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Ladder Diagram Editor (LD)

Overview

Ladder Diagram (LD) is the oldest of the three graphic IEC 61131-3 languages and follows the circuit-diagram metaphor: between a left and a right power rail, horizontal current paths (rungs) carry the signal. On every rung, contacts sit on the left (in series) and coils on the right; depending on the variable state they either “pass” or “block” the current. LD is well suited to simple control logic — limit switches, latching circuits, interlocks — and is highly readable to electrical planners.

Editor layout

The LD editor has the same structure as the FBD editor (toolbar at the top, QGraphicsView with grid + zoom + pan, variable table on the right), with two specifics:

  • Left power rail and right power rail are permanent items in the diagram. They cannot be moved and grow vertically with the number of rungs.
  • The toolbar adds buttons for LD symbols (contacts, coils, edge triggers) and an Add Rung button which inserts a new rung connection between the power rails.

Symbols

Contacts (left side of the rung)

SymbolMeaning
--| |--NO contact — passes when the variable is TRUE
--|/|--NC contact — passes when the variable is FALSE
--|P|--Rising-edge contact — passes for one cycle on a rising edge
--|N|--Falling-edge contact — passes for one cycle on a falling edge

Contacts in series act as logical AND, parallel paths as logical OR.

Coils (right side of the rung)

SymbolMeaning
--( )Standard coil — writes the current path state into the variable
--(/)Negated coil — writes the inverted state
--(S)Set coil — sets the variable to TRUE and latches it (even if the path opens later)
--(R)Reset coil — sets the variable to FALSE and latches it

Set/reset pairs implement a latching circuit without explicit IF-THEN logic.

Function blocks on the rung

Functions and function blocks from the library can be inserted inline between contacts and coils. The LD editor draws them as a horizontal box with pin lists on the right and left — semantically identical to the FBD block. Typical uses: timers (TON), counters (CTU), comparators (GT, EQ).

Example — latching circuit with stop priority

A classic relay circuit: a start button xStart turns on a motor qMotor, a stop button xStop turns it off. As long as xStart was pressed at least once and xStop is not pressed, the motor stays on (self-holding).

        |                                              |
        |   xStart      xStop                          |
   +----| |---+--|/|---+-----------------------( )----+
        |    |         |                       qMotor  |
        |    |         |                                |
        |   qMotor     |                                |
        +----| |-------+                                |
        |                                              |

Read as a sentence:

  • xStart (NO) or qMotor (self-holding contact, NO) — in parallel,
  • and xStop (NC) — in series,
  • drive the coil qMotor.

At compile time the LD compiler translates this rung to:

qMotor := (xStart OR qMotor) AND NOT xStop;

This is the simplest form of a latch with stop priority. If both buttons are pressed at the same time, xStop wins because the NC contact opens the path.