This is the equivalent of a parallel connection in an electric circuit. The output is then connected in line with the rung. For basic ladder logic programming we express binary events using normally open contacts NC and normally closed contacts NC. The five basic, yet essential, logic functions in ladder logic are:. You may be surprised, but when we utilize all these functions in our ladder logic programming we will be able to program the majority of automation control requirements.
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Why is Ladder Logic Popular? 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. Why is a ladder diagram used for PLC programming? How to Draw Ladder Logic Diagrams Ladder logic diagrams are drawn in a similar way to relay logic circuit. 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.
The tag names are the descriptions allocated to the addresses. Comments — Last but by not least, the comments are an extremely important part of a ladder diagram. Comments are displayed at the start of each rung and are used to describe the logical expressions and control operations that the rung, or groups of rungs, are executing.
Understanding ladder diagrams is made a lot easier by using comments. Parts of a Ladder Logic Diagram. What is Ladder Logic? Learn all about ladder logic. They are big and use lots of electricity, generating lots of heat in the process. In modern-day control terms, the elevator is a simple application that is well understood. Even so, a control system implementation in relays was a big, complicated affair. Other drawing conventions that survived into the software version include:.
Finally, each of the outputs is on a separate horizontal line or, in our ladder metaphor, rung. Another convention is that we always descend the ladder — we work from top to bottom.
Output contacts on real-life relays are usually called normally open and normally closed. Here is our English sentence written in ladder logic. Ladder Logic. With so much love and excitement,. The RealPars Team. Search for:. By David Richardson.
Automation Engineer. Posted on Aug 20, Checkout your email. Have questions? Sign in. People who are used to look at electrical diagrams and schematics. Just as in electrical diagrams ladder logic have symbols for contacts and relays which are called coils in ladder logic.
The symbols may look a little different from the ones you find in electrical schematics, but they have almost the same functions. This is a great read to level-up your understanding of PLCs. Knowledge is power. Another difference between ladder logic diagrams and electrical schematics is the way they are drawn.
Where electrical schematics are often drawn horizontal, ladder logic diagrams are drawn vertically. The best explanations for drawing ladder logic vertical instead of horizontal I can give you are these:. First of all, it makes ladder logic easier to read because it is natural for the eye to go from the left to right and then down to the next line.
Just like when you are reading. Of course, this applies only to people living in countries where the reading is done from left to right.
When you draw ladder logic on a computer you will make one line at a time. As you draw more and more lines in ladder logic called rungs they will stack on top of each other, making up what looks like a ladder. The best way to look at a large ladder diagram with many lines is to scroll vertically along with the screen.
The last reason for drawing ladder logic vertically is to set the order of execution. The order of execution is how the PLC will run your ladder logic. To be more precise in what order your ladder logic instructions will be executed by the PLC.
A PLC will always start at the top of your ladder logic and then execute its way down. As I said before ladder diagrams can look a lot like electrical schematics going vertical. Most people learn to draw ladder logic diagrams this way — by building them as electrical schematics.
But there are some differences. This is why I will advise you to learn it in a different way. The problem here is that electrical control systems and the PLC work in different ways.
Here are the biggest differences:. The first thing you will see when you create a new piece of ladder logic is two vertical lines. It is in between these two lines your ladder logic goes. When you draw ladder logic, you will draw vertical connections between these two lines. Each of those is called a rung. Just like on a physical ladder. Ladder Logic with Horizontal Lines called Rungs. In these rungs, you can put any of the ladder logic symbols to create the logic you want.
As you can see above, I have put numbers on each rung. This is to understand how the PLC hardware will execute the ladder logic. You may be familiar with the PLC scan time or scan cycle. Roughly said, the PLC will first scan all its inputs, then execute the program to set outputs. This might be one of the most important rules of ladder logic. The PLC only executes one rung at a time, then executes the next.
In fact, the PLC only executes one symbol at a time. Each symbol in ladder logic is an instruction. This can, in the beginning, be rather confusing. I will explain this with simple examples. Let me start by giving you a simple example. In this first example, you will be introduced to the two first ladder logic symbols. They are basically logic instructions, that make you able to create a piece of logic.
That piece of logic is your ladder logic or PLC program. If you take a closer look at the example below, you will see two instructions symbols. You can check out my video tutorial, and see how the basic PLC instructions work. I would still recommend you to finish this tutorial anyway since the video only gives you basic coverage. The first instruction here is called examine if closed.
The symbol for the instruction looks like this:. This is a conditional instruction. It means that you can use it to check if something is true. For example, check if a bit is on. This is the address of the specific bit, this instruction will examine.
In this case, a digital input. It could also just be an internal memory bit or even an output. Examine if closed is also known as normally open. It works basically the same way as a normally open contact in en electrical circuit. Of course, the normally open contact has no memory bit as a condition. The condition is whether the contact is activated or not.
So the condition could be a finger pressing a button. Yes, inputs and outputs are also bits of memory in the PLC. In the example above, the examine if closed instruction has been given memory address I0. This address belongs to the first input of the PLC.
The instruction itself even has a place in the PLC memory. What the PLC will put there is the result of the instruction. To see what the PLC uses that result for, we have to look at the next instruction:. As you can see, the symbol is placed on the right side of the rung. This means, that all the instructions that come before in the same rung act as a condition for that instruction. In our example that will be the result of the examination if closed instruction.
How ladder logic instructions work with the PLC scan cycle. In the animation above you can see that the PLC first scans all its inputs. The states of these inputs are then saved in a memory byte.
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