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  Robert J. Scoff, PE Copyright © 2007, Robert J. Scoff
  Copyright © Robert J. Scoff
  8 A Logic 'OR' Ladder Logic Program 3.9 Start Stop Circuit 3.1 Examine If Closed 3.1 Input Devices 2.5 Output Devices 2.5 Download the Program 3. 2. Introduction The PLC System 2.10 Documentation 3.12 The Garage Door Opener 3.2 Examine if Open 3.3 Output Energize 3.6 An One Rung Program 3.15 Counters Some (But Not All) General Considerations The Garage Door Program Conclusions
  3.14 Timers 3.13 The Silo 3.7 An One Rung 'AND' Ladder Logic Program 3.4 Logix Pro 3.2 Input Modules 2.Table of Contents
  1.3 The Processor 2. 6.4 Output Modules 2.6 Block Diagram of a PLC System Ladder Logic Programming 3.
  4. 5.11 Labeling the Ladder Logic Program 3.
  5 Figure 3.1 Figure 2.List of Illustrations
  Figure 1.9 Figure 3.1 Figure 3.10 Figure 3. Terminal 0 Layout of a Typical Output Module Showing Output Devices Information and Control Signal Flow in a PLC System The Examine If Closed Instruction The Examine If Open Instruction Output Energize Instruction Getting Started With a One Line Program 'AND' Circuit Done With Ladder Logic 'OR' Circuit Done With Ladder Logic Schematic of Hard Wired Start Stop Circuit Ladder Logic Rung for a Start Stop Circuit A Timer Example An Example of an Up Counter An Example of a Down Counter A Workable Garage Door Opener
  Copyright © Robert J.4 Figure 2.8 Figure 3.3 Figure 3.5 Figure 2.7 Figure 3.11 Figure 5.6 Figure 2.2 Figure 2.6 Figure 3.1 Figure 2.7 Figure 3.2 Figure 3.3 Figure 2.4 Figure 3. Scoff
  .1 Physical Layout of a Typical PLC System Symbols for Some Typical Digital Input Devices Layout of Typical Input Module Showing Input Devices Simplified Block Diagram of the Memory Map of a Typical PLC Layout of a Word in a Typical PLC Push Button Switch Connected to Input Module 1.
  the ladder logic would have to be rewired to implement the changes. But the technicians and electricians of the time understood relays and ladder logic well enough that they could get the systems to function again relatively quickly. Since that time. counting. such as pressure. and make new copies too replace the outdated versions. because everyone who was concerned with industrial control systems knew all about them. and Output Devices. it would help to look at the physical layout of a typical PLC. This was usually a big problem. most of the industrial processes in the world were controlled by relays and ladder logic. and other advanced operations. Introduction
  It would be hard to imagine today's industrial world without Programmable Logic Controllers (PLC. Common analog input/outputs are 0 to 10 Volts DC and 4 to 20 milliamps DC. comparisons. C. B. The power supply is necessary to operate the processor and the input and output modules. having only digital inputs. Processor. So the thought of using computers entered the picture. But Richard Morley of Bedford Associates was contracted by General Motors and invented the PLC and called it a MODULAR DIGITAL CONTROLLER. The initial conditions that the new computer controlled system had to follow were: A. Remember. a condition called 'Relay Race' could occur. over a range. and throw out the old. the Copyright © Robert J. This shows the physical layout of a typical PLC as it would be purchased. but the control system designers of the time would take that into account. A partial list of what they do consists of relay operations. Take up no more space than the relay ladder logic control systems. this was before Auto Cad. They are Input Devices. We can't really talk about the PLC itself without working on the PLC system. There was resistance to changing a system that everyone knew how to operate. Digital Inputs and outputs have nothing to do with digital computers. now wrong. water depth. many other functions have been added to the PLC.s). This happens to be modeled after an Allen Bradley Rockwell Automation SLC 500. Since digital computers work on 2 states. The MODICON was the first PLC. Examples are process variables. One big disadvantage was that when any changes were needed by the process.1. Scoff 4
  . The master drawings were done on velum.1. D. someone at Hydromatic Division of General Motors said that there had to be a better way. ect. Then. flow.. The PLC system consists of 5 basic parts. In 1968. They were also slow. The new system would adapt to change. These 5 basic parts will be described in detail. they would sometimes fail. copies. They did have drawbacks. math functions. Output Interface. change them. like on and off. Be able to be placed on the factory floor and keep functioning. Before PLC's. They are merely inputs or outputs that have 2 states or conditions. Although relays generally worked well. including the ability to handle analog signals. Note that for this particular PLC. data moves. Another thing that needs to be looked at is Digital Input/Outputs and Analog Input/Outputs. Before we do more on PLC's. timing. Analog input/outputs are continuously variable. copy them. This was usually a bigger problem. these 2 state input/outputs are referred to as digital input/outputs. This worked quite well. At first. In more complicated systems. the changes had to be documented. the PLC's were relatively simple. Let's look at Figure 1. 0 and 1. temperature. Input Interface. Someone had to find them. Be maintained by plant electricians. They are usually translated to an electrical variable such as voltage or current.
  It is possible to have more than 8 slots.4 to 2 volt by the input module. and want to do more. Analog inputs are the other type of input device. Pick the option with the 15 day free trail. Figure 2. There are isolation devices in the input modules that do the separation. and contacts from relays. The input module takes the signal from the input device and sends that information to the processor. The input and output modules can be either digital or analog modules. A typical input module would have 16 inputs. actual programming will be done on a on-line simulator.1 Input Devices
  The first thing that any control system needs is a way to get information into the system. This simulator is found on line at <thelearningpit. we will limit the system to 8 slots. It also protects the processor.2 Input Modules
  Input modules separate the input devices.1 shows the symbols for some digital input devices.com>. The important thing about all of these devices is that they are either off or on. The other 7 slots can be either input or output modules. This course will not cover analog inputs in any great depth.1 Symbols for Some Typical Digital Input Devices 2. Scoff 5
  . If you find it a lot of fun to do PLC programming. voltages. but for the purpose of this course. So we can say that an analog input is looked on as a voltage range by the processor. selector switches. The free 15 day option has certain limits. the cost of the full scale simulator is $30. digital inputs are push button switches. The PLC System
  2.1 Physical Layout of a Typical PLC System Finally. and conditions from the processor. Input devices are subjected to a lot of abuse. corresponding to the 16 Copyright © Robert J. For most industrial control systems.processor is in a special place called slot 0.
  2. proximity switches. but it will work for this course.
  Figure 2. We will say that analog inputs are usually 0 to 10 volts or 4-20 milliamps (ma). limit switches. Even the 4-20 ma inputs are turned into a 1 to 5 volt or 0.
  Each section is a number of 16 bit words. It consists of a number of words separated into sections.4. As an example. These modules fit in slots that are designed to accept them. Copyright © Robert J.3.3 Simplified Block Diagram of the Memory Map of a Typical PLC To help in this area.
  Figure 2.2 Layout of Typical Input Module Showing Input Devices 2. When using PLC's it helps to have a little understanding of how the words of this special purpose computer are arranged. The slots will accept input or output modules and are part of the PLC. Figure 2. but 8 or 16 inputs per module is more standard.
  Figure 2. It shows a simplified memory map of a typical PLC memory. Each word has an address. as shown in figure 2. There could be a number of input modules in a typical PLC.2 shows the layout of a typical input module with input devices connected. Scoff 6
  . let's look at the first word of the input table.bits of the words of the special purpose computer which is the processor. Only 4 inputs are shown for simplicity. let's look at Figure 2.3 The Processor
  The processor is a 16 bit special purpose computer. and each bit of each word also has an address.
  Copyright © Robert J. Figure 2.Figure 2.4 Layout of a word in a typical PLC There needs to be a way to identify each bit of each word of this little computer. Notice that we are counting from 0 to 15 instead of from 1 to 16. the first bit could be labeled [I:1/0]. Terminal 0 2. The letter I tells us that we are working with an input. each one of which can be connected to an input device such as a switch or relay contact.4 Output Modules
  Output modules look a lot like input modules. Scoff
  .6 shows an output module connected to various loads. Output devices are usually fused to protect the output modules in case of a short circuit. is that each bit of the input word I:1/0 is connected to a terminal on an input module.2 is also an example of this. Notice that the output devices all require some power to operate them. A big difference is that output devices are connected to output modules.5 shows how this is done. The fuses are shown in this typical layout to emphasize their importance. and solenoids. That particular bit is shaded in red in Figure 2. This is in deference to the way that a computer works. a failure of one output device could cause an entire system to shutdown. That address is very important. Without these individual fuses. Figure 2. the input module can have up to 16 terminals. Therefore. An important thing to notice now. The 0 tells us that we have the number 0 bit. Typical output devices include lights. The 1 tells us that this is the number 1 word in the input file and also the input module located in slot number 1 in the PLC. Figure 2.5 Push Button Switch Connected to Input Module 1. If this is an input word.
  Figure 2. relays. starters.4.
  Notice that slot number 2 was chosen for the output devices.
  Figure 2. we have used slots 0 and 1. control relays.6 Block Diagram of a PLC System. Again.6 Layout of a Typical Output Module Showing Output Devices 2. Scoff 8
  . motor starters.an output module.6 shows lights. let's look at how they are connected. In this case the two states are off and on. and solenoids. 2. Since. Figure 2. slot number 2 is the first available slot in our imaginary system. digital means devices that have only two states. These four devices cover most of the digital devices used in the industrial control world.7 shows this. Figure 2.5 Output Devices
  We have already shown typical digital output devices.
  Figure 2. in our examples.
  Now that we've seen the 5 parts of a PLC system.7 Information and Control Signal Flow in a PLC System Copyright © Robert J.
  instead of Examine If Closed. It is an input address.
  Figure 3. it's a pretty simple device. The symbol for this new word is shown in Figure 3.What really happens in our PLC system is that the input devices send information to the processor. It refers to an input device connected to an input module.
  Figure 3. With that in mind.2.1. The symbol for the second word in our new language is shown in Figure 3. 3. Ladder Logic Programming
  3. say instead Pass Logic If Closed.1. Instead of using the words 'Examine If Closed'.1 is associated with that Normally Open pushbutton switch at Input Number I:1/0.2 Examine If Open
  The second word in our new language is EXamine If Open or XIO. does something with it depending upon a program in the processor. and sets the output devices. The first word is EXamine If Closed. If we look at Figure 2. the XIC instruction is used to either pass logic or not pass logic. So why don't we just call the instruction Normally Open. Another way to think of this is instead of saying Examine If Closed every time you see the symbol of Figure 3. The reason is that the instruction is now passing logic.1 Examine if Closed
  There is a new language that we have to learn. the instruction will pass logic. Scoff
  . Looked at in this manner. Now notice that the instruction has an address associated with it.1 The Examine If Closed Instruction In the programming that is used in PLC's. let's see how to program this device. So if the pushbutton is closed and we Examine If Closed.
  3. to do PLC programming we only need to learn three words right now. the processor takes that information. it has a normally open pushbutton switch connected to input number I:1/0. Fortunately. So we can say that the XIC instruction shown in Figure 3.2. The instruction actually represents the push button switch.2 The Examine If Open Instruction
  Copyright © Robert J. or XIC. let's use the words 'Pass Logic If Closed'.
  5 Download the Program
  Copyright © Robert J. and 2 columns of output lights on the left hand side of the screen. With these three instructions. It will pass logic if the device connected to the input associated with it is open. Right click on the switch several times to see the different kinds of switches available.2 will pass logic if the switch is open.1 Mega Byte file. put the curser on the first switch. If you choose to buy it. This is one of the best PLC simulators available today. Light number 1 turns on. You should see one of the simulations on the left. we are now ready to do some simple programming. So. For right now we will work with columns 1 and 2.The Examine if Open instruction is used just like the Examine if Closed instruction. find Simulations in the upper left hand corner.2. Left clicking on it will change its state. When an input switch is closed. When it changes state the address will light up telling you that that particular input is now on. open it. You should be able to find selector switches. Real PLC's do something very similar. Next.3 Output Energize Instruction Something to notice now is that the Output Energize instruction is associated with an output device. but will not affect anything we do in this course. The free version has a few limitations. the cost is $30. and limit switches. the symbol in Figure 3.4 Logix Pro
  To do some simple simulations. normally closed pushbuttons. I:1/0. We can say 'Pass Logic If Open' in place of 'Examine If Open'. if we energize output 2/0. Output energize is used anytime we want to turn an output on.com> . and the second and fourth columns will be lights or outputs. Download Now. The first and third columns will be switches or inputs. Now put the curser on the switch again. For example. a light on the input module turns on to let you know that the switch is closed. One of the biggest disadvantages is that without paying for the software. Scoff
  . we will now download a free 15 day free trial simulator. Just go to the website <thelearningpit. Hence.3.
  Figure 3. Output 2/0 from Figure 2. This should open 2 columns of input switches. 3. It is free for 15 days. Its symbol is shown in Figure 3. normally open pushbuttons. find I/O Simulator. you cannot save any of the programs that you write.3 Output Energize
  The third word to learn right now is Output Energize. 3. Examine if Open is used if we want to pass logic when the switch connected to Input 1/0 is open. Under Simulations. 3. Just to see what happens. If we use I:1/0 from figure 2. and a place to write a ladder logic program on the right. and Download Now again on the next page.6 is controlling the light (Light number 1) connected to Output 2/0. You will download a 5. find Logix Pro PLC Simulator.
  7 An One Rung 'AND' Ladder Logic Program
  Figure 3. Let's now do some more complicated (but not too much more) logic. there is a rectangular box.4. When you left click on the light nothing happens. and then Enter. Find the XIC icon and grab and drag it down to the screen directly below the rectangular box. you are given the option to change the color of the lights. or anything in between. and then click on Run. To enter the addresses.4 Getting Started With a One Line Program Figure 3. 3. 3. When you right click on the outputs. You should get a line just like Figure 3. just below the crossed wrench and hammer icon. Now you should be able to make output O:2/0 turn on and off by turning input I:1/0 on and off.4 shows a one rung Ladder Logic program whose only function is to turn on the light connected to Output 2/0 when the switch connected to Input 1/0 is closed. Try these things and then we will make several one line programs to turn lights off and on. Click on 'User'.6 An One Rung Program
  Figure 3. Wait until it stops. When it does. Then grab an Output Energize icon and drop it the same way on the green square. That would be the first light in the second column. we have to follow a certain procedure to download and test our program. things have to be done in a very specific manner. but that was on purpose to give you an idea of how it all works. The screen will flash several times. The box will change. Find the small looped arrow in the upper right hand side of the rectangular box. In the center top of the screen.4. If you did. When a green square shows up on the line.Now put the curser on the output O:2/0. This same procedure is followed for a one line program. simply type in I:1/0 and then Enter. Scoff 11
  . Click on it. Because this is a computer. drop the XIC. This ladder logic program had only one line. double click on the XIC icon. Now. Click on Download. Do the same thing with the Output Energize and type in O:2/0. or a 100 page program. That is because the only way to change the state of the outputs is to use the program to do it. You should have something similar to Figure 3. you have just made a PLC ladder logic program. This has to be done in that sequence. and a '?' should appear.5 'AND' Circuit Done With Ladder Logic Copyright © Robert J.
  8 A Logic 'OR' Ladder Logic Program
  Figure 3.5. an XIO instruction.6. Let's do the same function with a PLC ladder logic Copyright © Robert J. do the download. It is located just to the left of the XIC instruction. Then when the coil of CR is energized.9 Start Stop Circuit
  Figure 3. Grab it and drag it down to next line in your ladder logic program. Scoff 12
  . current flows through the Start and Stop switches.6 'OR' Circuit Done With Ladder Logic To make an OR logic function.7 Schematic of Hard Wired Start Stop Circuit Figure 3. use the small looped arrow to change the rectangular box to element selection mode. and build a rung like Figure 3. 3. and an Output Energize instruction. Then. watch what happens to the light number 1 (O:2/1). Make switches 1 and 2 be selector switches. Then use the small looped arrow to get to the download box. Now grab 2 XIC's and put them in the two branches as shown in Figure 3. 3. and choose 'User'. that contact can be placed in parallel with the Start pushbutton. This is an AND circuit because both elements have to pass logic to turn O:2/1 on. Grab an Output Energize and put it at the end of the rung as before. When the Start pushbutton is pushed.Figure 3. and go to run mode. This is called a seal-in circuit. Do the Download – Run routine again. and test your OR circuit. We need to first get back into the program mode.7 is a schematic of a standard hard wired Start Stop circuit. Closing either input or both inputs will turn on output O:2/2. If CR is a relay with a normally open contact. the contact CR pulls in and allows current to continue flowing to the coil of CR.5 shows an XIC instruction in series with an XIO instruction. This causes coil CR to be energized. Then grab an XIC instruction. we need to use a new instruction called a Branch instruction. Lets go back to our Logix Pro program. As you turn switches 1 and 2 off and on.
  so we want it to pass logic if it is closed. From here on you can make more complicated ladder logic programs. That is correct.
  Figure 3. We still have to use the new word in our language. Then each input and output is assigned an address. the XIC contact. Then make Input Number I:1/6 be a Normally Closed pushbutton. Continuing where we left off in the PLC program that we're writing. and then goes to the first rung and repeats the operation. 3. It is important to notice this because in some operations the order in which rungs are done can influence the results. it does the rungs one at a time in sequence. 3. Scoff
  . it updates all of the outputs. Then when Output O:2/3 is energized. notice that no matter what you did the program generated an 'END' statement. EXamine If Closed. and test it to see if it really is a Start Stop circuit.10 Documentation
  As with most things in the industrial control world. the wiring person. documentation is very important. The Stop pushbutton is normally closed. Since it is a digital computer. from the person doing the parts layout. Control relay CR will be Output Number O:2/3. That is because when this kind of ladder logic program works. Doing this part of the PLC system properly helps everyone. First. it has to do things in sequence. Right click on Input Number I:1/5 until it is a Normally Open pushbutton. There are several things that need to be understood. The XIC contact labeled O:2/3 is similar to a relay contact. The next thing to notice is that the Output Energize has a contact associated with it.11 Labeling the Ladder Logic Program 13
  Copyright © Robert J. First.8. The next step is to add the rung shown in Figure 3. passes logic.8 to the ladder logic program that we are writing. starting at the top and ending at the END statement. For PLC programming lists of inputs and outputs are first made. When the program hits the End statement. O:2/3. When the engineer does his part properly at the beginning. everyone's job is made easier. let the Start pushbutton be Input Number I:1/5 and the Stop pushbutton be Input Number I:1/6. The length of time that it takes to do a complete loop is called the cycle time. The XIC only becomes open if the Stop pushbutton is pushed. and the programmer. It continues this loop as long as the controller is in the Run mode. and quite often the wire number.program. This address is also the address on the input and output modules. the Stop pushbutton is represented by a XIC.8 Ladder Logic Rung for a Start Stop Circuit There are several things to notice about this rung. Then program a ladder logic rung as shown in Figure 3.
  make the garage door open and shut. Do not worry about the A-B-C switch. If you use that particular address anywhere in the program. right click on the symbol. A red field will appear around it. There are also 5 Outputs. To edit rungs. and a Motor Run output. Copyright © Robert J. Go ahead and do this for your program. Step 1. and hit enter. Scoff 14
  . Left click on 'Edit Symbol'. Then right click on the red field. Moving the pointer will cause the door to move faster or slower. click on 'New'.So far we have done very simple things that we could keep track of in our heads. Notice that the door has 5 Inputs. You can speed up. They are Ajar Light. Note that even a simple process like this could have many possible solutions. Stop. So now we want to label the individual symbols in a program. and Full lights. go to program mode and erase all of the rungs that you have programmed. Choose 'Edit Rung Comment'. 3. Note that you must be in the program mode to edit symbols. 3. Shut Light. If you don't use the limit switches to turn the motor off at the top and bottom. and a table with 7 options will drop down. To give each symbol a name. you must also be in the program mode. or slow down the operation by finding the speed control on the right hand side of the Download screen. as well as the rungs. On the screen that appears. Make the left hand side of the screen big enough to show the entire door. and open 'Garage Door'. Simply type in your comment. Do this by grabbing the vertical line in the center of the screen between the picture of the door and the program. click 'OK'. I have included a program that will work at the end of this course. When your ladder logic area only has an END statement left. Step 3. You wouldn't be able to save your work unless you have paid for the software. There is just too much to keep track of easily. and a place to put information about that rung will appear.13 The Silo
  The Silo is a bit more difficult to make work properly. Close. Sometimes you can get the motor to burn out even if you use the limit switches. Open Light. A table with 6 options will appear. Go to the file menu. Top Limit Switch. hit enter. and a Fill Solenoid. Stop. Prox Sensor to detect the box. the motor burns out. Now is the time to have some fun. go to simulations. then right click on the red area that appears. Using the ideas that we have discussed so far. Ladder Logic Programs can be many pages long. Motor Up. and Bottom Limit Switch. Notice that the inputs consist of Start. The first step is to left click on the rung number. This is not usually the case. One way to erase is to left click on each rung number. and cut each rung. and open the Silo simulation. Step 2.12 The Garage Door Opener
  To get ready for the Garage Door Opener. and when you are ready we will make a garage door opener work. After you have gotten the Garage Door to work in a satisfactory manner. The outputs consist of Run. Your program should disappear into digital wonderland. You can either erase the program one line at a time or do the following sequence. and Motor Down. Go to simulations. the software will label it for you. A space will appear directly below the symbol where you can type in whatever you want to identify that symbol. They are Open. and Level Sensor to detect box full. Fill. After you type what you want to type there. erase your work (or save it if you have purchased the software) and load the Silo simulator.
  Look in the file menu.01 seconds). To do that.9 A Timer Example The easiest way to understand a timer is to build a ladder logic program that uses one.9. not very accurate.14 Timers
  Timers. These outputs can be Copyright © Robert J. were big. cheap (just three words in memory). A workable program will download into the programming area. as small as 0.
  Figure 3. After you struggle with this a bit. The full light turns on when the box fills up. The trick is to make the full light work right. Click on it and drag a On Delay Timer (TON) to the first line in your new program. they became small (just three words in a memory somewhere inside the PLC). before PLC's. first find the Timer/Counter tab to the right of the user tab that we have been using. After the advent of PLC's. and not very flexible. there is a program that will work in the software package.This program spills pink stuff on the floor if you don't do it just right. and open Silo. It is only 4 steps long. accurate (To within the time base of the PLC. This stands for ENABLE and DONE. Notice that the screen shows EN and DN outputs. and stays on until the box leaves the prox switch. Let's start by erasing all that we've done so far and loading the I/O simulator again. 3. Then look in Logix Pro. Download it and go to Run mode. The program will run. under open. Then build the ladder logic program shown in Figure 3. expensive. There is also a third output that is not shown called TIMER TIMING (TT). See what you can do with this box loading program. Scoff 15
  . and flexible (we'll get into that now).
  The timer starts timing when the input to it is made logically true. Scoff 16
  . Then run the program (after you download it) and watch the outputs change as you change the input. The Accumulator will count from 0 to 10.15 Counters
  The next useful function that most PLC's can do is to count events of one sort or another. and the accumulator counts up until it equals the Preset. The accumulator then resets.used just like output energize outputs. there is an instruction called RES or reset. Enable (EN) will be on as long as you hold the pushbutton switch closed. DN stays on. EN turns off. If you want to make the preset bigger to help you watch the timer operate. tells how many time bases have been counted when the timer is timing. This is important because the timer actually counts time bases to get the total time. and we will test a counter. Add the four rungs shown in Figure 3. the accumulator is set to zero. The last line. The time base 0. PRESET. Let's build an example right below the timer that we just built to see how it operates. Now let's see what happens to EN. It retains its value and keeps accumulating only when the input is logically true. I recommend that you just change the TON to a TOF. 3. These work very much like timers. RES means Reset and is one of the instructions in the Timer/ Counter set. just change it to the next one in sequence. and cut it out of the rung. Hit 'Enter' and the PLC will generate an address for you. DN. After copying the example from Figure 3. try 30 or 50 for the Preset. So. In our example. they look just like outputs. There is another Time Delay on Operate called RTO or Retentive Timer on Operate. Then Done and Timer Timing both turn off and the accumulator stays at the Preset value. the Enable output turns off. The Timer Timing output stays off or false. In the PLC they are actually three bits that are part of the three words that make up the timer. To program the On Delay Timer. and TT when the timer is activated. The next timer is called Timer Off Delay or TOF.10. the PRESET times the TIME BASE gives the total timer time. is programmed by the programmer to tell the timer how many time bases to count. TT turns off. and inflexible. and the Done output goes true. It starts out with the Accumulator equal to the Preset.9. it resets the accumulator. When RES goes logically true. If the input to the timer is stopped before it times out. inaccurate. EN stays on. first click on the '?' where the Timer Number is located. The next number. The big difference is that the Accumulator does not automatically reset when the input goes away. One way to do this is to delete the TON and replace it with a TOF in your program. To the PLC. This output was not usually available when timers were big. The timer is actually working as a counter of time bases. If you use more than one timer.1 seconds should appear in the next line. the Timer Timing output turns on. Make I:1/0 a normally open pushbutton switch. To make the accumulator reset. As soon as the input goes logically true. If the input to the timer is held true. download the program. When the input then goes false. Notice here that the enable output looks just like a relay or Output Energize. and go to Run mode. This is really your time delay on operate. The TT output is very useful in some applications. ACCUM. Then hold it closed and watch the timer. Copyright © Robert J. The Timer Timing output turns on whenever the accumulator is changing value. that happens when the switch represented by XIC contact I:1/0 is closed. the Enable output goes true. One way to delete a timer is to right click on it. and TT goes off. and the software gives the same number again. Done (DN) will turn on when the accumulator equals the preset. and the timer is ready to operate like a time delay again. and DN never turns on. expensive.
  however. that after programming. Actually using these capabilities is limited only by the imagination. We didn't test it here because we would have to count to 32767 to do that. Scoff
  . to determine the rate of a process. make sure that Inputs 1 / 1 and 1 / 2 are normally open pushbuttons. such as boxes of product shipped. and 2 / 4. You set the Preset when you programmed the Up Counter. an output bit is set. Put your program in the programming mode and add the Down Counter shown in Figure 3. Notice that Accum now equals 10.10 An Example of an Up Counter After programming this counter (Remember. It will do that until a condition called overflow occurs. That happens when the Accumulator equals 32767. It counts down. always resets the counter to zero. Another counter is called the Down Counter. It is called OV. It should be noted that the Accumulator can be set to any value between -32768 and +32767 while in the programming mode. download and put the processor in the Run mode). It can be used in the program as C5:0/OV. They can be used. A counter can be set to cause lubrication after a certain number of operations. Then push pushbutton 1 / 1 10 times while watching outputs CU. That is as high as the 16 bit word can count if one bit is reserved for the plus or minus sign. in conjunction with a timer. This makes Accum equal to the Preset. DN. Let's do an example of a down counter. Up Counters can be used to count events.
  Copyright © Robert J.11. Now press Input 1 / 1 again and notice that the Accumulator keeps incrementing and DN stays on. for overflow. Reset. 2 / 3.Figure 3. When that happens. instead of up.
  For the down counter in Figure 3. we can put a Reset just below the counter. the computer executes the first rung in your ladder logic diagram. DN only turns OFF when Accumulator becomes less than preset. in the programs that we have written. and then be negated on another line.10. and only does one step at a time. it cannot be reset to anything other than zero without going to programming mode.
  4. It then executes the second rung. Scoff 18
  . The underflow condition would be indicated by 'UF'. Activating Reset. but they are still done one step at a time. Some (But Not All) General Considerations
  Something that we haven't emphasized that is very important is the sequence of operations of the Programmable Logic Controller. that most times it does not affect the program. However. Also. the instruction 'C5:1/UF' could be used by the program to indicate an underflow condition. Notice that the Outputs do not change until the program has run through the entire ladder logic program. It does these steps pretty quickly.11 An Example of a Down Counter The Down Counter works a little differently than an Up Counter. This is so fast. Copyright © Robert J. The PLC is a small special purpose digital computer. When it gets to the END statement. and so on. turns the Accumulator value to Zero. So. If we look at the counters. then the third. As such. and in sequence. until it gets to the END statement. But there are times when something can happen on one line. The Accumulator value can also be set to any integer between -32768 and +32767 while in the programming mode. One of the big differences is that the Accumulator starts at zero and counts down. A condition called underflow (similar to overflow) occurs when the counter goes below -32678.Figure 3. DN is ON whenever Accumulator is greater than or equal to preset. and reset it with the same input that it is trying to count. it changes any Outputs that have changed as the program was being run through one cycle. This could lead to confusion. it works just like a computer.
  Then. without changing hardware. a workable garage door opener program will be shown. There are many math functions included in the PLC.1 shows this program. these comments are more and more useful. is that if you change the definition of ajar. Just change the table from Output to Input. add your comments to the elements and rungs. The door is ajar when it is stopped somewhere in the middle. The same is true of the compare functions. The Garage Door Program
  As promised earlier. lock the door down after a number of open close cycles. and then click on whatever instruction that you need help on.1. just for the fun of it add timers so that you have to hold the button for 3 seconds before the door opener operates. About 60 % of the problems are loose wires. and what they do. Then put a counter in the program that counts number of operating cycles. Conclusions This course is intended to be a start in your understanding of how PLC's actually work. This program implements that definition. there were no labels put on the elements or the rungs. As programs get more and more complicated. we can look at the condition of the timers and counters by changing the table to Timer or Counter.
  5. To make the program more understandable. This can be done on both the up and down directions. 6. A screen will appear that shows the output table in binary form. most of the problems with PLC systems have little to do with the actual PLC. If you haven't made the garage door opener work. Go ahead and see what options are available. just go ahead and copy this one and test it. In closing. and vice versa. Look at the compare functions available. Figure 5. not the hard wiring. and most of the rest of the Copyright © Robert J. Then. PLC's can be a lot of fun. Notice that the Radix of the data can be changed. A good description is given of how to use them. The use of them is beyond the scope of this course. The same thing can be done for the inputs. A special definition of door ajar was made . To determine what any instruction will do. To do that click on the crossed wrench and hammer icon near the top center of the screen. It prevents the door from trying to go in the opposite direction when it is moving. and how operations can be changed. With 0 meaning off and 1 meaning on. you can determine what all of the outputs are at a glance. There are other tables that can be examined from this menu. This is just one of the things that need to be addressed when programming a PLC. This shows the flexibility of PLC programs. go to the help menu. What would you change to make the Ajar light turn on whenever the door is not completely open or completely closed? Go ahead and try it and see what happens. If you get a real machine problem in the future. Scoff 19
  . Notice that this program takes into account the possibility of pushing the Up button when the door is going Down. you only need to change the programming. One of the nice things about PLC programming. Another thing that is very helpful in programming PLC's is to be able to look at the words in the memory that represent the Inputs and Outputs. you now have a good start on understanding what you need to do to make your automatic process work. For example. click on Rockwell RsLogix Instruction Help.The counter Accumulator would never get above zero. For the program of Figure 5.
  1 A Workable Garage Door Opener
  Copyright © Robert J. The PLC itself is relatively trouble free.problems are Input Devices.
  Figure 5. However. having an idea of how to read and understand ladder logic diagrams is a great help in finding the area of your automatic system that is causing problems at any given time. Scoff
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