Industrial Engineer · phase 3 of 16 · noun

Ladder Diagram (LD)

IEC 61131-3 graphical language that represents logic with relay-style contacts, coils, and function blocks.

The Explain card

Plain English
Ladder is the graphical language that draws logic the way an electrician draws a relay circuit. Two rails, rungs across them, contacts that test conditions and coils that set results.
Example
A rung with a normally open start contact in parallel with the motor's own run bit, in series with a normally closed stop contact, feeding the run coil. That is a seal in circuit, and every plant has a thousand of them.
Why it matters
Maintenance can read ladder at three in the morning with the machine down, because it looks like the schematic on the panel door. It is the language most of the installed base speaks, whatever the newer ones can do.
Hook
The wiring diagram that runs itself.

Word knowledge

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

In a sentence

During commissioning, the technician opens the Ladder Diagram online to trace the motor-start command from the field pushbutton input to the contactor output.

Why these words
  • ladder Old English hlæder, ladder or steps supplies the visual comparison to two side rails joined by horizontal rungs
  • diagram Greek diagramma, figure marked out by lines identifies the expression as a drawn technical representation
Where it came from
Origin
English compound formed from ladder and diagram, with ladder from Old English hlæder and diagram ultimately from Greek diagramma
Entered the language
20th century
What changed
Ladder broadened from a climbing structure to a name for drawings with a similar rail-and-rung layout, while diagram retained its sense of a figure drawn to show relationships.
How it is spelled
Pattern
Ladder keeps a doubled medial consonant after the short a sound.
Pattern
Diagram keeps the Greek-derived dia- opening and -gram ending.

Spelled like

  • adder
  • padded
  • telegram
  • monogram
Broken into chunks
  • ladder
    • stepladder
    • rope ladder
  • dia-
    • diagonal
    • diameter
  • -gram
    • telegram
    • monogram
What it sits beside

Same subject

Same shape

  • wiring diagram
  • block diagram
  • single-line diagram

IEC 61131-3 graphical languages

Ladder Diagram structure terms

Where it sits in the deck

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.

Neighbours in this phase