Instruction list, or IL, is the lowest-level of the PLC programming languages defined by IEC 61131-3. It is a compact, assembly-like textual language where each line is a single operation - load a value, perform an operation, store the result. Once common, IL has been deprecated in newer editions of the standard, but engineers still encounter it in legacy programs and older Siemens systems.
Instruction List in one line: Instruction list (IL) is a low-level, assembly-like textual PLC programming language defined in IEC 61131-3, where each instruction performs one operation on an accumulator; it is now deprecated in newer editions of the standard but persists in legacy code.
Instruction list works on an accumulator model, much like assembly language. Each line contains an operator and an operand: a LD (load) instruction places a value in the accumulator, an operation such as AND or ADD combines the accumulator with an operand, and an ST (store) instruction writes the result out. Programs are read top to bottom, one terse instruction at a time.
This low-level style makes IL extremely compact and, on older hardware, very efficient - it maps closely to what the processor actually does. But that same terseness makes it hard to read and easy to get wrong. A logic scheme that is obvious in ladder or function block diagram becomes a wall of cryptic mnemonics in IL, which is a large part of why the industry moved away from it.
The IEC 61131-3 committee removed instruction list from the third edition of the standard, effectively deprecating it in favor of the more readable languages - ladder diagram, function block diagram, structured text, and sequential function chart. Modern tools and modern hardware no longer need the efficiency IL once offered, and maintainability now matters more than saving a few processor cycles.
Despite that, IL has not vanished. It survives in older installations and was closely associated with the Statement List (STL) language in the Siemens world. Engineers maintaining legacy compressor packages, metering skids, or aging plant controllers still run into it, which is why understanding the basics remains useful. Whatever language the logic uses, the controller exposes its data as tags; a SCADA platform like Merobix reads those tags over the PLC's native protocol without needing to interpret the IL itself.
It is deprecated in the newer edition of IEC 61131-3 and rarely chosen for new projects, but it persists in legacy programs and older installations. Engineers maintaining aging equipment still encounter it, so familiarity with the basics remains worthwhile.
Instruction list is a low-level, assembly-like language working one operation at a time on an accumulator, while structured text is a high-level, Pascal-like language with IF statements, loops, and expressions. Structured text is far more readable and has largely replaced IL.
They are closely related. Statement List (STL) is the Siemens language that corresponds to the IEC 61131-3 instruction list concept, and the two share the same low-level, mnemonic style. Both are being phased out in favor of higher-level languages.
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