Ladder Logic – What Is It?

Ladder Logic – What Is It?

12th August 2014

At first glance, the phrase “ladder logic” seems simple enough – you‘re probably thinking that it means using ladders in a safe and logical manner, right?  No – wrong, time to think again.  

Originally, ladder logic was the term used to describe a written method of documenting the design and construction of relay racks used in manufacturing and process control.  Each individual device in the relay rack would be portrayed by a symbol on a ladder diagram that showed the connections between those devices.  Other items that were actually external to the relay rack (such as heaters, pumps, etc.) would also show on the ladder diagram.

The dawn of the computer age saw ladder logic evolve into a programming language that represents a program with a graphical diagram based on the circuit diagrams of the relay logic hardware.  This ladder logic is used to create software for programmable logic controllers (PLCs) used in industrial control applications.  It derives its name because the programs in this language look like ladders with two vertical rails and a series of horizontal “rungs” between them.  Although in the beginning ladder diagrams were the only available notation for recording programmable controller programs, today other forms are standardized in the international standard IEC 61131.

External Access LaddersLadder logic systems operate within manufacturing equipment and in some of the more sophisticated electronic devices.  PLCs offer several advantages such as being able to shut down power without using a switch.  They are easy to troubleshoot which means that mechanical downtime is reduced, leading to more cost effective operation.

Because ladder logic was designed to behave in the same way as relay logic (an older programming language that was widespread), engineers and computer technicians did not need to retrain or take additional certification courses in order to use it.  This meant that facilities already using ladder logic could remain on schedule.

Ladder logic control systems are able to operate fairly complex automated hardware functions like the circuits that control a bank vault door or an electric motor.  Ladder logic controls these types of systems in the most cost effective manner making them the ideal choice for modern hardware components that are constructed in the factory.

Because the circuits created using ladder logic are simple, they are more reliable than more intricate electronic components.  This has led to an increase in the life of products (whether mechanical parts or electric outlets) that incorporate ladder logic systems.

Ladder logic programming is flexible and versatile which means that engineers are able to rapidly adapt parameters to function over additional systems.

A programmable logic control program can be tested, validated and corrected in a lab which saves valuable time in the development stage.

When running a PLC programme, a visual operation can be seen on the screen, making troubleshooting a circuit a quick and simple undertaking.