Ladder Logic

What Does Ladder Logic Mean?

Ladder logic is a programming language that creates and represents a program through ladder diagrams that are based on circuit diagrams. It is mainly used in developing programs or software for programmable logic controllers (PLCs), which are used in industrial applications.


The language evolved from originally being a method for documenting the design and construction of relay racks used in manufacturing and process control, with each relay rack represented by a symbol on the ladder diagram that has connections to devices below them that look like vertical rails. The relay symbols themselves look like rungs in a ladder.

Techopedia Explains Ladder Logic

Ladder logic is described as a rule-based language rather than a procedural or imperative one. Each "rung" in the ladder represents a rule, so when implemented to relays and various electromechanical devices, these rules execute simultaneously and immediately. But if the program is applied to PLCs, the rules are executed sequentially through software and in a continuous loop. By executing the loop quickly enough, the effect still seems like a simultaneous and immediate execution within the required time tolerance. The capabilities of the PLC being used have to be considered during programming as the electromechanical nature of the devices connected to it might not be able to keep up with the instructions, and it may seem that some rules are being skipped when the devices really just cannot keep up.

Ladder logic is widely used in industrial settings for programming PLCs where sequential control of manufacturing processes and operations is required. The programming language is quite useful for programming simple yet critical systems or for reworking old hard-wired systems into newer programmable ones. This programming language is also used heavily in highly sophisticated automation systems such as electronics and car factories.

The idea behind ladder logic is that even personnel without programming backgrounds can quickly program since it makes use of conventional and familiar engineering symbols for programming. But this advantage is quickly negated since manufacturers of PLCs often also provide ladder logic programming systems with their products, which sometimes do not use the same symbols and conventions as those made for other models of PLCs from other manufacturers; in fact, the programming system is usually meant only for specific models, so the programs cannot be ported easily to other PLC models or must be outright rewritten.


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Margaret Rouse
Technology Expert

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.