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This article mainly explains "how to realize the ladder diagram of PLC programming". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Next let the editor to take you to learn "PLC programming how to achieve the ladder diagram" bar!

What is a ladder diagram?

Ladder diagram is a PLC programming language, also known as ladder logic (Ladder Logic). It is called a ladder diagram because the program is made up of horizontal lines that look like ladders.

Ladder diagram is invented for electrical engineers, it is a graphical programming language, which means that the programming process is not the use of text, but the combination of different graphical symbols, because the target user is an electrical engineer, so the symbols used are very similar to circuit symbols for electrical engineers to understand.

Ladder diagram is mainly used for bit logic operation, and its specification is made by PLCOpen, so ladder diagram is a standardized PLC programming language, which is IEC 61131-3.

How to read the ladder diagram

One of the differences between ladder diagrams and circuit diagrams lies in the different methods of compiling and drawing. Ladder diagrams are usually compiled from left to right and from top to bottom because:

1. It's easier to read pictures.

Our eyes naturally read the picture from left to right and then move on to the next line, just as you do when you read.

two。 It is easy to program on computer.

When compiling a ladder diagram on a computer, you can compile one line at a time. As more and more rows are drawn, they will be superimposed to look like a ladder. The best way to view a ladder diagram with a large number of rows is to scroll up and down the screen.

3. Order of execution

The final reason is the order of execution, that is, the order in which the PLC runs the ladder diagram, or more specifically, how the instructions in the ladder diagram are executed in turn: PLC always starts at the top of the ladder diagram and then executes downward.

The ladder diagram looks very much like a circuit diagram. This is how most people begin to learn to make ladder logic diagrams. But there are still some differences, the main differences are:

PLC executes one line of the ladder diagram each time before continuing to the next line

In electrical systems, many circuits are activated at the same time.

Now that we understand these key differences, let's start learning ladder logic.

Ladder diagram foundation

When you create a ladder diagram, the first thing you see is two vertical lines, that is, the ladder diagram is compiled between the two vertical lines, each horizontal line is called a Rung (horizontal line):

Trapezoidal logic symbols are placed on these horizontal lines, and as you can see in the figure above, I marked numbers on each horizontal line to understand how PLC performs trapezoidal logic. You may have heard of the PLC scan time or scan cycle. Simply put, PLC first scans its input, then executes the program, and finally sets the output.

But how does PLC execute our trapezoidal logic?

One horizontal line at a time.

This is probably the most important rule of trapezoidal logic: PLC can only execute one horizontal line at a time, and then the next one. In fact, PLC can only execute one logical symbol at a time.

Write trapezoidal logic

Each symbol in trapezoidal logic is an instruction, which seems rather confusing at first glance. But don't worry, I'll explain it with simple examples. Let me give you a simple example in which two trapezoidal logic symbols will be introduced.

So, what exactly are these symbols or instructions?

They are basic logic instructions that allow you to create a small piece of logic, that is, your PLC program. If you look closely at the following example, you can see two instructions (symbols):

Closed circuit inspection instruction

The first instruction is called closed-circuit check, and the symbol of the instruction looks like this:

This is a conditional instruction, which means that it is used to check whether a condition is met, such as whether a data bit is in the state of ON.

The closed-circuit check instruction is used to check the location of an address. In the figure above, it refers to a special location of a digital input, which can also be a memory bit or even an output bit.

It is important to note that each closed-circuit check instruction must set an address in the PLC.

Both input and output are memory points in PLC. In the above example, the closed-circuit judgment instruction sets the address I0.0 as a condition, which belongs to the first input of the PLC.

Its working principle is as follows:

When the PLC scan cycle starts, PLC first checks the status of all inputs and then writes the input status (0 or 1) to memory. If the input is LOW, the corresponding memory location is 0, and if the input is HIGH, the corresponding memory location is 1.

Output coil instruction

Each instruction itself has a location in PLC memory, and PLC stores the result of the instruction. To understand what PLC does with the results, let's look at the next instruction:

The output coil instruction is used to turn on or off a data bit.

In our example, the previous instruction is a closed-circuit check instruction.

Let's examine the final result of the directive to understand how it works:

PLC scan | enter-> I0 bytes

Program execution | I0.0-> Xic result

In the above animation, you will see that PLC first scans all input and stores the input state in memory. A memory byte is 8 bits adjacent to each other.

Once PLC has saved the state of all inputs, the program will start running. The first instruction to be executed is a closed-circuit check instruction, the result of which is consistent with the memory bit state-- which is why the instruction is called a normally open instruction-- in the normal state (memory bit is 0), the contact will remain normally open and the result will be 0, but if the memory bit is 1, the contact will be closed and the result will become 1.

Finally, let's look at the output of this line:

XiC result-> output coil

Output coil-> output byte

Now, the output coil instruction uses the result of the preorder instruction as a condition. This is called RLO (logical operation result). The results of logical operations are saved in a specific location in PLC memory. In the Siemens S7 series PLC, this position is called the status word.

In PLC terminology, a WORD is 16 bits, or 2 bytes.

The output coil instruction is simple, it just sets its result to the same value as the condition.

All digital output in PLC is also mapped to memory addresses. We call it the output bit, so the address Q0 corresponds to Q0.0-Q0.7. The result of the output coil instruction will be written to the memory bit Q0.0.

When PLC finishes executing the entire program, it sets the output. Each output is set to a state consistent with the output memory bits.

The concept of scanning cycle is very important and must be kept in mind when you are compiling trapezoidal logic. Otherwise, your program may behave strangely. We will demonstrate this in the next example, introducing three new trapezoidal logic instructions.

Output latch

In the previous example, we learned how to read the state of the digital input and set the digital output to the same state. You need to point out that the digital input is a transient button because it has a spring inside, which means that the button will remain active only if you keep pressing it.

The ladder diagram above works, but you may notice that the output is activated only when the input is active. So you don't have to press the button all the time without using your fingers to keep the output active. But come to think of it, if the output controls a fan in a ventilation system, it would be unreasonable to require the operator to press the button all the time. We need a way to keep the output active, even if the operator has released the button.

In trapezoidal logic, there are two ways to do this:

If you are familiar with circuits, you will find that this is similar. This cup is called Latching or self holding.

The name reveals how it works: the coil simply maintains its state of the previous scan cycle. Let's take a step-by-step analysis:

Interesting things happen when the logic is run later. Because this is a transient button, it will not activate all the time. Depending on the total running time of the PLC program, the button may no longer be activated on the second, third, or fourth run.

Let's enter the first scan cycle after the button is released.

The output is still active because the previous scan cycle button was pressed. At this point, PLC will read the input again and store it in the corresponding memory bit. The memory bit I0.0 "will be stored in" 0 "this time. Therefore, the result of the closed-circuit judgment instruction of i0.0 is false or" 0 ".

But as you can see, there is another parallel closed-circuit judgment instruction, but the condition of this instruction is to output memory bits, so the result is true or "1" because the output is still active. As long as the output memory bit is "1", the output will be activated, just like its own condition.

The reason why the self-hold instruction is in parallel with other instructions is to construct an OR condition, in which either I0.0 or Q0.0 is true will activate the output.

Open circuit check instruction

You have just learned how to write a useful PLC ladder diagram program. A button to activate the output. In our example, this may be a contact connected to a fan and the output can be self-sustaining.

But there is a problem with this program. How to turn off the fan?

We hope to turn off the fan again. The easiest way is to add a stop button that connects to the second input of PLC, so its memory address is I0.1.

The question is, what instructions do we use for the stop button? More importantly, where should we put it on the ladder diagram?

The answer to the first question is another trapezoidal logic instruction: the open circuit check instruction, which looks like this:

This instruction works exactly the opposite of the closed-circuit inspection instruction, and the result is a reversal of the condition. This means that if the condition is "0", the result is "1" and vice versa.

If you think about it, this is exactly what we want the stop button to do. To close the output coil, we must give the condition "0".

Now comes the second question, where to put this instruction?

We need to put it after the self-locking instruction, or rather, concatenate it. Otherwise, "1" will still be output when the stop button is pressed.

Now the trapezoidal logic is as follows:

You can see that the open circuit check instruction passes the reversal result of its condition to the output coil. To activate the output again, you need to press the start button again.

In the above example, I used an open circuit determination instruction as the stop button. This is not good practice!

We finally follow the best practices, and the modified ladder diagram is as follows:

At this point, I believe that everyone on the "PLC programming how to achieve the ladder diagram" have a deeper understanding, might as well to the actual operation of it! Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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