PLC scan cycle with Ladder Logic Part II

What is Ladder Logic?

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Ladder Logic (also known as Ladder Diagram or LD) is a programming language used to program a PLC (Programmable Logic Controller). It is a PLC graphical programming language that uses ladder diagrams to express logical operations with symbolic notation, much like the rails and rungs of a typical relay logic circuit.  Ladder logic is made by rungs of logic, which form what looks like a ladder, hence the name “Ladder Logic”.

Ladder logic is mostly used for bit logic operations. In more complex PLC programs and particularly in SCADA system programming, even basic bit logic operations may sometimes be of use.

In the IECuino online ladder platform you can use the blocks to draw your diagram

For PLCs, ladder logic is not just a programming language. It is one of the most commonly used and generic languages for programming PLCs. This essentially suggests that a norm represents ladder logic. IEC 61131-3 is the name of the standard.

There are the best examples for drawing ladder logic vertically rather than horizontally:

  • Easier to read: Firstly, it allows ladder logic to be easier to read because going from left to right and then down to the next line is normal for the eye. Just like when you are reading. This refers, of course, only to individuals coming from countries where reading is carried out from left to right.
  • Drawn on a device: You can generate one line at a time as you draw ladder logic on a screen. They will stack on top of each other as you draw more and more lines (called rungs in ladder logic), making up what seems like a ladder. The easiest way to look at a large ladder diagram of several lines is to scroll vertically along with the device.
  • Execution Command: Setting the order of execution is the last justification for drawing ladder logic vertically. The order of deployment is how your ladder logic will be run by the PLC. To be more accurate, the PLC will execute the ladder logic instructions in sequence. At the top of the logic of your ladder, a PLC will still start and then run its way down.

A programming language used to program a PLC Arduino Industrial
A programming language used to program a PLC

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.
But certain variations still remain.
The issue here is that the electrical control systems and the PLC work in numerous ways. Here are the biggest distinctions:

  • One ladder logic line (rung) is taken by the PLC and executed 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 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 technicians, electricians, and 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 (which are called coils in ladder logic). The symbols may look a little different from those found on electrical diagrams, but they have almost the same functions.

The symbols may look a little different than those found on electrical diagrams, but they have almost the same functions. As we can see the example of IECuino

Example of different symbols for Ladder Programming with Arduino