PLC Programming Tutorial - Ladder Logic #2: Timers and Counters

What's up, automation nation? Welcome back to LMaster Tech. And in today's video, we're going to continue our tutorial series on ladder diagram, basic ladder programming for automation and controls engineering. Today, we're going to be looking at the most common types of timers and counters used in PLC programming. We're going to look at TO's which are timer on delay to F which is timer off delay. T NR which is often also abbreviated RTO or retentive timer in other platforms. And then we're also going to look at counters. Count up countdown. They're pretty much the same. So we're going to cover all of these today. We're going to be using ladder diagram. Of course the syntax is quite different but the concept is exactly the same if you're using these in structured text or another PLC programming language. This is meant as a standalone tutorial. So the concepts in here don't require prior knowledge of any of the past tutorials, but we are also compounding automation and controls and ladder programming knowledge as we go through this series. And as we dive into these tutorials, if you're really enjoying the content, be sure to leave a like on the video, subscribe to the channel, and if you want to help me grow faster, do more hardware related projects, feel free to check out my Patreon down below as well. So to start, as a reminder, my physical setup, uh, which we kind of covered in the last video, I have a 24volt power supply that is powering a Seaman Sematic S71200 PLC. It's running TIA portal version 17. We're covering ladder diagram, not Seaman's TIA portal. So the instructions that we're covering in this video will work for most platforms, but that's what I'm using. And then for demonstration purposes, I have a control box with uh two hand switches and four output lights that are going to be used for the examples in this video. So if you want to pause on this, my face is probably in the way. If you want to pause on this wiring schematic and build some similar uh test bench, go ahead. I'll link the PLC that I'm using in the description below the video. Okay, so let's dive right into it. Uh I'm going to go ahead and delete out everything I have in this code. Okay, so I've got all functional code out of this program. The only thing we're calling in main is the ladder tutorials binary function, which is what we're programming here, and it has no code in it. And then if I show all the PLC tags, all I have are my four output tags for my lights and my two input tags for my hand switches. I added those in the last video if you're curious. Um, but there's nothing on this PLC. So, now let's dive into the timer. And we're going to drop the uh empty box function in here. And we're just going to start with a to. The n is timer on delay. It is the most common uh timer. And honestly, you can do almost everything you need to do in code with this. So, we'll call it example to n. Um, let's do ex all caps just cuz I'm crazy like that. We're going to do an example timer on delay here. Okay. And it's kind of nice. It populates with the four parameters you need to know about for timers. There's on what condition do we want this timer to start. That's this in. Some platforms will call that en for enable. Some platforms will have in for input. But they're sort of universally on what conditions do you want to start the timer. Now PT process timer, preset timer, uh it doesn't really matter. That's your set point. Okay? It's called PT in almost every platform, but that's your set amount of time you want it to count up to, right? So, a uh a timer is going to be something that starts on these input conditions, counts up to a value, that's your PT, and then it when the timer finishes, that will actuate your output bit, which is just true, false, has the timer finished? And that's my Q. If you remember most European and international standards to avoid O's that look like zeros call outputs Q's but uh like Rockwell Allen Bradley uses an O there. They just say like hey you'll figure it out um for output but the Q is going to be what happens when the timer is done. And then the only other thing we haven't talked about here is this ET which is our elapsed time. So as the timer is counting up to that preset time value, it's continuously keeping track of how much time has passed in the elapse time field. So that's everything that makes up a timer object. And you might have seen when I made this to block, it created a database of this example timer. And that's because you have to create a little pocket of memory to store these variables. There has to be the input bit, the Q bit, the output bit, the preset timer, and the elapse timer stored somewhere. So that's living in a timer database that's called example timer. So let's say what do we want uh to have actuate this timer to have it start. Simplest thing we can do is just say when I turn hand switch one on. So while hand switch one is on. And if you missed the previous video on binary conditions, um this means that the whole time hand switch one is on, I want the timer counting up. So not just when I turn it on, which would be like this P, which would be the positive transition of it turning on, but the whole time hand switch one is on, I want this timer counting up. Now, we're not going to put anything in elapsed time because it's a purely internal thing that's keeping track of how long the timer has been active. But we'll put two two seconds in preset time. So we'll say 2,00 because the default unit is going to be milliseconds. But what you see when I hit enter is that it says t pound sign2s. Okay. So the seaman's PLC is taking that value inferring that I meant milliseconds and it's making it t pound 2S which is the seaman's PLC standard for a time parameter. in like Alan Bradley, it's just going to leave the millisecond value you punch in there. If I were to say 2 and a half seconds, if I were to type in 2500 here, it's going to give me this t pound sign 2 seconds underscore 500 milliseconds. It's honestly kind of annoying. Um, but you can even delete the s and the underscore and just say t pound sign 2500 milliseconds. That part doesn't matter too much because if you want, you can always just throw a millisecond value in there and TIA portal generally is going to translate it for you. Most PLC timers are expecting you to punch in a millisecond value there. So, let's go ahead and change it back to 2 seconds, 2,000 milliseconds. We're not going to put anything in the elapse time. And then we're just going to add when the output finishes, when the timer finishes, let's go ahead and turn on output light one. Okay, so I've got my box here. I think it looked pretty good last video just keeping it on screen. Let's download this to the PLC. We're going to hit these little glasses and play button which is live monitoring. And uh every PLC platform generally has some form of live monitoring the logic. So just make sure to hit the button for yours. And now we're going to look when I turn on hand switch one, we should see a timer start counting and after 2 seconds the light should turn on. Okay. And it's beautiful when it works. When I turn the hand switch off, it's going to turn off right away. And what you could see when I turn this on is the timer starts counting. Maybe you could see the elapsed time value starts counting up there. So elapsed time is the number on top that's counting up 0, 250 milliseconds, 1 second, 2 seconds until the light turns on. So that is the basic functionality of a timer on delay is conditions trigger it to start counting up to some preset time and when the preset time finishes you can use the output bit of that timer to turn on the light. Now an interesting derivation off of this but super useful if you want your timers and your commands for your timers in different spots you can reference this example timer.done done bit, you can reference that bit on a separate line of code. This will work exactly the same way. So in a really large ladder program, you might want to create a timer based on a certain input in one spot, but then use the output of that timer in another spot. You can do that with this uh sort of referencing the timer output elsewhere. So if I turn the hand switch on, you'll see the timer still counts up, the timer done bit will become true, and then the output light will still turn on. So you can reference that output light elsewhere. One additional thing to mention while we're here, you can look for a uh let's say greater than block uh greater than and we'll say get rid of the x timer.done bit here. We'll say now if x timer it uhelapsed time is greater than 1 second. Uh and now let's also say just to get tricky with it, let's say and the timer is not done. Okay. Then we want the light to turn on. This is us looking at the elapsed time and saying, "Okay, it the timer's been active for more than 1 second, but hasn't yet finished. That's when we want to turn on the light." So, we're creating a program that if it's working correctly, should turn on when the timer gets to 1 second. So, halfway finished, and then turns off after 1 second. So, let's take a look. Okay, that's kind of a cool example of using elapse time and the done bit and you kind of get a uh multi-state output out of that. So that's sort of every component of the basic to n timer on delay block. Now let's look at to this is going to go really fast because it's very similar. Okay, so let's go ahead and put this back just as the very simplest version which was just when the timer finishes, we'll turn output light one on. And now we'll start making something that instead is going to look at hand switch 2. And here's where it's kind of nice to have the box. We can leave all our previous timers in place. And now we're going to type in to okay timer off delay. And we'll do example to. Okay. Tof though timer off delay. And uh similarly, but to show that it's different, we'll make this one 3000. Okay. So we'll make this 3 seconds. It has the exact same in uh preset time, elapse time, and Q. It It is still a timer block. It has all of the same uh values, inputs, and outputs and uh variables as the TO. It's a timer block. Main difference between the TO and the TOF is that this output will become true as soon as the input conditions are true regardless of how long the preset time is. But they will stay active for a period of time after the input conditions are active and then go away after that timer. So timer off delay means outputs energize immediately when the input conditions are true and then the output is retained for a period of time afterwards. So timer off delay. Let's go ahead and send this one that's going to turn on output light two just so you guys can see exactly what that looks like. Okay, so I'm going to go ahead and trigger output uh hand switch two. And you can see the light turns on immediately. And now I'm going to turn hand switch 2 off. And now you can see the elapse time starts counting up. And 3 seconds later, output light 2 turns off. So if we were going to do something like we did a little bit earlier, which was use the elapse time. So if we do a greater than and we use the elapse time on our example to example to f.elapse time and we say that when it's greater than500 then we want to turn let's use output light three so we can leave output light 2 exactly how it is. When the timer the elapse timer is greater than 1500 we want uh output light three to turn on. So let's go ahead and send that. Right. So with that active, you can see the hand switch is uh not active right now, but output light three is on because the elapse time has hit 3 seconds. So the timer finished and elapse time has hit 3 seconds. Let's go ahead and cycle hand switch 2 real quick and talk about what's happening. You can see elapse time goes to zero and then when I turn the hand switch off, it starts counting up. It gets above 1500 and then it stays on. So, what you would actually want to do in this case is rather than looking for a lapse time uh greater than 1500 seconds, we'd want to look at it elapse time less than 1500 seconds. This is what'll mean when the timer starts counting, which means at the point the input conditions have fallen false after half of that countdown, that's when the light will turn on. So, let's go ahead and trigger hand switch two again. And here you can see both lights turn on at the same time. And they're going to stay on because elapse time stays zero the whole time the input conditions are true. And then when I trigger it off, you can see the timer starts counting up. When I hit 15 seconds, the red light goes away. Or maybe you can't cuz I moved the box out of the way. But you get the idea. The interesting thing here is elapsed time is zero the whole time that timer is energized because it's not counting. It's just saying the input conditions are currently true. Therefore, my output is currently true and my elapse time is zero. And then once the sort of falling edge of the input conditions are detected, the timer starts counting up. And that's when elapse time starts trending towards preset time. And once elapse time hits preset time, it stays at or just above preset time and output stays false. So to kind of count for a certain period of time after a condition has become false. TO is sort of wait for a trigger and then count up a period of time. All right, so let's move on from the TOF and look at what could really be considered kind of the third type of timer. The only other kind of timer really, which is a retentive timer. All right, let's go ahead and this in Seammens is a TR. In Alen Bradley, it's RTO or retentive timer. So, uh, we'll call it toner example. And there's a new bit on this one. There's reset. And that's because retentive timer is counting up while conditions are true. Um, but it's not going to forget that value. So if I have hand switch one true, this to elapse time will start counting up. But if I turn hand switch one back off before it reaches the preset time, elapse time goes back to zero. It resets by default every time input is no longer true. The same thing but in reverse for this tof. If I turn hand switch two off, this timer is going to start counting up and elapse time is going to start counting up. But if I turn hand switch two back on, it's going to forget how long its elapse time was and just go back to zero. Sometimes like if you might want over the span of a day you want to trigger some sort of clean in place or reset cycle every time uh something has happened a valve has been opened for a full hour or something like that you might want to retain time over time. So you might want to use a trigger that's like let's say um both hand switches being on no let's say both hand switches being off are going to be what's actually going to reset the timer. Um, but both hand switches being on will be what triggers the timer and then the timer being done. Okay, so let me write this and then re-explain it a little bit better. It's hard to write code and explain it super well um at the same time. But let's look at how a retentive timer works by using when both hand switches are true, this will count up. Let's give it a higher uh target value like 10,000 milliseconds or 10 seconds. And then we're going to do nothing with elapse time. We're just going to let it count up. And we're going to have this be what triggers output light number four. Okay. Uh and maybe yeah, we're going to have some of the other lights uh triggering during this time because I'm not going to delete those timers. But let's talk through what this is saying again real quick. This is saying when hand switch one and hand switch two are both active, count up. And we'll see elapse time counting up while both of them are active. But if I turn either of those two hand switches off, this is going to remember, it's going to retain the elapse time so far, and it's going to store that in elapse time until someone comes along and resets it. And the reset conditions I have are when both hand switches are off and this timer has already finished. So, basically, no matter what, this timer is going to hold on to how long both hand switches have been active until it hits 10 seconds, and then I can turn both hand switches off to get it to reset. Let's go ahead and download this and see it in practice. Okay, so output light four is this one that my index finger's on right here. I'm just going to keep the index finger on so you guys don't lose it amidst the other timers we already made. And I turn one hand switch on, the timer is not counting up yet. I turn the second hand switch on. It starts counting up. It gets to three seconds. And I turn one of the hand switches off. And you can see just above the timer block, it's holding on to 3 seconds and 441 milliseconds. Now I'm going to turn uh both of them back on. And I'm going to turn the other hand switch off this time. And you can see I got to 6.7 seconds, but it remembers how long it was active. Now I turn them both on. And I'm going to let it get up to 10 seconds. And now you can see it's done. And output light four turned on. And I'm going to turn one hand switch off. That didn't reset it. I'm going to turn the other hand switch off. That didn't reset it because my conditions to reset are both hand switches being off and the timer done. So now if I turn both hand switches off, you see output light 4 turns off. Okay. So, a retentive timer is sort of uh like I was describing, useful for the total amount of time something was active being true. If you have a heating jacket or a cooling jacket and the total heating time is the thing that matters, but it could start and stop and start and stop numerous times during a batch, but you need to make sure that something is mixing for a minimum of 1 hour or something like that. A retentive timer can be a really useful tool. Now, I'm going to go ahead and delete all three of these networks and showcase one more thing that is closely related to these. So, I think it's super useful to cover here. Um, and I'm going to delete all these networks and we're going to do a counter. Okay, so a counter is not necessarily uh a timer because you're counting up just total conditions and the number of times a condition has been met, but it's not time bound. So, for example, let's go ahead and say I want to count by using the positive rising edge of hand switch one. I want to count the number of times I've cycled hand switch one. And I don't care how long it was on or off in between cycles, just total number of times that hand switch one has been turned on. I need to make a memory bit so that I can remember the last state of hand switch one. We covered this in the last video. If you haven't seen a rising edge like oneshot trigger before, uh not a bad idea to go check out the boolean logic video. But basically, if I want to say how do I keep track of the total number of times that hand switch one has turned on, I want this instruction called CTU, count up. Other types of counters are count down and then a CT, which is count up and down. Um, but we're just going to cover count up because similar to TOF and RTO or TOR R, they're all iterations on a theme. So, if we cover count up, you should have a pretty good idea what you're doing here. Um, and so we'll say let's make a counter example data block. And you're going to see count up, which is actually the uh trigger to count something. And then we see the reset bit, just like the retentive timer. We see PV, which is the preset value. And in this case, uh let's say we want this counter to be done when we get to 10 uh cycles. Okay, so 10 times oscillating that hand switch on. Uh and then when this is finished, let's just go ahead and turn all of the lights on. Okay, so uh let's just use all the lights for a cool light show. And we'll say output light two, three, and four are all going to turn on when this counter finishes. All right. And now let's add to the reset condition uh what we want to have turn it off. And we'll say how about the rising edge of hand switch two is what's going to uh reset this. Um and let's go ahead and make a hand switch to memory bit. Uh just like we did for hand switch one. Boolean global memory. Global memory is easier so I don't have to stick it in a database. And let's go ahead and say that I only want the rising edge of hand switch two to reset my counter if the counter has also finished. Okay, so um this one totally optional. You don't uh you don't have to add this bit for this to work. It would work just by looking at hand switch 2, but I'm going to say I don't want to reset this counter unless it's finished. No matter what, I want to keep track of how many times this has occurred. Uh so let's go ahead and download this and view it in practice. Okay, so let's get our box up here. Let's look at hand switch one right here. And uh let's start cycling it. All right, I turn it on. There's the positive edge. And you can see the value of my counter turned to one. I turn it back off. Doing hand switch two does nothing because the counter hasn't finished. But now if I get to two, three, four, five, six, seven, eight, nine. Okay, the next high transition of this hand switch is going to complete the counter because you can see it got to its target value. Now it's at 10 and all four of the lights turned on. What's cool about the count up block is it's actually going to keep counting even though it's already finished. So, you can use counters to just track total number of times something has happened and not worry too much about that PV value where you're like, I don't actually have a target finish in mind. I'm just generally curious how often this valve is cycling or things like that. You can use a counter just to track number of strokes. Okay. But now let's say even with hand switch one uh in the true position which is obviously the the signal to count up. If I make hand switch two now, oh, it's not going to reset it because I used uh I used the wrong bit. Rather than using done, I used count up, which is a trigger uh that is not going to be helpful. So, we're going to use QU, which means output count up has finished. Uh, I used count up, which is only true for one cycle when the rising edge of hand switch one is uh true. PLC's are fast enough that there's no way I was going to get that to work. Um, so this is the output bit saying count up is uh true. And you can see it's already highlighted green because we're well above the set point of 10. So, I'm going to go ahead and trigger hand switch to on again. Where's the box? There it is. And you can see all my lights turn off. Okay, so count up is very similar to a timer. Um, but it's not time bound. It's sort of conditional bound, but it's super useful to include timers in counters. Okay, so I know these are action-packed tutorials. It's impossible to cover any of these instructions in every possible use case, but I think we're building a really excellent skill set and fundamental understanding of ladder as we go through these. I hope you found this useful. Drop uh in the comments any questions that you might have and what you'd like to see next on the channel. I hope you've been finding them useful. If you really love the channel, consider leaving a like on the video, subscribing to the channel. My Patreon is down below. And as always, thank you for watching and see you next time.