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Ladder Diagram Editor (LD)

Planned for release 0.3

LD is planned for release 0.3 and is not compiled yet. For runnable programs use Structured Text (ST) today. See also the roadmap.

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.