Working in ladder logic programs is getting further complicated especially for large-scale projects. So, it is very beneficial to know shortcuts for writing ladder logic simpler, more readable, and easier for others to develop. Using this variety of logic gates, enrich the logic and fluency of writing the logic in different situations. For example, using XOR logic is very common for comparing two inputs to decide if they are equal or different. Therefore, we found that it is crucial to go through all logic gates and do simulations for them all in this tutorial. for coherently we get the knowledge to use them fluently in ladder logic programming to solve different logical problems and scenarios. There are seven basic logic gates which are: “AND”, “OR”, “NOT”, “NAND”, “NOR”, “XOR”, and “XNOR”. We have gone through “AND”, “OR”, and “NOT” logic gates including simulation work. So let us go through the other four gates for learning and practicing all basic logic gates.
The “NAND” logic gate is the invert of the “AND” gate like you invert the output of an “AND” logic gate as shown in Fig. 12. Table 5 lists all combinations of the inputs and the output of the “NAND” logic gate.
Fig. 12: NAND symbol
Table 5: the truth table of the “NAND” logic gate
| Input A | Input B | Output |
| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
In addition, the timing diagram of a “NAND” logic gate is shown in fig. 13, it shows the output goes low when both of the inputs A and B are true which is the inverse of the “AND” logic gate. Also, Fig. 14 shows a ladder logic of a “NAND” logic.
Fig. 13: The timing diagram of the “NAND” logic gate
Fig. 14: Ladder logic sample of a “NAND” logic
The NAND logic gate can be considered as the invert of the “AND” logic gate. So as listed in table 1, the truth table of AND and NAND logic gates shows how the NAND gate is the reverse of the AND gate. Also, it shows the NAND gate should come out to your mind when you want the output always low except when both inputs A and B are high.
The NAND gate can be implemented by connecting the negate of the output in series to the inputs A and B. Another way to implement NAND is by inverting both inputs and connecting them in parallel as shown in fig. 1. Rung 1 and rung 2 respectively.
Fig. 1: The NAND ladder logic in two ways
Let’s test our ladder logic in both methods. According to the truth table of NAND gate, we have four test cases. figure 2 shows the first test case when both inputs A and B are false. In this case, the output should be true as shown in fig. 1.
Fig. 2: NAND ladder logic when both inputs are false
figure 3 shows the second test case when inputs A goes true while B is false. In this case, the output should be true as shown in fig. 3.
Figure 4 shows the third test case when inputs B goes high while B is low. In this case, the output should be true as shown in fig. 4.
Fig. 4: NAND ladder logic when input A is true and input B is false
Figure 5 shows the last test case when both inputs A and B become high so the output goes false as shown in fig. 5.
Fig. 5: NAND ladder logic when both inputs are true
Thanks to performing the simulation of the NAND gate, we now can conclude that we should be looking for a NAND logic when we want to shut down an actuator i.e. motor whenever two inputs are in a true logical state simultaneously. For a practical example, when we have three pumps and we want to run them in the mode of two of three. There should be only two of the three pumps to run at any time. In that case scenario, the run condition of any motor can be a NAND of the status of the other two motors.
The “NOR” logic gate receives two inputs and has one output. It is the same as the invert of the “OR” logic gate. Like you follow the output of an “OR” gate by a “NOT” logic gate. Fig. 15 shows the symbol of a “NOR” gate. In addition, the truth table is expressed in table 6. It shows the output becomes false only when one of the input A or input B or both goes high which is the reverse logic of the “OR” logic gate.
The “NOR” logic gate can be formed by connecting the “OR” logic gate to the inverter “NOT” logic gate. Or by inverting the inputs by using “NOT” logic gates and connecting them to the “AND” logic gate as shown in Fig. 16.
Fig. 16: structure of “NOR” logic gate
Table 6: the truth table of the “NOR” gate
| Input A | Input B | Output |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 0 |
Figure 17 shows an example of a ladder program for implementing the “NOR” logic gate. It shows that the “NOR” gate can be implemented in ladder logic by connecting two contacts of type NC in series.
Fig. 17: A sample ladder for a “NOR” logic gate
On the other hand, the timing diagram of the “NOR” logic gate is depicted in Fig. 18. It shows the output is false as long as either input A or input B or both are true.
This logic gate can be considered as the negate of OR as you can notice in the truth table as listed in table 2. You can now feel when we may need to use the NOR logic? Yes! Exactly you want it when you design for output which is all time off except when both inputs are false.
Table 2: the truth table of NOR versus OR
| Input A | Input B | OR | NOR |
| 0 | 0 | 0 | 1 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 1 | 0 |
Figure 6 shows two ways to implement the NOR logic in ladder logic. To make that happen, we connect the invert of the two inputs in series to the output as shown in the top part of fig. 6. The other way is to connect the two inputs in parallel to form OR and then connect to negate the output as shown in the lower part of fig. 6.
Let’s practice simulation of the NOR gate, in fig. 7, the first test case is when both inputs are false, the output is true as shown in fig. 7.
Fig. 7: The Simulation of NOR ladder when both inputs are false
Figure 8 shows the second case when input A is true and input B is false, the output is false as shown in fig. 8.
Fig. 8: The Simulation of NOR ladder when input A is true and input B is false
Figure 9 shows the third test case when input B is true and input A is false, the output is false as shown in fig. 9.
Fig. 9: the Simulation of NOR ladder when input B is true and input A is false
Figure 10 shows the last test case of NOR ladder logic, it shows the output is false
One practical example of using the NOR logic is that, imagine friends we drive some machine with two motors. And it is required to have at least one of them or both are running all the time otherwise alarm should be energized. The NOR logic is the best to manage that alarm to get energized if and only if both motors are off.
Despite this logic gate having two inputs and one output like the “AND” and “OR” logic gates, this logic gate is a bit more complicated than the previous logic gates. Table 4 lists the truth table including all combinations of the inputs and the output. By noticing the truth table of the XOR logic gate, you can see the output becomes low when the two inputs are equal like both are high or both are low. But the output goes high when there is a difference in the state of the two inputs. Imagine my friend, how much this logic gate is very beneficial for comparing two signals.
| Input A | Input B | Output |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
On the other hand, Figure 10 shows the symbol of the XOR logic gate and its schematic. See how the basic logic gates OR, AND, and NOT can be utilized to build the logic of XOR logic gate.
Fig. 10: The XOR logic gate symbol
Figure 11 shows a sample of a ladder logic program that implements XOR logic. In addition, it shows the timing diagram of the inputs and output, it shows the output goes high when there is a difference between the two inputs and becomes low when they are equal i.e. both are low or both are high.
Fig 11: Sample of the ladder logic for XOR logic and the timing diagram
The XOR is used to compare two signals if they are equal or different. Table 3 lists the truth table of the XOR. It shows that the output comes to true when inputs are different and becomes false when they are equal.
Table 3: The truth table of XOR logic
Figure 11 shows the construction of XOR ladder logic. It shows that it is composed of, two parallel branches and each branch is forming AND logic of the two inputs in the opposite logical state.
Figure 12, shows the simulation results of XOR when input A and B are false, the output is false.
Figure 13, shows the simulation results of XOR when input A is true and input B is false, the output is true.
Figure 14, shows the simulation results of XOR when input B is true and input A is false, the output is true.
Figure 15, shows the simulation results of XOR when input A is true and input B is true, the output is false.
In a conclusion, the XOR logic in simulation shows that the output is low whenever both inputs are equal and goes high when the inputs are different in the logical state. A very good practical example for utilizing the XOR logic is that imagine friends we have a motor that is energized by two different destinations, and it should be requested by only one at a time. So we can get the run signal from the XOR of the two input switches. So, the only case to run the motor is by requesting from one source.
This logic gate is the invert of the XOR gate. So it is equivalent to applying an inverter to the “XOR” logic gate. Table 7 lists the combination of its two inputs and its output. It shows clearly that, the output becomes true when inputs are equal i.e. both inputs are true or both are false.
| Input A | Input B | Output |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
Fig. 20 shows the symbol of the “XNOR” logic gate, it shows clearly how it is the invert of the XOR logic gate. This logic gate is very useful to validate if two signals are equal or not.
Fig. 20: The symbol of the “XNOR” logic gate
On the other hand, Fig. 21 shows a sample ladder logic of an “XNOR” logic gate implementation. It shows that there are only two ways to have the output in the TRUE state which are by setting both inputs TRUE or set both FALSE.
Fig. 22 depicts the timing diagram of the inputs and output of the “XNOR” logic gate and clearly shows the output goes high when both inputs have the same state.
Fig. 22: the timing diagram of the “XNOR” logic gate
The XNOR is the invert of the XOR and it is used to compare two input signals. Table 4 lists the cases of the truth table of XNOR logic. It shows the output goes high when both inputs are equal i.e. both are high or both of them are low.
Figure 16 shows the construction of XNOR ladder logic. It shows that it is composed of, two parallel branches and each branch is forming AND logic of the two inputs in the same logical state.
Figure 17, shows the simulation results of XNOR when input A and B are equal i.e. both are false, the output is high.
Fig. 17: The Simulation of XNOR ladder when both inputs are false
Figure 18, shows the simulation results of XNOR when input A is true and input B is false i.e. they are different. So the output goes false.
Fig. 18: The Simulation of XNOR ladder when input A is false and input B is high
Figure 19, shows the simulation results of XNOR when input A is false and input B is true i.e. they are different. So the output goes false.
Fig. 19: The Simulation of the XNOR ladder when input B is true and input A is false.
In the last case when both inputs A and B are high as shown in Fig. 20, the output becomes true.
Fig. 20: The Simulation of XNOR ladder when both inputs are true
We can conclude that the XNOR is marked by the true status of its output whenever both inputs are equal and vice versa. One of the most common scenarios for the best practice of XNOR is the protection of the operator's hands in the cutting machine. In that machine, the command for running the knife driving motor is an XNOR of two switches on the left and right hand of the operator. In that way, it is guaranteed that to run the motor, the operator should use both hands at the same time.
I am very pleased to see you up to this point of our tutorial, Now you are familiar with all logic gates and practiced their logic on the simulator. In addition, you can feel the importance of mastering all these logic gates to ease your programming and enrich your programming skills. In the next tutorial, we are going to go deeply through the edge signal including rising and falling edge. We are going to introduce the benefits of these edge signals and how they can be utilized in ladder logic programming to solve a lot of problems. So be ready for more learning and practice with simulation in ladder logic series.
We are discussing these logic gates because they are the main building block of complicated logic. Normally, complex logic is designed using multiple logic gates. So, today, we will simulate the basic logic gates i.e. AND, OR, and NOT, while in the next lecture, we will simulate NAND, NOR, XOR and XNOR in PLC Simulator. So, let's get started:
In very simple language, it is a Boolean decision that has one of only two values either “TRUE” or “FALSE”, not both. For instance, the decision to run or shut down a motor, open or close a valve etc. Well! For deciding such Boolean nature thing, there are two things, inputs and logic to apply on those inputs. On the other way, logic gates apply some sort of logic to the inputs to determine the state of the output.
It’s a table that lists all possible combinations of the inputs and the state of the output for each record. For example, a gate with two inputs has four possible combinations of the inputs and four states of the output. inputs.
There are seven basic logic gates. Some of them have only one input while others have two inputs. There are seven basic logic gates which are “AND”, “OR”, “NOT”, “NOR”, “XOR”, “XNOR”, and “NAND”. So let us enjoy a short journey with them having a fast stop at each one’s station. Our trip will include, how they work, design, timing diagram, and connection with ladder logic programming.
Table 1: Truth table of the AND, OR, NOT logic
| Switch A | Switch B | Motor |
| AND LOGIC | ||
| 0 | 0 | 0 |
| 1 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 1 | 1 |
| OR LOGIC | ||
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
| NOT LOGIC | ||
| Switch | Output | |
| 0 | 1 | |
| 1 | 0 | |
The “AND” logic gate has two inputs and one output. Like its name, the only condition for having the output become true, is by having both inputs, input A and input B are true. Table 1 lists the truth table of the “AND” gate and Fig. 1 images the symbol of the “AND” gate. In addition, Fig. 2 shows a sample of ladder logic rung that uses “AND” gate logic. It decides the status of the motor based on two switches. The two switches must be in true status for running the motor. ‘to sum up, the logic of the “AND” gate, is that, the output comes to true when and only when both inputs A and B are true.
| Input A | Input B | Output |
| False | False | False |
| True | False | False |
| False | True | False |
| True | True | True |
Fig. 1: symbol of “AND” logic gate [1]
In the ladder logic rung shown in Fig. 2, there are two contacts I1 and I2, they are of normally open (NO) type, these two contacts are connected in series, so the only way to set the output to true is that both contacts I1 and I2 must set to true. For full imagination, please notice the timing diagram of the inputs and output signals shown in Fig. 3. It shows the output is only high when both inputs are high.
Fig. 3: The timing diagram of the “AND” logic gate
Figure 19: Simulating AND logic
This logic gate has two inputs and one output like the “AND” gate. Like its name, the output comes true when either input A or input B comes true as shown in Fig. 4.
Fig. 4: The symbol of “OR” logic gate [1]
Table 2 lists the truth table of the “OR” gate. It lists all possible combinations of inputs and the output status as well. It shows that the output comes to true when input A or input B comes to true.
| Input A | Input B | Output |
| False | False | False |
| True | False | True |
| False | True | True |
| True | True | True |
Figure 5 shows an example of a ladder logic rung that implements the “OR” logic. We can implement this by connecting two inputs I1 and I2 in parallel branches and to the output. like this way of connection, the output can be set to true by simply setting I1 or I2 or both true. Once more, let us see the timing diagram in fig. 6, it is clearly shown that the output goes high as long as either one or both of the inputs are true.
Fig. 5: sample ladder logic rung for “OR” logic [2]
Fig. 6: the timing diagram of the “OR” logic gate
Figure 20: Simulating OR logic
This logic gate has only one input and one output. In a very simple language, the output is the invert logic of the input. So when the input is true, the output would come to false and vise versa as shown in Fig. 7.
Table 3 lists the truth table rows of all possible combination of input and output.
Table 3: the truth table of the “NOT” logic gate
| Input | Output |
| True | False |
| False | True |
Figure 8 depicts a very simple example of a ladder logic rung that shows the output Q1 is the reverse logic of the input I1. In addition, Fig. 9 shows the timing diagram of input and output of the “NOT” logic gate. It shows clearly that, the output is the reverse of the input.
Fig. 8: Sample of the ladder logic rung representing “NOT” logic [2]
Fig. 9: The timing diagram of the NOT logic gate
Before going further with the logic gates, I want to let you know the good news that, you can implement any logic by using the aforementioned three logic gates “AND”, “OR”, and “NOT”. However, for simplification, the other logic gates are designed based on using these three logic gates in different topologies to perform a specific logic functions.
Figure 21: simulating Not logic
Now! I appreciate your follow-up to our PLC tutorial. I am very happy to feel that, by moving further in our plc tutorial our experience is getting increasing bit by bit. However, some questions may come to our mind like does the operator needs to keep pressing input like the push button to keep the motor running? What happens if he released it, does the motor stop? Well! By asking such questions, I can affirm you start your way to master PLC programming and its logic. And let me say the answer to your questions is yes the operator needs to keep pressing the input push-button until the motor has done its task. But that is not the best practice in the real life. There are other techniques to keep the motor running by one touch of the push button, thanks to latching, setting, and resetting techniques as we will show you in the next sections.
Table 2: The first three scan cycles of latching operation
| Scan cycle | Run (I0.0) | Motor status (Q0.0) | Motor coil (Q0.0) |
| 1 | 1 | 0 | 1 |
| 2 | 0 | 1 | 1 |
| 3 | 0 | 1 | 1 |
We may be sure of the logic we wrote for coding the ladder logic of the latching technique. However, at this point how about going to the simulation lab to work out our latch ladder logic program to enjoy validating our ladder code by putting it in the simulator and see how far it match what it is designed for.
Figure 24: simulation result of the first ladder program
We will concentrate on moving forward with ladder coding which is our target. However, we just tried to show you at any time you can validate your ladder at any point to enjoy and confirm you are on the right track as long as you are working on your project.
Let’s use another approach for latching which is based on using set and reset coil. Figure 25 shows the set and reset methods.
Well! The rational expectation is that the motor won’t be able to start. However, the good thing is there is a magic solution to differentiate between the situation of this is a normal stop request by the operator or the button is hold pressed unintentionally or due to an issue with the switches. The one-shot technique can magically recognize the event of pressing or releasing the pushbuttons. Therefore, when it is held for a long time or forever that is only one button press event and for triggering it needs to release and pressed once again. That’s amazing but how does it work? Well! Let’s go demonstrate the concept of how it works, implementation using ladder logic, and give an example to understand it consistently and enjoy the magic of one-shot action.
Figure 25: set and reset for easy latching output
Two edges happened when a pushbutton pressed and released which are falling edge and rising edge as shown in figure 26. It depicts the rising edge when the button is pressed and the falling edge when it has been released. Now, let's move to ladder logic, there are two equivalent rising and falling edge contacts that can be used to tell the PLC this is a one-shot signal. Figure 27 shows how the use of the rising edge of the reset pushbutton |P| at address I0.3. it shows that despite the reset being pressed, its effect in the moment of pressing and then it needs to be released and pressed again to reset the valve at Q0.1. in the next section, let’s get to business and work out one practical example which represents a real problem in the industry just to harvest the fruit of what we have learned so far.
Figure 26: The rising and falling edge [2]
Figure 27: The effects of one-shot technique in ladder logic
So, that was all for today. I hope you have enjoyed today's lecture. In the next tutorial, we will simulate Advance Logic Gates using Ladder Logic Programming. We will design NAND, NOR, XOR and XNOR gates in the next lecture. Thanks for reading.
After this article, you will have a complete understanding of PLC contact and coil including their types and possible causes. Because they are the building block of any rung of a ladder logic program. So let us start with ladder logic rung components.
Figure 1: Normally Open (NO) contact [1]
Figure 2: Normally open contact or switch in a circuit [2]
Figure 3: Normally Closed (NC) contact
Figure 4: Normally close contact or switch in a circuit [2]
Figure 6: active and inactive coil
To our fortune we no longer need wires and devices to practice what we have been learning together, thanks to the simulator, which we have installed in the previous lecture. Let's create a new project on TIA portal software and test it with the PLCSIM simulator.
As this is the first time to use our software to write and simulate a ladder logic code, let us go step by step creating our very first project on the TIA portal software.
Figure 7: Creating a new project on TIA portal software
Figure 8: adding PLC controller
Figure 9: adding program block
I just want to say well done! And congratulate you that you are now all set to start writing your first ladder logic rung as shown in Figure 10. It shows on the left the project components including hardware i.e. devices and controllers, networking devices, configurations, program blocks etc. The most important thing you need to know for now is the program blocks which contain the only main block and other blocks as the project needs. Now! please stare your eye toward the right to see the icon bar that contains every ladder symbol. You can see the contact of normally open and normally closed. Furthermore, you should see the coil and more which we are going to go into detail later in our upcoming articles of PLC tutorial.
Figure 10: starting writing ladder code
Figure 11: writing the first ladder logic program
Figure 12: compiling ladder logic program
Figure 13: Example of an error in compilation
Figure 14: calling simulator and downloading program
Figure 15: the wizard of downloading the ladder program to plc controller
But wait! Will you continue pressing the push button for our motor to keep running? For sure No, there should be a way to let it keep running by just hitting the button thanks to the latching technique.
Figure 16: Simulating the first PLC code
Figure 17: forcing the inputs on and off
Figure 18: operating using simulator full control window
Now, how do you see your progress so far? I can see you have just completed the most basics of ladder logic programming. You are now very familiar with the ladder basic components, using the editor to write a ladder logic program, simulate your work for verifying your logic correctness. So you are doing progressively and that’s great to hear that. However, we still have a lot to learn to master ladder logic programming. For example, using blocks like timers, counters, mathematical blocks, data comparison etc. So we hope you have enjoyed what we have reached so far in our PLC tutorial and please get yourself ready for the next part. In the next part, you will learn about types of Timers and how you set their configuration and how you utilize them to achieve the timing-based tasks accurately.
As PLC is an Industrial Controller, it comes with built-in relays/transistors(with protection circuitry) and thus is quite expensive as compared to microcontrollers/microprocessors i.e. Arduino, Raspberry Pi etc. Moreover, if you are working on a real PLC, you need to do some wiring in order to operate it. So, in order to avoid these PLC issues at the beginning, instead of buying a PLC one should work on a PLC Simulator. Using PLC Simulator, we can program our PLC controller and imitate its real behavior without having the hardware, saving both time and money as now we don't need to buy a new PLC and can start right away.
To sum up, by completing this article you will have a complete lab that includes the software you are going to use, the simulator that plays as the hardware, and certainly, you will be familiar with installing a PLC programming environment by which you can program, configure, moving the program to the PLC hardware, retrieving the program from the PLC to the software environment, and testing your program on the simulator. In addition, we are going to test our environment setup with a very basic program and take the chance to show you how to program, configure, upload, and test your program on the simulator.
As I mentioned in the last tutorial, we are going to work on Siemens PLC throughout this tutorial, as it's one of the most common PLC controllers. So, we are going to install PLC Environment designed by Siemens and is called Total integrated automation (TIA). Along with this software, we will also need to install a PLC Simulator called S7 PLCSIM, again designed by Siemens. At the time of this writing, their most stable versions are 15.1, so download these two applications from below links:
After downloading the TIA and the simulator, we extract the package by double click on the file we downloaded, and then it will be self-extracted and initiate the setup wizard as shown in the below figure. The image shows many steps. Moving our eyes from left to right, on the first part, the downloaded package has been extracted. In the next part of the picture in the middle, the setup wizard gets started by general settings in which you can set the preferred language and select the preferred installation location. The third part shows the setup goes on progressively and takes you to the end of the installation of the software IDE. Congratulation! You know have the programming software IDE installed on your computer and the good news is, all packages of Siemens go with the same scenario, you download the software package files. Click them to be extracted. And then, the installation wizard is launched by the end of file extraction which is a very systematic and easy way.
Figure 2: TIA portal version 15.1 setup wizard
Well done so far! After having the programming software IDE completed, the next step is to install the simulator package which is PLCSIM version 15.1. Download PLCSim from the above link and then double-click the downloaded file of the simulator package as shown in Figure 3 to start extracting the packed file. You will be asked for the language and the location you prefer to have the installation folder. So you can leave it as the default or go with your preferences.
Figure 3: PLC simulator PLCSIM version 15.1 package extraction
After file extraction has been completed, the setup wizard will start automatically as shown in Figure 4 with the general setting screen by which you can set the preferred language and the location to install the simulator software. So you can use the default setting or update with your preferred choices.
Figure 4: the simulator setup general settings
Figure 5 shows the simulator setup configurations screen which helps you to configure and customize your installation. In this configuration screen, you can go with the typical options of installation in which all software components are selected to be installed or you can customize your installation to select or deselect components of the package. And by hitting the next button of this window, the installation will go on as shown in figure 6 until the end of the installation. During the installation progress, Siemens show off the features you may find in the software and the facilities you will enjoy by using this software. At the end of the installation, the wizard will request you to restart your computer now or later for completing the setup wizard by saving settings and registry values related to the installed software as shown in figure 7. That’s great! As for now, you have everything is ready and you are all set to get started and enjoy practice and learning the ladder logic programming and simulating your work.
Figure 5: the simulator setup configurations
Figure 6: the simulator installation screen
Figure 7: Simulator setup completion screen
Before going any further let us check the successfulness of the installation process of the software and PLC simulator. Simply go to start and open TIA portal 15 and S7-PLCSIM you will see the software opening with no problem as shown in Figures 8 and 9. In figure 8, you can see options to create a new project or open an existing project. Also, there is an option to migrate projects from one version to another version by upgrading or downgrading the version of the projects. In addition, you can enjoy the welcome tour to know about the software programming tools and be familiar with its components. In addition, there is an option to check the installed software to validate the packages you select to include within your installation. for any further information you can click help to search and inquiry about any doubts.
Figure 8: Opening the TIA 15 software for testing installation successfulness
Moving to the S7 PLCSIM simulator software, as you can see in Figure 9, it is a very smart and simple interface. It shows a power button by clicking it you can shutdown the PLC controller or turn it on. Also, all indicators like the real PLC controller are included. You can see the RUN/STOP indicator. In addition, the ERROR indicator blinks red for any faults with the CPU of the PLC. In addition, you can see the run and stop buttons to start and stop the controller. Also, the MRES button to reset the PLC to the default values at any time. In addition, there is a detailed interface of the S7 PLCSIM simulator as shown in Figure 10. You can launch the detailed or maximized interface of the simulator by hitting the top-right icon on the shortcut window version of the simulator.
Figure 9: Opening the PLC simulator PLCSIM 15 for testing installation successfulness
The shortcut or the small version of the simulator shows the basic functions of the simulator like starting and stopping the controller or resetting the PLC and showing the status of the controller i.e. Run, stop, in fault status. But, the maximized or the detailed window simulator interface shows more options and facilities of the simulator. For example, you can create a simulation project to link it with a PLC project. Showing status of all input and output channels on the Input and output modules. In addition, it enables you to set and reset any of the inputs as we will elaborate in detail later in the next articles.
Figure 10: Opening the PLC simulator PLCSIM 15 in detail mode
After completing the installation successfully of the programming tools software TIA version 15.1 and the simulator PLCSIM version 15.1, it should be validated to make sure all components are installed and working properly. Let us validate by going through the functions and wizard. You now get in the Lab by opening the TIA portal and hitting create a new project as shown in Figure 11. On the right, you just need to name your project like for example let it be “first_ladder_prog” and you may leave the default location of projects or alter the data to the project file location as you prefer.
Figure 11: Creating a new project on the TIA portal
By hitting create, the creating project wizard comes out as shown in figure 12. As you can see you have many options to do on this screen like configuring the hardware, designing visualizations by designing and programming a human-machine interface (HMI) screen, motion control, or writing a ladder logic program. As for now do not worry about all these options as they are all not our scope in this series except those are relating to ladder logic programming like writing program option and configuring PLC device and network which we will come to them later in the next articles. For testing the installed software, you can simply select write a plc program for now.
Figure 12: creating project wizard
By choosing to write a PLC program, the wizard takes us to add the controller on which we are going to run the designed program as shown in Figure 13. If you are not familiar with the type of PLC controller models and hardware for now. That is not an issue because we are here to learn Ladder logic programming which is general for most PLC controllers of all brands i.e. Siemens, Schneider, Rockwell Automation, Allen Bradley, Beckhoff, WAGO, et cetera. So, for now, let us for testing purposes select S7-1200 which is one of Siemens PLC controllers to use in our project. By hitting the yellow small cross icon to add a device. You will see the list of the Siemens controllers that have appeared. For each controller, you can see many versions. Each version represents firmware for example, by selecting S7-1200 CPU 1211C AC/DC/RLY, you will see three versions. Each version represents a specific controller CPU in the market i.e. the selected one if of firmware ver 4.2 as shown in figure 14. By seeing this long list of CPUs and models, that means the software has been installed successfully and is ready to be used in our projects. So congratulation for successfully setting up the working environment for our Lab of ladder logic programming and being ready for utilizing this environment including programming software TIA 15.1 and simulator S7 PLCSIM version 15.1 in our learning and practice.
Figure 13: adding PLC controller wizard
Figure 14: selecting S7-1200 CPU v4.2
Ladder Logic Programming has been derived from relay logic electrical circuits. This language of PLC programming is called ladder logic or LD as it looks like a ladder of many rungs. Each rung represents a line of logic by connecting inputs logically to form a condition on which the output is determined to be on or off. By completing this article, you will have enjoyed understanding the basics of the LD programming language. Consequently, you will have been able to read a ladder logic code and translate the logic in your mind or the electrical circuit between your hand into ladder rungs.
Relay logic control was the old fashion classic control in which input switches and sensors are connected between the hot voltage and relays’ coils to energize these coils and in turn, activate their contacts and thereby connect or disconnect the actuators i.e., motors, lamps, valves etc. You cannot imagine how complicated that control was besides its limitation in functionalities. In addition, it has a huge number of wires and components to achieve a simple logic. Furthermore, there are no chances to change the logic or the sequence of operation without destroying, rebuilding, and rewiring everything from scratch. When it comes to troubleshooting and maintenance, you should be very patient and generous for your time and efforts to pay to keep tracking hundreds of wires and checking up a bunch of components to figure out the problem.
To image the difference between old fashion classic control and PLC, figure 1 shows the case of a very simple process that contains four motors, and four sensors that are connected via four relays and a timer for performing a very simple logic. Let’s say we need to run motor 1 at the start and after a while, we plan to run motor 2 and 4, and at last run motor 3 considering one constraint that no more than two motors can run simultaneously. In addition, sensors will be used as protection to emergency stop motors at any time. Now on the left of the figure, you can see the relay logic control. you can imagine how many components and punch of wiring work for connecting sensors, timers, relays, and motors. On the other hand, PLC-based control shown on the right, you can notice only the PLC and input devices and output actuators are connected to the PLC. To sum up, the number of wires is reduced significantly, the effort of wiring is immensely reduced. In addition, when it comes to modification of logic or process sequence you need to destroy all old wiring and start over rewiring according to the new requirements while you can do this by modifying the program in PLC without touching the wiring. To sum up, PLC reduced the number of components, wires, time of implementation. In addition, the processing is faster thanks to PLC processing and modification becomes programmatically.
Before opening the door to enter our tutorial on ladder logic programming, let’s have a brief idea about how PLC works and what ladder logic program has to do with PLC. Well! Let me briefly say that PLC has input modules, output modules, and a processor. The input modules are connected to input switches and sensors while the output modules are connected to actuators i.e., motors, valves, lamps. And for sure processor runs the ladder logic program that we going to learn here! In the below figure, I have tried to clear it visually.
The processor works in scan cycles, in each scan cycle, it gazes into all inputs and records them in its memory and then executes the ladder logic to determine the new status of outputs and update them and go to the next cycle, and so on. So now you can tell me what ladder logic has to do with this? That is great to hear you say that it is the logical connection between inputs to determine the output status. I really can not wait to go ahead and hit the nails on the head and open the door to let us get started with our tutorial about ladder logic programming. In the below figure, I have shown PLCs of different manufacturers:
Each programming language has a structure and building blocks. The building block of a ladder program is a rung. Yes, rungs of a ladder go step by step to do the designed logic and repeat every scan cycle. Each rung forms a complete piece of logic like one complete circuit. so let us go to know how to form this rung and get to know what components of these rungs and how they are connected logically.
In the above section, we have designed a simple ladder logic program, where we have used just a single input to control our output. These contacts/inputs in their two configurations can be connected in series (AND logic), parallel (OR logic), or negation (NOT) to form logical combinations. In the below figure, I have designed three rungs of Ladder Logic, let's understand them one by one:
In the below figure, you can see a slightly complex Ladder Logic Program, having different components, let's discuss them one by one:
If we define the ladder and its building blocks which are rungs how about the word “logic” the second word in ladder logic? Well! Table 1 reviews the logic gates and their truth table. For you to expect the incoming status of your output based on the status of the inputs and the logical combination pattern. to decide the results logic output ( RLO) which is the status of the logical combination of contacts that precede the output coil. For instance, the AND gate/logic function is applied between two or more inputs when all of the inputs should be true to get the output to be true. On the other hand, the OR gate/logic function between two or more inputs is used when we need at least one of the inputs to be true to get the output to be true.
Let’s see one simple example to understand the reading ladder logic program. Figure 3 shows a very simple complete ladder logic rung that connects a normally open pushbutton to start a motor and another normally closed pushbutton to stop the motor. You can see each component of the inputs and outputs should have an address to be uniquely identified by PLC. For instance:
Now, how do you see your progress so far? I can see you are doing progressively and that’s great. However, we still have a lot to learn to master ladder logic programming. However, we need to set up the environment for simulating a Ladder logic program to be able to validate our programs and enjoy seeing its execution typically as if we have a PLC controller and to verify how far our designed ladder performance matches the real-time execution on the simulator.
In today's post we are gonna implement few complex logical gates. Its not gonna be much difficult if you have the basic concepts. I am just pointing out few important points here. While implementing any gate in ladder logic, always consider rung as an electrical line having HIGH voltage at one end and LOW voltage at the other, while the inputs are simple switches. Voltage will be supplied to the output only when switch is closed i.e. input is HIGH, otherwise the output will remain OFF. You should also have a look at Introduction to Logic Gates.
You have seen in previous post, while implementing OR gate we have used a second switch in parallel which ends at the first rung so overall its a single rung having two inputs in parallel so input can come either from first switch or from second one. So, now let's start implementing some complex logical gates in Ladder Logic for PLC. Today, we are gonna implement these logic gates:
Ladder Logic is different from the usual programming language of Microcontrollers like Arduino, PIC Microcontroller etc. Microcontrollers programming usually compiled from top to bottom i.e. the compiler first capture the first statement and then moves downward till it reaches the end line but that's not the case with Ladder Logic Programming for PLC. In ladder logic, the compiler moves from left to right and it gets all the lines at the same time. It seems bit difficult to understand at first but be with me and you will get it at the end. :)
Ladder Logic is a programming language used for PLC as C for Microcontrollers. Ladder logic is a combination of rungs. Each rung is executed from left to right. For example, have a look at the below figure, a single rung of ladder logic is shown in it.
Today. I am gonna give an overview about PLC. We will have a look on basics i.e. what is PLC? Why we use PLC instead of microcontroller like Arduino or PIC Microcontroller? What's its advantages and disadvantages? I will try to cover all about the basics. After reading this tutorial, you must have a look at Introduction to Ladder Logic for P L C, Ladder Logic is programming language for PLCs.
There are different types of PLCs available in the market manufactured by different companies, so its impossible to cover all of them. In this tutorial, I am gonna discuss Fatek PLC as I have worked on it during my project. The model I have used is Fatek PLC Fbs-20MA. The reason I used this model because it was cheap and has enough input/output ports sufficient for my project. That's why I preferred it as its engineers' task to optimize the cost as well. Let's get started with PLC.
Its a basic question, which is normally asked by all the starters so I am gonna reply it first for the newbies.
What's inside PLC, which makes it so cool ? That's a good question and normally engineers wonder about it. PLC can be divided into 3 sections, which are as follows:
I think now you have the idea about PLC, so now I am getting started with PLC. I am gonna explain the functioning of Fatek PLC as I have used that one but if you are using another model of PC then no need to panic as all PLCs have same functionality. So, it doesn't matter which one you are using. If you check the below image then you will see I have marked three sections in it.
In the below image, I have indicated Section 4 and 5, these are the input/ output section. If you have a look at it closely then you can see there are two rows of screws, where you plug your wires for inputs and outputs and above them, they are also labelled with white color. So, it goes like that, first row of labelling is for first row of screws and second row is for second row of screws.
That's all for today. I hope you got the basic idea of Programmable logic controller and now its time to have a look at Introduction to Ladder Logic for P L C, ladder logic is programming language for PLC. Your feedback are warmly welcome. In the next tutorial, I am gonna cover about ladder logic and will show you how to program a PLC. Till then Take care and have fun.
