Instruction List :Pros, Cons, and Niche Applications
Introduction
Instruction List (IL) was once a dominant PLC programming language, favored for its simplicity and resemblance to assembly code. Though deprecated in the latest versions of the IEC 61131-3 standard, IL still holds a place in the history and occasionally in the niche of industrial automation.
As an automation experience of hands-on industry, I’ve worked on legacy systems that heavily relied on IL. In this post, I’ll take you through what IL is, why it was widely used, its benefits and limitations, and when you might still encounter or apply it today.
Whether you’re maintaining older PLC systems or curious about low-level logic control, understanding IL adds depth to your automation knowledge.
Table of Contents
- What Is Instruction List (IL)?
- Why IL Was Popular
- Basic Syntax and Structure
- Instruction List vs Other IEC Languages
- Pros and Cons of Instruction List
- Niche and Legacy Applications
- Best Practices When Working with IL
- Conclusion
What Is Instruction List (IL)?
Instruction List (IL) is a low-level, text-based language used in PLC programming. It resembles assembly language with short, mnemonic-based instructions that execute line by line.
Each instruction performs a specific task like loading a value, performing a logical comparison, or setting an output. These are executed in a top-down scan, much like traditional PLC logic.
Sample IL Code
LD StartButton
ANDN StopButton
OUT MotorRunThis short routine checks if the Start button is pressed and the Stop button is not pressed, then turns on the Motor.
Why Instruction List Instruction List IL Was Popular
In the early days of automation, hardware limitations required efficient and compact logic representation. IL was ideal because:
- It executed quickly with minimal memory
- Was simple to parse and compile by early PLCs
- Matched closely with the instruction set of early microprocessors
- Allowed for precise control of scan execution and logic flow
Historical Use Cases
- Early Allen-Bradley and Siemens PLCs
- DOS-based PLC development tools
- Low-memory embedded controllers
Basic Syntax and Structure
IL uses three-character mnemonics similar to assembly language.
Core Instructions:
| Mnemonic | Description |
|---|---|
| LD | Load a value or input |
| AND | Logical AND with next value |
| OR | Logical OR with next value |
| OUT | Output to a digital output |
| ST | Store to variable or memory |
| NOT | Invert logic |
| JMP | Jump to another label |
| LBL | Label for jump destinations |
| RET | Return from subroutine |
Extended Instructions:
Depending on the platform, some IL implementations support:
- Timers (TON, TOF)
- Counters (CTU, CTD)
- Arithmetic (ADD, SUB, MUL)
Instruction List vs Other IEC Languages
| Feature | Instruction List | Ladder Logic | Structured Text | Function Block Diagram |
| Language Type | Low-level text | Graphical | High-level text | Visual block-based |
| Readability | Low | High | Medium | High |
| Programming Speed | Fast (for experts) | Medium | Fast | Medium |
| Debugging Ease | Hard | Easy | Medium | Easy |
| Best Use Case | Compact logic | Discrete I/O | Complex algorithms | Analog & sequential |
IL excels where speed, compactness, and simplicity are prioritized over readability and scalability.
Pros and Cons of Instruction List
Pros
| Advantage | Why It Matters |
| Compact codebase | Perfect for low-memory PLCs |
| Faster execution | Ideal for high-speed scanning or embedded use |
| Predictable scan control | Minimal logic ambiguity |
| Platform-agnostic basics | Many IL instructions are universal |
Cons
| Limitation | Impact |
| Difficult to read | Troubleshooting is hard for multi-line routines |
| Hard to scale or document | Poor maintainability |
| Deprecated by IEC 61131-3 | Not supported in modern environments |
| Steep learning curve | Requires deep familiarity with instruction mnemonics |
Niche and Legacy Applications
While modern PLCs favor Ladder, ST, and FBD, there are still areas where IL is relevant:
1. Legacy Systems Maintenance
- Older PLCs in refineries, utilities, and OEM machines may still rely on IL
2. Small Embedded PLCs
- IL can reduce code footprint on compact controllers
3. Academic Use
- Teaching low-level logic control and scan cycle understanding
4. Reverse Engineering
- Understanding IL is helpful when translating old logic into modern equivalents
Best Practices When Working with Instruction List IL
| Tip | Why It Helps |
| Use comments generously | Explains intent behind compact code lines |
| Keep routines short | Improves readability and traceability |
| Modularize with jumps and labels | Break large logic into small, manageable blocks |
| Use descriptive variable names | Aids in long-term support and upgrades |
| Maintain documentation externally | Link diagrams, tables, or flowcharts for clarity |
Conclusion
Instruction List may no longer be the language of the future, but it was undeniably the language of the past. It helped shape the early evolution of PLC programming by offering a fast, efficient way to program logic on memory-constrained devices.
While its use today is limited, understanding IL is invaluable for professionals maintaining legacy systems, working with compact embedded platforms, or simply seeking a deeper appreciation of control system fundamentals.
If you’re working in an environment with aging infrastructure or exploring the roots of automation languages, IL is worth a second look even if only to bridge the past with the future.
