Industrial Engineer · phase 3 of 16 · noun

Instruction List (IL, Legacy)

Legacy mnemonic PLC language found in older systems but not included in the current IEC 61131-3:2025 language suite.

The Explain card

Plain English: Instruction list is an old text language that reads like assembly, one operation per line with an accumulator. It was part of the standard for years and has now been dropped from it.
Example: LD Start, OR Run, ANDN Stop, ST Run. That is the same seal in rung as ladder, written as four mnemonics.
Why it matters: You will meet it in old Siemens and small controller projects that are still running lines today, so you need to read it. You should never write anything new in it, since the tooling is going away.
Hook: Read it like a museum label, never a blueprint.

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

The technician reads the Instruction List rung that checks the motor overload input before energizing the contactor output.

Why these words

instruction Latin instructio, arrangement or teaching identifies each command or operation
list Middle English liste, a strip or written series presents the commands as an ordered sequence

Where it came from

Origin: English technical compound from instruction and list
Entered the language: 20th century
What changed: Instruction moved from arrangement and teaching to a command executed by a machine, while list developed from a strip into an ordered series of entries.

How it is spelled

Spelled like

Broken into chunks

What it sits beside

Same subject

Same shape

PLC language abbreviations

compound terms ending in list

Phase 3: PLC Program Organization and Execution

How the logic is arranged and how the scan actually runs it, which is the difference between code that works and code that works every time.