Sequential Function Chart :Designing State-Based Workflows

Introduction

In industrial automation, complex processes often require step-by-step sequencing, conditional transitions, and clearly defined states. When conventional ladder logic or function block diagrams become too cumbersome to handle multi-phase workflows, Sequential Function Chart (SFC) programming offers a structured, visual solution.

As an automation experience in the field, I’ve relied on SFCs for everything from pharmaceutical batch processes to packaging lines and robotic systems. In this blog post, we’ll unpack what SFC is, when to use it, how to structure it effectively, and best practices for building scalable, state-driven logic that works reliably.

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Table of Contents

1. What Is Sequential Function Chart (SFC)?
2. When and Why to Use SFC in PLC Programming
3. Core Elements of SFC
4. SFC Workflow: Step-by-Step
5. Real-Life Example: Batch Mixing Process
6. SFC vs Ladder Logic vs Structured Text
7. Best Practices for SFC Programming
8. Common Mistakes to Avoid
9. Conclusion

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What Is Sequential Function Chart (SFC)?

Sequential Function Chart (SFC) is a graphical programming language defined in IEC 61131-3, used to model state-based control logic. It breaks a process into steps, actions, and transitions, making it easy to visualize and manage sequence-dependent workflows.

Imagine a flowchart, but built for real-time logic execution.

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When and Why to Use SFC in PLC Programming

SFC excels in systems with:

• Multiple operational states
• Conditional transitions between phases
• Step-by-step tasks such as batching, pick-and-place, or machine cycles

Key Advantages

Feature	Benefit
Visual flow representation	Easy for teams to understand logic
Modularity	Steps and transitions can be reused
Improved diagnostics	Pinpoint which step failed or stalled
State-driven logic	Ideal for machines with defined operating modes

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Core Elements of SFC

Element	Description
Step	Represents a state in the process. Can hold actions (e.g., open valve).
Transition	Condition that must be true to move from one step to the next.
Action	Code or function block executed when a step is active.
Initial Step	First active state in the sequence at startup.
Branching	Allows parallel steps for concurrent operations.

These elements create a readable and maintainable control structure for state-based systems.

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SFC Workflow: Step-by-Step

1. Define States (Steps)

Break your process into discrete operational states:

• Idle
• Fill Tank
• Mix Contents
• Heat Product
• Discharge

2. Create Transitions

Define what must happen to move from one state to another:

IF (TankLevel >= 80%) THEN Transition_1 := TRUE;

3. Attach Actions

Assign logic to each step using structured text or function blocks:

Action_1: PumpON := TRUE;

4. Structure Branches (if needed)

Support parallel activities (e.g., start agitator while heating).

5. Monitor Step Activity

Most PLCs highlight the active step in real time—useful during commissioning.

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Real-Life Example: Batch Mixing Process

Application

A mixing skid for a cosmetics plant had to:

1. Fill tank to 80% level
2. Start mixer for 5 minutes
3. Heat to 60°C
4. Discharge after temperature is reached

SFC Implementation

Step	Action	Transition
Fill_Tank	Open Inlet Valve	IF Level >= 80%
Start_Mixer	Motor := TRUE	IF Timer >= 5 minutes
Heat_Mix	Heater := ON	IF Temp >= 60°C
Discharge_Product	Open Outlet Valve	IF Discharge_Complete = TRUE

This SFC replaced over 50 rungs of ladder logic and cut debugging time by 70%.

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SFC vs Ladder Logic vs Structured Text

Feature	Ladder Logic	Structured Text	SFC
Visual Format	Relay-based	Text-based	Flowchart-based
Best Use Case	Discrete I/O	Complex logic/math	Sequencing & state machines
Learning Curve	Low	Medium	Medium
Real-Time Clarity	Good	Low	Excellent

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Best Practices for SFC Programming

Best Practice	Why It Helps
Use descriptive step names	Improves clarity for team members and HMI mapping
Include entry and exit actions	Simplifies data resets and prep for next steps
Combine with structured text	Enables complex logic within steps
Modularize large charts	Break into subcharts to manage complexity
Document transitions clearly	Avoids logic gaps or unexpected stalls

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Common Mistakes to Avoid

Mistake	Recommendation
Skipping initialization steps	Always define a clean starting state
Poor transition conditions	Use well-defined Boolean logic
Unmanaged parallel branches	Ensure synchronizing conditions are in place
No watchdog timers	Add timeout logic to avoid endless waiting
Ignoring restart logic	Handle power-loss and mid-sequence recovery properly

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Conclusion

Sequential Function Chart (SFC) is a game-changing tool for designing clear, maintainable, and scalable control systems. It enables automation professionals to build process logic that’s intuitive, testable, and easy to commission—even in the most complex state-driven systems.

With its combination of visual logic flow and structured action execution, SFC bridges the gap between engineering documentation and real-world code, turning process maps into programmable logic.

If your project involves sequences, steps, or state machines, SFC might just be the smartest move you make in your PLC toolkit.