Ladder Diagram Example A manual mixing operation is to be automated using sequential process control methods. The process composed of three steps: a.) filling a tank to a predetermined level b.) agitating the liquid for 30 minutes c.) draining the tank for use in another part of process Does the ladder logic schematic that follows perform. SolidWorks Electrical Schematics. SolidWorks Electrical Schematics is a professional drawing software for collaborative diagram and design tools which can be used to make complete electrical systems. With the help of the symbol library, 3D manufacturing parts, and component models, you can create a pro electric drawing using this software.

Ladder logic is a programming language that is used to program a PLC (Programmable Logic Controller). It is a graphical PLC programming language which expresses logic operations with symbolic notation using ladder diagrams, much like the rails and rungs of a traditional relay logic circuit.

Ladder logic is a fast and simple way of creating logic expressions for a PLC in order to automate repetitive machine tasks and sequences. It is used in a multitude of industrial automation applications. Some industrial automation application examples where PLC ladder logic is used include….

Material Handling Conveyor System.
Pallet Packing and Strapping.
Ball Mill Lubrication System.
Logistics Package Conveying and Sorting.
Cement Batching.
Beverage Bottling and Labeling.
Hopper and Tank Level Control.
Air and Liquid Flow and Pressure Control.

In the good ol’ days, machine and process automation was accomplished using a hard wired control system known as relay logic. With the advent of microprocessors and the invention of the PLC, relay logic quickly became superseded by programming languages such as ladder logic.

Why is Ladder Logic Popular?

Ladder logic is the most popular method of PLC programming because it is a graphics based programming language that resembles electrical schematic drawing. Engineers, electricians and students that are already familiar with electrical schematic drawings find making the transition from an electric circuit to ladder logic relatively easy, compared to other text based programming languages.

When programming ladder logic in a PLC, the graphic, drag and drop nature of ladder diagrams helps you formulate code quickly and easily. Ladder logic also helps you easily trouble shoot your code because you can visually see the flow of logic from the LHS start rail, through the logic symbols and to the RHS end rail.

Learning the Basics of Ladder Logic

It’s relatively easy to learn the basic concepts of ladder logic programming, even if you don’t have experience with electric circuits. Take comfort in knowing that ladder logic is the quickest and easiest PLC programming language to learn.

In order to help you learn the basics of ladder logic we will cover the following….

Introduce the ladder diagram.
Examine the seven basic parts of a ladder diagram.
Identify the binary and logic concepts used in ladder logic.
Reveal the hidden ladder logic functions that are automatically built into the structure of the ladder diagram.
Discover the five fundamental logic functions that are essential to know.

So let’s begin….

What is a Ladder Diagram in a PLC?

A ladder diagram is the symbolic representation of the control logic used for ladder logic programming of a PLC. Ladder diagrams have horizontal lines of control logic called rungs and vertical lines at the start and end of each rung called rails. The structure of a ladder diagram actually looks like a ladder, hence the name “ladder diagram”.

There are two differences between an electrical schematic and aladder diagram. The first difference is the control logic in an electricalschematic is represented using components whereas in a ladder diagram symbolsare used. The second difference is the control logic execution in an electricalschematic is as per the operation of an electrical circuit whereas in a ladderdiagram it relies on the methodical nature of the PLC scan.

Why is a ladder diagram used for PLC programming?

The reason why ladder diagrams are used for PLC programming is because theearly control system designers were accustomed to relay logic control circuits andladder diagrams closely mimic these. They preferred to use ladder diagramsinstead using text based programming languages ofthe day like C, BASIC, Pascal and FORTRON. The other reason ladderdiagrams are used is because factory maintenance staff already understand howto read relay control circuits so using ladder diagrams for programming a PLC meantthey were easily able to troubleshooting control system problems.

Ladder diagrams help you to formulate the logic expressions in graphical form that are required to program a PLC. They represent conditional, input and output expressions as symbols. So writing a PLC program using ladder diagrams is similar to drawing a relay control circuit.

Ladder Diagram (LD) is the official name given in the international PLC programming standard IEC-61131. But, these days the terms ladder diagram, ladder logic diagram, ladder drawing, ladder control, ladder circuit, control logic diagram and logic diagram (to name a few) are all used to describe relay logic circuits and ladder logic programming.

So don’t get too caught up in the specific definition of each of these expressions, they kind of generally all mean the same thing. At the end of the day most people will know what you are talking about anyway. Personally, I use the term ladder logic for PLC programming and relay logic for relay control circuits.

To access the body responsible for maintaining the IEC-61131 standard please click here.

How to Draw Ladder Logic Diagrams

Ladder logic diagrams are drawn in a similar way to relay logic circuit. However, ladder logic diagrams express logic operations using symbolic notation rather than circuit components.

The rails in a relay logic circuit represent the supply wires of a relay logic control circuit. However, in ladder diagrams, the rails represent the start and end of each line of symbolic code.

The rungs in a relay logic circuit represent the wires that connect the components together. However, in a ladder diagrams, the rungs represent the logic flow through the symbolic code.

When implementing a ladder logic program in a PLC there are seven basic parts of a ladder diagram that critical to know. They are rails, rungs, inputs, outputs, logic expressions, address notation/tag names and comments. Some of these elements are essential and others are optional.

To help understand how to draw ladder logic diagrams the seven basic parts of a ladder diagram are detailed below…..

Rails – There are two rails in a ladder diagram which are drawn as vertical lines running down the far most ends of the page. If they were in a relay logic circuit they would represent the active and zero volt connections of the power supply where the power flow goes from the left hand side to the right hand side.
Rungs – The rungs are drawn as horizontal lines and connect the rails to the logic expressions. If they were in a relay logic circuit they would represent the wires that connect the power supply to the switching and relay components. Each rung is numbered in ascending sequential order.
Inputs – The inputs are external control actions such as a push button being pressed or a limit switch being triggered. The inputs are actually hardwired to the PLC terminals and represented in the ladder diagram by a normally open (NO) or normally closed (NC) contact symbol.
Outputs – The outputs are external devices that being are turned on and off such as an electric motor or a solenoid valve. The outputs are also hardwired to the PLC terminals and are represented in the ladder diagram by a relay coil symbol.
Logic Expressions – The logic expressions are used in combination with the inputs and outputs to formulate the desired control operations.

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