Instruction List (IL) in PLC Programming

Instruction List (IL) is the assembly-like programming language defined in IEC 61131-3 — terse mnemonic instructions executed top-to-bottom, similar to writing assembly for a stack-based virtual machine. The 2013 revision of IEC 61131-3 deprecated IL in favour of Structured Text. New projects use ST; you only encounter IL when maintaining legacy Siemens STL code or older Mitsubishi/Allen-Bradley projects from the 1990s and early 2000s.

Important context: IL is deprecated

The IEC 61131-3 standard deprecated Instruction List in the 2013 revision. The successor is Structured Text. New projects should use ST; modern PLCs (Siemens S7-1500, Allen-Bradley ControlLogix 5580, Beckhoff CX) still compile IL for backward compatibility, but tooling support is being phased out. Siemens TIA Portal still includes STL (their IL-equivalent) for migrating S7-300/400 code to S7-1500. Beyond migration, there is no reason to write new IL.

IL syntax

IL operates on a single "current result" (CR) — equivalent to the accumulator on a stack-based CPU. Each instruction either loads a value, modifies the CR, or stores it.

(* Boolean rung: Out := A AND NOT B *)
LD   A          (* load A into CR *)
ANDN B          (* AND CR with NOT B *)
ST   Out        (* store CR to Out *)

(* Arithmetic: result := (a + b) * c *)
LD   a
ADD  b
MUL  c
ST   result

(* Conditional: jump if zero *)
LD   counter
EQ   0
JMPC TARGET
LD   counter
SUB  1
ST   counter
TARGET:
LD   counter
ST   output

Instruction reference

Group	Instructions	Purpose
Load/Store	LD, LDN, ST, STN	Move data into/out of current result
Set/Reset	S, R	Latch and unlatch booleans
Boolean	AND, ANDN, OR, ORN, XOR, XORN, NOT	Boolean operators on CR
Arithmetic	ADD, SUB, MUL, DIV, MOD	Math on CR
Comparison	GT, GE, EQ, NE, LE, LT	Compare and store boolean in CR
Jump/Call	JMP, JMPC, JMPCN, CAL, CALC	Control flow (conditional based on CR)
Return	RET, RETC, RETCN	Return from POU

Why IL was deprecated

Hard to read. Each line is one operation; even simple expressions span 4-5 lines. Code review and maintenance are slow.
No standard data types beyond basics. Strings, structures, arrays are awkward to manipulate.
Vendor variants diverge. Siemens STL, Mitsubishi MELSAP, Allen-Bradley legacy IL — all looked similar but compiled differently.
Modern CPUs don't need it. ST compilers produce machine code as efficient as hand-written IL on modern 32-bit ARM/x86 PLC CPUs.
OOP extensions don't fit. The 2013 revision added classes, inheritance and methods to ST — IL syntax couldn't support them cleanly.

When you'll still see IL

Legacy Siemens S7-300/400 STL code. Siemens STL is functionally equivalent to IL. TIA Portal still compiles it for S7-1500 backward compatibility, and migrations from S7-300 to S7-1500 typically preserve STL blocks rather than rewriting them.
Older Mitsubishi MELSEC Q/L series projects in industrial Asia.
Allen-Bradley legacy SLC-500 / PLC-5 code compiled before Studio 5000 — pure ladder, but with SCP / structured equivalents that read like IL.
Pulse-by-pulse motion code — some older servo controllers expose IL-style microcode for tight motion loops.

If you encounter IL today, treat it as legacy code: read it, document it, but don't extend it. Convert to Structured Text when you next have to make significant changes.

Frequently asked questions

What is Instruction List in PLC programming?

Instruction List (IL) is one of the original IEC 61131-3 PLC programming languages — assembly-like mnemonic instructions executed against a single "current result" register. It uses commands like LD, ANDN, ST, JMP. IL was deprecated in the 2013 revision of IEC 61131-3 in favour of Structured Text.

Is Instruction List still used?

New projects rarely use IL. You still encounter it in legacy Siemens S7-300/400 STL code, older Mitsubishi MELSEC projects, and 1990s/2000s Allen-Bradley installations. Modern PLC compilers continue to support IL for backward compatibility but new development uses Structured Text.

What is the difference between IL and Structured Text?

IL is low-level (one instruction per line, single accumulator) while Structured Text is high-level (Pascal-like, expressions, control structures, named function blocks). Both compile to the same machine code on modern PLCs, but ST is dramatically easier to read, maintain, and extend with OOP. The IEC 61131-3 standard now considers ST the successor to IL.

Is Siemens STL the same as IEC Instruction List?

STL (Statement List) is Siemens' implementation of IL with Siemens-specific extensions for S7-300/400/1500 hardware (BLOCK_DB references, jump labels, accumulator manipulation). The core IL instructions (LD, AND, OR, ST) work the same; STL adds Siemens-specific functionality. Pure IEC IL is portable; Siemens STL is not.