Glossary · PLC and control programming
Instruction list (IL)
Also known as: IL, Statement list (STL), AWL
German: Anweisungsliste (AWL)
In IEC 61131-3, instruction list (IL) is a low-level textual programming language similar to assembler, in which each line holds one operator acting on an accumulator; the 2013 edition of the standard marked it as deprecated.
- PLC programming
- Standards
In one sentence
Instruction list (IL) is an assembler-like PLC language with one operator per line; IEC 61131-3:2013 marked it as deprecated.
Example
An older machine program contains IL lines such as LD Start, AND NOT Stop, ST Motor to switch a motor on.
How it applies
- Legacy systems: Many existing machines, especially older Siemens S7-300 and S7-400 projects, contain large amounts of IL or its Siemens variant statement list (STL, German AWL). Maintaining them requires people who can read it.
- Migration: Because IL is deprecated in IEC 61131-3 and not available on every newer controller platform, migrations often rewrite IL code in Structured text (ST) or a graphical language. The rewritten code must be tested as new code, not assumed equivalent.
- Engineering: New projects should not use IL. Its compactness comes at the price of readability and tool support.
- Documentation: For legacy code, document the logic at the function level (what the network does, which signals it uses) so that the knowledge survives a later migration. A Deprecation path for IL code belongs in the lifecycle plan of the machine.
IL vs. STL
IL is the IEC 61131-3 language; STL (AWL) is Siemens' own, richer statement list. They look similar but are not identical, so code cannot be moved between them unchanged.