) Type instance upon the completion or specific step transition of a primary SFC Type in SIMATIC PCS 7, you must interconnect the primary instance's state outputs to the control inputs of the secondary instance in CFC. In sequential batch and multi-phase continuous operations, cascaded execution is controlled using standard SFC Type control parameters: enabling automated commands via ENSTART := 1 and pulsing the execution trigger START := 1 when the master sequence enters the target step or asserts its COMPL (Completed) status output. This ensures deterministic phase progression without requiring manual operator intervention or external recipe coordination.

Table of Contents

SFC Types Operating Architecture and State Logic

In SIMATIC PCS 7, Sequential Function Charts (SFC) manage state-based, sequential process procedures (e.g., vessel CIP cleaning, reactor heating profiles, product transfer phases):
* SFC Charts vs. SFC Types: While SFC Charts are individual standalone programs, SFC Types are reusable, standardized sequential templates instantiated in CFC with dedicated instance data blocks and OS faceplates.
* The ISA-88 State Model: Every SFC Type conforms to the international standard ISA-88 / NAMUR state model:
- Idle -> Starting -> Running -> Completing -> Completed.
- Exception states: Holding, Held, Aborting, Aborted, Stopping, Stopped.
* Autonomous Coordination: In multi-unit operations, Phase 1 (e.g., Agitation and Heating) must automatically trigger Phase 2 (e.g., Dosing) when specific physical criteria are satisfied, without requiring an operator click on the WinCC faceplate.

Control Parameters for Automated Execution: ENSTART and START

Every SFC Type block instance exposes a standardized set of control inputs and state outputs in the CFC editor:

Parameter NameDirectionTypeFunction in Cascaded Sequence ControlENSTARTInputBOOLEnable Start: When TRUE, permits the SFC instance to accept automated start commands.STARTInputBOOLStart Trigger: A positive edge (0 -> 1) transitions the SFC from Idle to Starting.AUTOInputBOOLSets the SFC operating mode to Automatic (controlled by interconnected program logic).MANUALInputBOOLSets the SFC operating mode to Manual (controlled by operator via faceplate).READY_TCOutputBOOLReady to Start: Indicates the SFC instance is in the Idle state with interlocks healthy.COMPLOutputBOOLCompleted: Asserts TRUE when the sequence reaches the final step and enters the Completed state.

Cascading Strategies: Step-Based vs. State-Based Triggers

Depending on process requirements, engineers deploy two cascading patterns:

Pattern 1: State-Based Handshake (End of Sequence Trigger)

The second SFC starts only after the first SFC completes its entire execution cycle:
* SFC1.COMPL pulses SFC2.START.
* SFC1 is configured to automatically reset from Completed back to Idle via parameter AUTORST := 1, preparing it for future cycles.

Pattern 2: Step-Based Mid-Sequence Trigger (Parallel Overlap)

The second SFC starts while the first SFC is still actively running (e.g., starting an exhaust fan when a heating furnace reaches 200 °C at Step 3):
* A characteristic boolean flag inside Step 3 of SFC1 asserts a dedicated output (e.g., Step3_Active).
* This output feeds a pulse generator (PULSE_P) that triggers SFC2.START.

Method: Step-by-Step CFC Interconnection Workflow

To configure automatic cascaded execution between two SFC Type instances:

  1. In SIMATIC Manager, open the CFC chart containing the two SFC Type instances (e.g., SFC_Dosing and SFC_Reaction).
  2. Configure Operating Modes:
    - Connect a static TRUE (or central plant Auto signal) to AUTO on both instances.
    - Set ENSTART := TRUE on both instances.
  3. Configure the Cascading Interconnection:
    - If starting after completion: connect output SFC_Dosing.COMPL to input SFC_Reaction.START.
    - Ensure an interlock safety check: route the start signal through an AND block with SFC_Reaction.READY_TC to verify the second unit is physically idle and ready.
  4. Prevent Continuous Trigger Lockup:
    - The START input requires a rising edge (0 -> 1). If COMPL remains asserted continuously, SFC_Reaction will not restart on subsequent cycles. Insert a standard pulse block (PULSE_P with pulse time T#1s) or set SFC_Dosing.AUTORST := 1 to ensure COMPL drops back to FALSE after the transition.
  5. Compile CFC charts and compile OS.

Verification

To verify that cascaded automatic starting operates smoothly:

  1. In the CFC editor, open both SFC Type instances in online test mode.
  2. Verify that both instances report Mode: Auto and State: Ready / Idle.
  3. Start the first SFC instance (SFC_Dosing) via test command or simulated operator trigger.
  4. Observe the sequence progression:
    - Monitor SFC_Dosing as it transitions through active execution steps.
    - When SFC_Dosing reaches the final step, verify that COMPL asserts TRUE.
  5. Observe SFC_Reaction:
    - Confirm that within one CPU cycle, SFC_Reaction transitions from Idle to Starting, then to Running.
    - Confirm that the second sequence executes all defined phase steps without stalling.

Limitations and Operational Precautions

  • Edge Detection Requirement: Never apply a continuous static TRUE signal to the START parameter. SFC Types require a positive signal edge to initiate a state transition. A static TRUE prevents the block from registering future start requests.
  • Exception State Handling: If SFC1 is held or aborted due to a plant trip, ensure SFC2 is commanded to pause or abort. Interconnect SFC1.HELD and SFC1.ABORTED to SFC2.HOLD and SFC2.ABORT to maintain synchronized shutdown.
  • SIMATIC BATCH Coordination: If the plant runs SIMATIC BATCH, do not use hardcoded CFC interconnections to chain SFC Types. SIMATIC BATCH manages recipe phase progression at the supervisory level; manual CFC cross-wiring conflicts with BATCH recipe allocation.

Engineer FAQ

Why does my second SFC fail to start even though START is receiving a signal?
The second SFC will ignore START if ENSTART is FALSE, if the instance is not in the Idle state, or if an operating interlock (INTLOCK) is active.

Can an operator manually stop an SFC Type running in Auto mode?
Yes. Operators can issue Stop, Hold, or Abort commands from the WinCC SFC faceplate at any time, regardless of whether the sequence was started automatically or manually.

What is the function of the AUTORST parameter on SFC Types?
When AUTORST := 1, the SFC automatically transitions from the Completed state back to Idle without requiring an explicit reset command, making it immediately ready for the next run.

Technical References

  • Siemens Industry Online Support Entry ID 109742323: SIMATIC Process Control System PCS 7 SFC for S7 Manual
  • SIMATIC PCS 7 SFC Visualization and Engineering Manual (Document No. A5E02779836)
  • Siemens Industry Online Support Entry ID 109809264: ISA-88 State Model Implementation in SIMATIC PCS 7

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  • https://industrialautomation.net/search?query=sfc-types
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  • https://industrialautomation.net/search?query=sequential-control
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