Relay Ladder Logic

Ladder diagram or Relay ladder logic (RLL) is the fundamental programming language for programmable logic controllers (PLCs). Electrical schematics can look a lot like ladder diagrams going vertical. By designing them as electrical schematics, most individuals learn to draw ladder logic diagrams this way. Although clearly, certain variations persist that we will see below.

Electromechanical relays can be interconnected to perform logic and control functions, acting as logic elements in much the same way as digital gates (AND, OR , etc.). A closely common form of a schematic diagram showing the interconnection of relays to perform these functions is called a ladder diagram. In a “ladder” diagram, the two poles of the power source are drawn as vertical rails of a ladder, with horizontal “rungs” showing the switch contacts, relay contacts, relay coils, and final control elements.
The problem here is that the electrical control systems and the PLC work in many different ways so it is difficult to find a general one. Here are the biggest distinctions:

  • The PLC takes a ladder logic line (rung), executes it, and then moves to the next line.
  • Many lines (current paths) can be executed (activated) at the same time in electrical systems

Introduction to Ladder Logic

Ladder logic is a graphical programming language, which means that instead of text, programming is done by combining different graphical elements. These graphic elements are called ladder logic symbols.

One of the clever things about ladder logic symbols is that they are made to look like electrical symbols. Ladder logic was originally created for people with electrical experience. People who are used to looking at electrical diagrams and schematics.

As in electrical diagrams, ladder logic has symbols for contacts and relays called coils in ladder. The symbols may look a little different from those found on electrical diagrams, but they have almost the same functions.

Example ladder logic symbols.

You can take a look at the IECuino available tools ⚒️ to know more about the online ladder editor.

1 thought on “Relay Ladder Logic”

  1. Pingback: Ladder Logic Basics I - Online Ladder Editor for Arduino - Still Beta 😉

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