IL

Instruction List (IL) is a textual programming language for PLCs based on an assembler-style instruction structure. The language was part of the original IEC 61131-3 standard for industrial automation and was used for compact, efficient and low-overhead PLC programming within Industrial Automation and Process Automation.

IL was designed for:

• Fast logical processing
• Compact PLC programs
• Low memory footprint
• Direct hardware control
• Deterministic execution

Although IL was historically very popular, the language has officially been deprecated in recent versions of IEC 61131-3 due to limited readability and maintainability compared to more modern languages such as:

• Ladder Logic
• FBD
• ST
• SFC

IL is still found in legacy PLC environments, older SCADA systems and industrial installations with long lifecycles.

⚙️ What is Instruction List

IL is a text-based language built from simple instructions that are executed sequentially.

The structure closely resembles assembly programming.

Example:

LD StartAND NOT StopST Motor

Meaning:

The instructions are processed line by line by the PLC runtime.

🧱 Structure of IL programs

An IL program consists of:

• Operations
• Operands
• Labels
• Jumps
• Memory locations

Example:

LD SensorAND Pressure_OKST Pump

Accumulator-based processing forms the core of IL.

🔄 Accumulator-based operation

IL typically operates on an internal accumulator.

Process:

LD input↓Accumulator filled↓AND next value↓Store result

This model resembles classic CPU register processing.

⚡ Common instructions

Load instructions

For loading values:

Example:

LD StartButton

Logical instructions

For boolean logic:

Example:

LD Sensor1AND Sensor2ST Output

Store instructions

For storing results:

Jump instructions

For program control:

This allowed state machines and sequences to be built.

⏱️ Cyclical execution

Like other PLC languages, IL operates cyclically.

PLC scan:

Read inputs    ↓Execute IL code    ↓Write outputs    ↓New scan

Important properties:

Thanks to compact instructions, IL was highly performant on older PLC hardware.

🏭 IL in industrial automation

Historically, IL was used in:

• Production machines
• Process installations
• Embedded PLC systems
• Power plants
• Building automation

Applications:

• Interlocks
• Start/stop logic
• Machine cycles
• Alarm processing
• Basic process control

🔌 Direct hardware control

IL offered relatively direct control over PLC resources.

Benefits:

• Low overhead
• Fast execution
• Efficient memory use

This was important for older PLC generations with:

• Limited RAM
• Low CPU capacity
• Small program storage

🧠 IL versus Ladder Logic

Maintenance teams often preferred visual languages.

🔄 IL versus Structured Text

Structured Text has largely replaced IL on modern PLC platforms.

⚡ Performance characteristics

IL was known for:

• Very fast execution
• Low memory pressure
• Compact code
• Deterministic behaviour

Benefits for older hardware:

• Small program size
• Less CPU load
• Faster scan times

This was important in real-time OT processes.

🛡️ IL in safety systems

IL was also used in older Safety PLC systems.

Examples:

• Safety interlocks
• Shutdown logic
• Emergency stop functions
• Machine protection

Important standards:

Today, safety applications are more often developed in modern languages for better validation and maintainability.

📡 Integration with SCADA and HMI

IL programs deliver process data to:

• SCADA
• HMI
• Historian
• MES

Communication uses:

• Modbus TCP
• ProfiNET
• Ethernet IP
• OPC UA

🧪 Diagnostics and troubleshooting

A major drawback of IL is limited readability.

Example:

LD AAND BOR CST D

In larger programs, troubleshooting becomes complex.

Problems:

⚠️ Common design errors

Excessive use of jumps

Complex jump structures cause spaghetti code.

Poor documentation

Many IL systems lack clear comments.

Hardware-dependent design

Some implementations are strongly vendor-specific.

No modular structure

Older IL programs are often monolithic.

Best practices:

• Clear labels
• Comment lines
• Modular routines
• Limited jumps

🔐 Cybersecurity risks

Legacy IL systems present an elevated cybersecurity risk.

Risks:

• Outdated PLCs
• No modern authentication
• Legacy engineering software
• Difficult to patch

Attacks can:

• Manipulate outputs
• Change process logic
• Disrupt safety functions

Known OT malware such as Stuxnet demonstrated how PLC programs can be modified without direct visibility.

🧱 Security measures

Important measures:

Legacy PLC systems often require additional compensating controls.

🌐 Why IL disappeared

IEC 61131-3 has officially deprecated IL.

Main reasons:

• Poor readability
• Difficult maintenance
• Limited scalability
• Higher error rates
• Difficult validation

Modern automation calls for:

• Modular architectures
• Better debugging
• Complex data handling
• Software reusability

Languages such as ST became more important as a result.

🏗️ IL in legacy OT environments

Despite being obsolete, IL still runs in:

• Old production lines
• Energy infrastructure
• Water installations
• Legacy machine control

Migration is often complex due to:

• Downtime risk
• Vendor lock-in
• Insufficient documentation
• Old hardware

IL therefore remains relevant within Lifecycle Management of OT systems.

📈 Benefits of IL

Key benefits:

• Compact code
• High performance
• Low memory pressure
• Deterministic behaviour
• Direct hardware control

⚡ Limitations

Key limitations:

• Low readability
• Difficult troubleshooting
• Limited Scalability
• Little visual structure
• Difficult to maintain

For these reasons, IL is rarely used for new projects today.