The output coil works in a simple way. It simply sets the bit to the same value as its condition RLO. In the PLC all the digital outputs are also assigned to bits in memory. The result of the output coil will be put in memory bit Q0. When the PLC has executed the whole program, it will set the outputs. The state of each output is set to the same state as the output bits. Otherwise, your program might act a bit strange. This will be illustrated in the next example. At the same time, you will also learn about 3 other ladder logic instructions.
In the previous example, you learned how to read the state of digital input and set a digital output to the same state. It is called momentary because it has a spring inside. This means, that the pushbutton will only be active as long as you press it. The ladder program above works just fine. But as you might have noticed, the output will only be active as long as the input is active.
You will have to hold your finger on the button to keep the output activated. It would not be very convenient for the operator to hold down the button all the time. We need a way to keep the output active, even though the operator releases the pushbutton.
If you are familiar with electrical schematics, you may find this familiar. This is called a latch or a self-hold. The name reveals how this works. The coil simply holds itself. When the PLC runs this ladder logic program the first time with the button pressed , the output will be activated. This is just like the example before.
The fun happens the second or third time the PLC runs the ladder logic. Since this is a momentary pushbutton, it will not be active for long. Depending on how long time the PLC takes to execute the program, the button might be deactivated again the second, third or fourth time. The output is still active since the pushbutton was pressed in the last scan cycle.
This time the PLC will, again, read the inputs and save them in the memory byte. In memory bit I0. The first examine if closed instruction with I0. But this one has the output memory bit as a condition.
It acts as a condition for itself. The reason that the self-holding instruction is put in parallel to the other instruction is to make it an OR condition. I will come back to that later. Important to know here is that either I0. You just learned how to make a functioning ladder PLC program.
A pushbutton that activates an output. In our example, this would be connected to a contactor giving supply to a fan. The output then holds itself. We want, somehow to be able to turn off the output again. The simplest way to do that would be to add a stop button. The button will be connected to the second input. Thereby giving it the memory address I0. To answer the first question, let me introduce you to another ladder logic instruction: examine if open. This instruction works the exact opposite way of the examine if closed instruction.
The result of this instruction will be an inverted condition. If you think about it, this is precisely how we want to stop button to work. We have to place it after the self-holding instruction. Said in another way — serial connected. You can see that it inverts the condition to the output coil. This will break the latch. To activate the latch again, the start button has to be pressed. You can read more about why you have to use normally closed contact for stop buttons in my article about it.
In short, it is to make sure that the system stops when a wire to the button breaks. Although we changed the instruction, the ladder will still work in the same way. Following the program, which shows a normally-open X1 contact in series with a Y1 coil, no "power" will be sent to the Y1 coil. Thus, the PLC's Y1 output remains de-energized, and the indicator lamp connected to it remains dark.
Any and all X1 contacts appearing in the program will assume the actuated non-normal state, as though they were relay contacts actuated by the energizing of a relay coil named "X1". In this case, energizing the X1 input will cause the normally- open X1 contact will "close," sending "power" to the Y1 coil. When the Y1 coil of the program "energizes," the real Y1 output will become energized, lighting up the lamp connected to it:.
Take for instance this next system, a motor start-stop control circuit:. The parallel Y1 contact will also "close," thus latching the "circuit" in an energized state:. Since PLC program elements are typically designed by single letters, I will call the internal control relay "C1" rather than "CR1" as would be customary in a relay control circuit:. To make the lamp turn off, we will have to actuate press all three switches, like this:. Open navigation menu.
Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks. Bestsellers Editors' Picks All Ebooks. Explore Audiobooks. Bestsellers Editors' Picks All audiobooks. Explore Magazines. Editors' Picks All magazines. Explore Podcasts All podcasts. Difficulty Beginner Intermediate Advanced. Explore Documents. PLC Training. Uploaded by Arun prakash. Did you find this document useful? Click OK. Example 1 — Motor Control Place the cursor below on line Vertical Drill Motor Motor up and down L2.
Place the cursor at the position you want to add a Timer. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel.
Carousel Previous. Carousel Next. What is Scribd? Explore Ebooks. Bestsellers Editors' Picks All Ebooks. Explore Audiobooks. Bestsellers Editors' Picks All audiobooks. Explore Magazines. Editors' Picks All magazines. The cycle then repeats itself for each part that comes down the line. Sequence of Operation Here is a more detailed explanation of the drilling process: When the machine starts, the stop gate lowers and the part is moved into position by the main conveyor.
Optical sensors photoeyes determine when the part is in place. When the part is positioned correctly, a clamp extends to hold the part in place. A sensor in the drill press spindle tells the PLC when the spindle has reached the end of its travel. After the hole is drilled, the spindle retracts, the clamp retracts, the stop gate is lifted and the part is carried out of the machine by the main conveyor.
Though the device name, such as PB1, would not show up on the actual station, it is a good idea to show them on your drawing. Do not skip this crucial step. The text in the fixed font is basically the information that you would see if you were looking at the monitor of the computer or a printout.
In actual practice, the fonts used in PLC software vary widely. For the purposes of this book, we want to easily differentiate the program logic from our explanations of the logic. For purposes of this manual, I have placed additional explanations between rungs. Use a title to name the program and include any general information. This type of bit is what we call an internal coil. It has no hardwired connection to the outside world. If the emergency stop is clear, and the machine guard is in place, and there is no system fault the operator may press the start button to set the latch.
Most of the time, the order of the bits in a rung doesn't matter. We could have rearranged any of the bits in this rung, though we would still have to put the latch around the Start pushbutton. The PLC wouldn't care and the output coil would still respond the same.
However, to make the rung easier to read, I try to place bits from left to right in order of importance. If the E-Stop is not cleared, then nothing else should matter anyway. Having the safety guard in place is more important than a system fault. Now, if those requirements have been met, we can press the start button. And we don't care about the stop button until we have pushed the start button. Note the instruction used for the input of PB3, the Stop System bit.
It may seem backwards at first, since a hard-wired circuit would use the normally closed contacts of the switch. The E-Stop and the guard limit switch are called "safety interlocks. Use additional contacts from the switches and wire them to inputs of the PLC so that it knows the machine is to be stopped, or has stopped. It is very important to label the bits properly. Arrange the verbs and nouns correctly.
If you do, the rung will read like a sentence. There are some simple rules that I always follow when I am writing a description for a bit: - Descriptions for bits portray an action.
We can then place just that bit in the rung that controls the spindle motor and know that we have met all the criteria to allow the spindle motor to run. The idea is to turn on all the pilot lights for a couple of seconds so you can verify that all the lights work. This feature is very handy when you are troubleshooting a machine. When the Emergency stop is first cleared, Timer 0 is started.
The result is that all the pilot lights will turn on for two seconds after the E-Stop is cleared. This bit is then used throughout the program.
You want to enable certain machine functions in Auto Mode, and disable some in Manual Mode, and vice versa. Notice how the System Running bit is used. If we lose that bit, such as when the emergency stop is pressed or the machine guard is opened, neither mode is valid. A manual mode is provided to allow ease of set-up. These photoeyes are positioned so that if they "see" a part, they will turn on the input.
A part will break the beam, the input will turn on and you know you have a part present.
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