SFC

Sequential Function Chart (SFC) is a graphical programming language for PLCs and industrial control systems under the IEC 61131-3 standard. The language is designed for sequential process control, where processes are broken down into steps, transitions and actions.

SFC is widely used in Process Automation, machine building, batch systems and complex industrial installations where processes pass through a clear sequence of states.

Typical applications:

• Batch processes
• Machine cycles
• Start-up procedures
• Shutdown sequences
• Recipe management
• Robot sequences
• Process automation

SFC combines visual process structures with other IEC 61131-3 languages such as:

• Ladder Logic
• FBD
• ST

SFC therefore often acts as the higher-level orchestration layer on top of underlying logic and control engineering.

⚙️ What is SFC

SFC describes processes as a series of consecutive steps.

The basic components are:

Basic example:

Start  ↓Start motor  ↓Conveyor active  ↓Detect product  ↓Stop

Each step represents a process state.

🧱 Structure of an SFC program

An SFC consists of:

• Initial steps
• Active steps
• Transitions
• Actions
• Sequences

Example:

[Step 1]    ↓ Condition TRUE[Step 2]    ↓ Condition TRUE[Step 3]

A transition only occurs when the transition condition is true.

🔄 Steps

A step represents a state within the process.

Examples:

A step can:

• Execute actions
• Activate outputs
• Start timers
• Evaluate interlocks

⚡ Transitions

Transitions determine when the process moves to the next step.

Example:

TankLevel >= 90%

Or:

TimerExpired = TRUE

Transitions are crucial for controlled process flows.

🎛️ Actions

Actions contain the actual control logic.

Examples:

• Start motor
• Open valve
• Activate alarm
• Change PID setpoint

Actions are often written in:

• Ladder Logic
• FBD
• ST

SFC therefore typically acts as the higher-level sequence layer.

🏭 SFC in industrial automation

SFC is widely used in processes with fixed sequences.

Batch processes

In batch installations:

Fill ↓Mix ↓Heat ↓Cool ↓Drain

Widely used in:

• Chemicals
• Food & beverage
• Pharmaceuticals
• Water treatment

Machine control

For:

• Production cycles
• Robot sequences
• Packaging machines
• Assembly lines

Process installations

For:

• Start-up procedures
• Shutdown sequences
• CIP processes
• Safety procedures

🧠 Parallel sequences

SFC supports parallel process flows.

Example:

        ┌→ Conveyor 1Start ──┤        └→ Conveyor 2

This allows several processes to run concurrently.

Applications:

• Parallel production lines
• Multi-tank installations
• Robot coordination

🔁 Alternative branches

SFC supports conditional process paths.

Example:

IF ProductType = A    → Route AELSE    → Route B

This enables flexible production processes.

⏱️ Timing and real-time behaviour

SFC runs within the cyclical PLC scan.

Important parameters:

Real-time behaviour remains important for:

• Machine safety
• Synchronisation
• Process stability

⚙️ State machines and SFC

SFC closely resembles state machine models.

Each step represents:

• A state
• A process status
• An operational phase

Benefits:

• Clear process structure
• Clear state models
• Easier troubleshooting
• Visual process flows

🔌 Integration with PLC architectures

SFC is often combined with other programming languages.

Typical architecture:

This hybrid model is standard in modern PLC platforms.

📡 Integration with SCADA and HMI

SFC processes provide status information to:

• SCADA
• HMI
• MES
• Historian

Typical visualisations:

Operators can see in real time which process phase a plant is in.

🛡️ Safety functions

SFC is regularly used in safety procedures.

Examples:

• Controlled shutdown
• Emergency sequences
• Safety interlocks
• Start-up validations

In safety systems, additional requirements apply under:

Safety-critical sequences often run on a Safety PLC.

⚡ SFC in batch control

SFC aligns well with ISA-88 batch architectures.

Examples:

• Recipe steps
• Batch states
• Procedure management
• Unit phases

Many batch systems combine:

• SFC
• MES
• SCADA
• Historian systems

🧪 Diagnostics and troubleshooting

SFC provides strong visual diagnostics.

Operators can immediately see:

• Active steps
• Stuck transitions
• Process status
• Sequence errors

Common problems:

⚠️ Common design errors

Over-complex sequences

Large monolithic flows become hard to read.

Poor transition logic

Insufficient validation causes instability.

No fault handling

Fault states are often missing.

Too many parallel branches

Parallel processes greatly increase complexity.

Best practices:

• Modular sequences
• Clear state names
• Fault handling
• Timeout protection
• Clear transition conditions

🔐 Cybersecurity risks

SFC programs are an important target in OT attacks.

Attackers can:

• Manipulate process flows
• Block shutdowns
• Skip safety steps
• Disrupt production

Risks:

• Unauthorised engineering
• Malware
• Manipulation of PLC code
• Insider threats

Advanced malware such as Stuxnet demonstrated how process sequences can be modified invisibly.

🧱 Security measures

Important security measures:

PLC engineering workstations often fall under strict Change Management.

🌐 SFC in Industry 4.0

In Industry 4.0, SFC remains important for process orchestration.

New applications:

• Virtual PLCs
• Digital twins
• AI-driven workflows
• Edge orchestration
• Flexible production

SFC supports the transition to software-defined OT architectures.

📈 Benefits of SFC

Key benefits:

• Very clear process structures
• Strong for sequential processes
• Good troubleshooting
• Clear status models
• Modular design
• Good batch support

⚡ Limitations

Key limitations:

• Less suited to complex calculations
• Large flows can become complex
• Parallel logic quickly becomes hard to read
• Less suited to purely continuous Control Loop

SFC is therefore usually combined with:

• Ladder Logic
• FBD
• ST