The Engineering Projects

Designing Logic Gates in PLC Simulator

Hello friends, I hope you all are doing great. In today's tutorial, we are going to design logic gates in PLC Simulator. It's our 4th tutorial in Ladder Logic Programming Series. We come today to elaborate the logic gates with comprehensive details for their importance in PLC programming. you can consider logic gates as the building blocks of ladder logic programming. Like every time we start with telling what you guys are going to have after completing this session? For those who like to buy their time and calculate for feasibility, I’d like to say by completing this article, you are going to know everything about what types of logic gates, how they are designed and how they work, how you can translate the logic in your head into the logic gate and some about logic calculation which is so-called logic algebra, and for sure the connection with examples between logic gates and the Ladder logic programming. In our previous tutorial, we have Created First Project using Ladder Logic, where we designed a simple logic by using contact and coil. Today, we are going to extend that code and will design different logic gates in ladder logic.

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:

Logic gates

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.

Truth table

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.

Basics of logic gate

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.

Simulating ANR, OR, and NOT logic

• The AND, OR, and NOT logic are considered the basic building block logic for designing the complicated logic to decide the output status.
• By using two switches A and B and one output representing lamp or MOTOR, we can design and program these logics and simulate them on the PLCSIM simulator.
• Table 1 lists the truth table of the three logic AND, OR, and NOT.

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

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.

Table 1: the truth table of “AND” logic gate

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. 2: sample ladder logic rung for “AND” logic [2]

Fig. 3: The timing diagram of the “AND” logic gate

AND logic in PLC simulator

• Let us once more enjoy learning further by validating and practicing on the simulator, here you can see in figure 19, on the right the AND logic has been programmed by connecting two switches A and B in series.
• The motor status is the result of the AND logic between the two switches.
• On the left, you can see the results of the simulation by setting the status of switches to simulate all truth table conditions and see the motor status changed accordingly.
• In addition, you can see the truth table of the AND logic on the most right of the figure. So you can review and validate what is going on in the simulator.

Figure 19: Simulating AND logic

The “OR” Logic Gate

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.

Table 2: The truth table of the “OR” gate

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

OR logic in PLC Simulator

• You can see in figure 20, on the right the OR logic has been established and programmed by connecting two switches A and B in parallel.
• The motor status is the result of the OR logic between the two switches.
• On the left, you can see the results of the simulation by setting the status of switches to simulate all truth table conditions of the OR logic and see the motor status charged accordingly.
• In addition, you can see the truth table on the most right of the figure. So you can review and validate what is going on in the simulator.

Figure 20: Simulating OR logic

The “NOT” logic gate

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.

Fig. 7: The symbol of the “NOT” logic gate [1]

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.

Not logic in PLC Simulator

• Also, the NOT logic is one of the primary logic functions, you can see in figure 21, on the right the NOT logic has been designed and programmed by connecting switches A in negative logic in series with the motor.
• The motor status is the result of the NOT logic of switch A. On the left, you can see the results of the simulation by setting the status of the switch to simulate the two-state of the NOT logic truth table and see the motor status charged accordingly.
• In addition, you can see the truth table on the most right of the figure. So you can review and validate what is going on in the simulator.

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.

Latching output

• Figure 22 depicts the latching technique that we simply use to keep the motor running by pressing the input push button and having it keep running even after releasing the button.
• As you can see, I have used the Output as a Virtual Input and placed it in parallel with actual input.

Figure 22: Latching output

• Table 2 lists the First three scan cycles to show the sequence of operations and how the latching process works when someone will press the Input.
• In the first scan cycle, when the input gets HIGH, the plc will scan the input "Run (I0.0)" and will find it pressed/ON and thus will make the output "Motor (Q0.0)" HIGH.
• In the second scan cycle, the input "Run (I0.0)" turned off after being released, but the motor contact is still ON from the previous scan cycle.
• So, the compiler won't change the status of the OUTPUT and we can say it's latching the output.

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

• Now let’s add a way to terminate the latching and stop the motor as per request.
• Well! Simply figure 23 shows a stop button is added for terminating the latching condition.
• So in table 2, the RLO for letting the motor running will be unfulfilled by hitting the stop push button in the third scan cycle.

Figure 23: latching with stop button

Simulation of the latching in ladder logic

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.

Latching Ladder code simulation

• Now let’s try our latching ladder program in the PLCSIM simulator, by entering our ladder logic and starting the simulator.
• Figure 24 shows the first four scan cycles. Notice on the left we can set the inputs on and off and see the effects on the right part.
• In the first scan, every single input and output is at its initial state, so the output is not energized.
• In the next scan cycle, you can notice we switch on input at I0.0 which is the start push button.
• Therefore, the motor has been started and running. In the third scan cycle, the start button is switched back off.
• However, the motor still runs thanks to the latching technique. WOW, we can see our logic is working as we designed for.
• In the last scan cycle, we tried to test stop latching by hitting the stop pushbutton and indeed it stopped latching and the motor stop running.

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.

Latching using set and reset

Let’s use another approach for latching which is based on using set and reset coil. Figure 25 shows the set and reset methods.

• By hitting set_valve at address I0.2, the valve at Q0.0 will be set ON until a reset command is present by hitting the reset_valve pushbutton at I0.3.
• It is very easy but you need to take extra care while using set and reset as the last set/reset command will overwrite the previous commands.
• But wait, what’s if an operator keeps pressing the rest or set button for a long time or if the pushbuttons are the stuck and same thing for the stop button.

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

The signal edges

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.

Creating the First Ladder Logic Program in PLC Simulator

Hello friends, I hope you all are doing great. In today's tutorial, I am going to create the first Ladder Logic Program in PLC Simulator. It's 3rd tutorial in our Ladder Logic Programming Series. In our previous tutorial, we have installed PLC Simulator and now we can say our lab is ready to learn and practice. So let us get to work and get familiar with the ladder logic components.

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.

Ladder Logic Contact/Input

• In ladder logic programming, a contact represents the input of the system and it could be a button press by the operator or a signal from the sensor.
• Examples of contacts are toggle switches, pushbuttons, limit switches, sensors like level, pressure, proximity switches et cetera.
• There are two types of contacts normally used, which are:
  1. Normally Open Contact.
  2. Normally Closed Contact.

1. Normally Open Contact

• A normally open contact is Open/LOW by default and it gets Closed/HIGH by pressing or getting signal from any external source i.e. sensors.
• As shown in the first row of figure 1, the contact is open or disconnected by default and then the operator turns it to closed or connected status, shown in the second row.

Figure 1: Normally Open (NO) contact [1]

• Let's understand it with its equivalent electrical circuit, imagine you wire a switch in series to a lamp as in figure 2.
• After you complete wiring and connect L1 to the hotline and L2 to the neutral.
• See that at the start the lamp is off until you come and press the pushbutton then it is turned on.
• So, here the switch is acting as a normally open switch.

Figure 2: Normally open contact or switch in a circuit [2]

 

2. Normally Closed Contact

• A normally closed contact is at HIGH/Closed state by default and gets Low/Open if pressed by the operator.
• Figure 3 shows the symbol of normally close contact.
• So it flows current at the very beginning and disconnects the current flow by being pressed by the operator to become like an open circuit or contact.

Figure 3: Normally Closed (NC) contact

• For elaborating the behavior, let us wire a circuit that is depicted in figure 4.
• The contact is connected in series with a lamp to convey the current and let it turn on.
• So initially, the lamp started in ON status when the contact is not activated by the user.
• And, when the operator activates the contact it turns off.
• So, the switch is acting as a normally closed switch.

Figure 4: Normally close contact or switch in a circuit [2]

 

Ladder Logic Coil/Output

• The coil in ladder logic represents the actuator or the equipment we aim to turn on or off.
• A good example of a coil is a lamp and motor.
• Typically it is located at the most right side of the ladder logic rung.
• Same as contact has two types based on the initial state and the next state after user activation, also the coil comes in two forms which are:
  1. Normally Active Coil
  2. Normally Inactive Coil as shown in figure 5.
• An inactive coil is normally not energized until it gets connected by connecting the left side to the hot wire thanks to a contact.
• In contrast, active or negated coil type comes initially On status or energized and turned off when the left side is connected to the hot wire.

Figure 6: active and inactive coil

 

Create First Ladder Logic program

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.

Creating a new project on TIA Portal

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.

• You now get in the Lab by opening the TIA portal and hitting create a new project as shown in Figure 7.
• On the right, you just need to name your project like “new project” and you may leave the default location of projects or alter the data to the project file location as you prefer.

Figure 7: Creating a new project on TIA portal software

• You will have to select a PLC controller whom we are going to use. So you simply select one PLC controller as shown in figure 8 and click okay.

Figure 8: adding PLC controller

• The wizard now goes on asking you to add a program block.
• You can see in Figure 9, the default program block is the Main block which has the main program and other blocks are additional blocks.
• So for now let us go with the essential requirements for our program which is the main block and you just double click on the Main block to go to the next step.

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

Writing First program on the TIA Portal

• WOW! You are a superb learner as I can see you can follow figure 11 and by dragging a contact and dropping it on the blue line, you added a start button of normally open (NO) contact type.
• For identifying contacts and coils, the compiler assigns a unique name & address to each component and can recognize it anywhere in the program.
• Therefore, you just set the address and name for every component you add to your rung.
• The address of components has a specific format that is very logical and easy to understand.
• For example, the contact address “I0.0”, the first character is “I” which denotes input and it is followed by the number of the input module in the rack that holds all inputs and outputs modules.
• Then a number of the input channel as each input module has many channels.
• For instance, an eight channels input module can have numbers from 0 to 7 while 16 channels input module can have numbers from 0 to 15.
• A period is used to separate between the number of input modules and the channel number.
• So by set address I0.0, this refers to the very first channel in the first input module in a PLC rack.
• In addition, a name is used as a tag to easily identify the input i.e. “start” to refer to a start switch.
• Similarly, you add a stop button of the type normally closed (NC) with address I0.1 which means the second input channel in the first input module.
• Furthermore, you double-clicked the coil for the motor and set address Q0.0 which means the first output channel in the first module.
• I know you wonder what is “Q”? Yes! “Q for denoting output like “I” is denoting an input. Well done!. Now let us enjoy simulating the very first code you just have done yourself.

Figure 11: writing the first ladder logic program

 

Compiling Ladder Logic Program

• Like any programming language, the first thing to do after writing a program is to compile, to make sure it is free of error and ready to be downloaded into the PLC controller to run.
• Figure 12 shows the very simple steps to compile your program by clicking the compile icon in the toolbar which is highlighted in yellow.
• And you can notice in the lowest window below the results of compilation in blue showing that the code is free of error and warnings.

Figure 12: compiling ladder logic program

• To let you imagine how the compiler can help you to find the error location and type, we have done one mistake in the code and compiled as shown in figure 13.
• You can notice that compiler is telling the rung that has the issue which is network 1.
• In addition, the message clarifies the error by telling, you missed the required data for operand which is the address of input is missing.

Figure 13: Example of an error in compilation

Simulating First ladder logic program

• After compiling our program successfully, now the next step is to download it to the PLC controller.
• Yes for sure our simulator will act as the plc controller.
• So, by clicking the simulator button on the toolbar, the simulator window comes out and also another window to download the program to the controller as shown in figure 14.

Figure 14: calling simulator and downloading program

• You simply hit the “start search” button to search for the connected PLC controller.
• In our case, the simulator will appear in the search results.
• So, you just select it and click load to proceed with the wizard of downloading your program as shown in figure 15.

Figure 15: the wizard of downloading the ladder program to plc controller

• By reaching the last screen and clicking finished you have downloaded your first program to the simulator.
• Well done! And let's move forward with simulating our first program to validate our code and enjoy tracing the logic behavior same as a real-time 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.

Simulating our first PLC Program

• After downloading the program and pressing the run button on the very small window of the PLCSIM simulator, we can notice the run/stop indicator turned on in green showing the running status of the PLC as shown in figure 16.
• Now, click on the monitor icon on the toolbar highlighted in yellow on the most right of figure 16, you can notice the rung shows every status of each contact and coil in our program.
• I am very happy to reach this point at which you can see the normally closed contact is showing a green connection as we described above and the normally open contact showing disconnect status and can not wait until the operator press it down to connect and energize the output.
• But how do we press the buttons or switches when we are simulating? There is no physical switches or button to press!!! No friends that are not the case. Let us see how that can happen thanks to the great simulator that we have between our hands.

Figure 16: Simulating the first PLC code

Simulating the operator behavior

• This section is more than exciting, it shows you how the simulator not only does imitate the PLC controller but also it has the facility to imitate devices, switches, push buttons besides showing outputs’ status and values.
• In addition, we will go further in plc programming to show the series and parallel connections of contacts in branches and utilize simple logic AND, OR, NOT to form simple and complicated logics.
• The first way to set inputs on and off is by right-clicking on any contact and modifying the status to 0 or 1 as shown in figure 17.

Figure 17: forcing the inputs on and off

• The other way is to go to the expert mode of the full functional simulator, by hitting the which icon on the very small simulator window.
• A full version of the simulator control window will open up, where you can add inputs and outputs on the right as you can see in figure 18(left side).
• You can notice the inputs have an option in form of a check button to set it on or off.
• As a result, the contact will be turned into the selected status and the program perform according to the new status and the designed logic of your program as shown in figure 18 on the right side.
• It shows the output coil is turned to true status and highlighted in green.
• At this point, I would like to thank you my friends to follow up on our PLC tutorial series and let us move forward to learn further and do more practice with our simulating lab.

Figure 18: operating using simulator full control window

 

What’s next

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.