Ladder Diagram (LD)

Ladder Diagram

LD borrows its visual form from electrical relay schematics drawn between two vertical “power rails”: each horizontal rung runs from the left rail to the right rail and contains a sequence of normally-open / normally-closed contacts on the input side and one or more coils on the output side. A coil energises when current can flow from the left rail through its rung to the right rail.

Despite the visual heritage, LD has been a programming language in IEC 61131-3 since the first edition. Function blocks can appear inline as boxes between contacts and coils — the LD program is a graph, not just a contact-coil diagram.

ForgeIEC exposes LD via the same graph editor as FBD; the persisted form is PLCopen XML.

Sub-pages

• Elements — contacts, coils, inline FBs, the power-rail topology.