Automatically Chaining Sequencers and Control Strategies in PCS 7 SFC Types

To automatically chain sequencers within a SIMATIC PCS 7 SFC Type so that the second sequencer executes immediately upon completion of the first, configure the start condition of the second sequencer to evaluate the completion status of the first sequencer along with the desired control strategy. In the final step of Sequencer 1, set the 'Prepared Control Strategy' (QCSP or CS_LL) to Strategy 2. In Sequencer 2's start conditions, configure 'RUNNING AND (QCS = 2 OR QCSP = 2)' or link the step transition to 'READY_TC' so that the SFC state engine transitions smoothly without stopping the phase.

Executive Summary

A frequent design requirement in PCS 7 recipe and equipment phase engineering is the automatic, uninterrupted progression between distinct operating sequencers inside an SFC Type. Engineers often encounter situations where Sequencer 1 finishes its programmed steps, but Sequencer 2 fails to trigger, leaving the equipment unit stalled in the 'Run' or 'Completed' state. This article provides the definitive engineering methodology for chaining sequencers within an SFC Type, covering start condition syntax, continuous execution settings, control strategy handshakes, and testing procedures.

Architecture and Fundamental Mechanics

Sequencer Execution and State Transition Mechanics in SFC Types:

Sequencer Start Conditions:
An SFC Type contains multiple sequencers (e.g., Sequencer 1 for 'Heating', Sequencer 2 for 'Dosing', Sequencer 3 for 'Holding').
Each sequencer has an independent Start Condition evaluated cyclically by the SFC runtime engine while the SFC is in the Run state.
Operating Modes and Execution Logic:
Standard Execution: Once a sequencer reaches its termination step (END), the SFC transitions to the Completed state and halts unless re-triggered.
Continuous Execution: By checking 'Continuous operation' in SFC Type Properties or using auto-transition logic, the SFC transitions immediately to the next eligible sequencer whose start conditions are met, without traversing the Completed and Ready to start states.
Handshaking Parameters:
QCS: Current active Control Strategy integer output.
QCSP: Prepared Control Strategy output for the subsequent sequencer.
READY_TC: Ready for Transition of Control Strategy (asserted when the active sequencer completes and is ready to yield control).

Step-by-Step Implementation Procedure

Configuring Automated Sequencer Chaining in SFC Type:

Open the SFC Type in the SFC Editor:
- Navigate to the project's Component view, open the Master Data Library, and open the SFC Type.
Define Control Strategies and Sequencers:
- Under 'Control Strategies', define CS 1 (e.g., 'Phase_A') and CS 2 (e.g., 'Phase_B').
- Under 'Sequencers', create 'SEQ_PHASE_A' and 'SEQ_PHASE_B'.
Configure Start Conditions for Sequencer A:
- Select SEQ_PHASE_A -> Properties -> Start Condition:
RUNNING AND (QCS = 1)
Configure Final Step of Sequencer A:
- In the last active step before END in SEQ_PHASE_A, add statement:
SELF.QCSP := 2 (or assign SELF.CS_LL := 2).
Configure Start Conditions for Sequencer B:
- Select SEQ_PHASE_B -> Properties -> Start Condition:
RUNNING AND (QCS = 2 OR QCSP = 2)
Configure SFC Type Execution Properties:
- In SFC Type Properties -> Options, enable 'Continuous mode' or configure the SFC instance in CFC with CONT := 1.
Compile and Download:
- Compile CFC/SFC changes and perform an online download to the automation station.

Verification

To verify sequencer chaining:
1. In CFC test mode, open the SFC chart instance online.
2. Start the SFC in Auto mode with Strategy 1 selected.
3. Monitor SEQ_PHASE_A steps progressing. When the final step is reached, confirm QCSP changes to 2.
4. Verify that SEQ_PHASE_B immediately becomes active and executes step 1 without the SFC dropping out of the Run state.

Limitations and Operational Precautions

Operational precautions:
- Ensure that transitions between sequencers evaluate all physical process interlocks (e.g., tank empty, valve closed) before starting the chained sequencer.
- In recipe-driven plants controlled by SIMATIC BATCH, multi-phase transitions should generally be arbitrated by Batch recipes rather than hard-coded internal sequencer jumps.
- Avoid mutual recursion: do not create circular chaining between Sequencer A and Sequencer B without an explicit exit condition, which would create an infinite run loop.

Engineer FAQ

Can two sequencers run simultaneously in the same SFC Type?

No. An SFC Type can only execute one sequencer at a time within a single instance.

What does the 'Continuous' execution option do?

It allows the SFC to immediately cycle into the next sequencer or restart upon completion without pausing in the 'Completed' state.

What parameter is used in SFC Type steps to change the prepared strategy?

The internal parameter SELF.QCSP or SELF.CS_LL.

Why did Sequencer 2 not start even though its start condition was true?

Verify that the SFC is still in the Run state and that READY_TC has asserted allowing the strategy handover.

Can sequencers be chained across different CFC charts?

No, sequencer chaining via start conditions occurs within a single SFC Type instance; inter-chart chaining is coordinated via CFC boolean interconnections.