What is a PLC?

A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software, that gives the desired results.

PLCs are used in many "real world" applications. If there is industry present, chances are good that there is a plc present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a plc.

For example, let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switches individually turned on? We need a lot of external counters.

As you can see the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time.

This site gives you enough information to be able to write programs far more complicated than the simple one above. We will take a look at what is considered to be the "top 20" plc instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence.

 

That's right, more than 80%! Of course we'll learn more than just these instructions to help you solve almost ALL your potential plc applications.

An Example of Animated PLC Working:

FIND MORE ON REMAINING POSTS............

PLC History

In the late 1960's PLCs were first introduced. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was based upon the PDP-8. The MODICON 084 brought the world's first PLC into commercial production.

When production requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, possibly hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices! These relays would be individually wired together in a manner that would yield the desired outcome. Were there problems? You bet!

These "new controllers" also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment. That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones.

In the mid70's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microprocessors evolved, larger and larger PLCs were being based upon them. However, even today some are still based upon the 2903.(ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.

Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling. They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!

The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. Today the world's smallest PLC is about the size of a single control relay!

The 90's have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980's. The latest standard (IEC 1131-3) has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time! PC's are also being used to replace PLCs in some applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system.

Inside plc

The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data. We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. Do these counters, timers, etc. really exist? No, they don't "physically" exist but rather they are simulated and can be considered software counters, timers, etc. These internal relays are simulated through bit locations in registers. (more on that later)

What does each part do?

• INPUT RELAYS-(contacts)These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.
• INTERNAL UTILITY RELAYS-(contacts) These do not receive signals from the outside world nor do they physically exist. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored.
• COUNTERS-These again do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down. Since they are simulated they are limited in their counting speed. Some manufacturers also include high-speed counters that are hardware based. We can think of these as physically existing. Most times these counters can count up, down or up and down.
• TIMERS-These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.
• OUTPUT RELAYS-(coils)These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen.
• DATA STORAGE-Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!!

PLC Operation

A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. There are typically more than 3 but we can focus on the important parts and not worry about the others. Typically the others are checking the system and updating the current internal counter and timer values.

Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step.

Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.

Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.

After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above.

Response Time

The total response time of the PLC is a fact we have to consider when shopping for a PLC. Just like our brains, the PLC takes a certain amount of time to react to changes. In many applications speed is not a concern, in others though...

If you take a moment to look away from this text you might see a picture on the wall. Your eyes actually see the picture before your brain says "Oh, there's a picture on the wall". In this example your eyes can be considered the sensor. The eyes are connected to the input circuit of your brain. The input circuit of your brain takes a certain amount of time to realize that your eyes saw something. (If you have been drinking alcohol this input response time would be longer!) Eventually your brain realizes that the eyes have seen something and it processes the data. It then sends an output signal to your mouth. Your mouth receives this data and begins to respond to it. Eventually your mouth utters the words "Gee, that's a really ugly picture!".

Notice in this example we had to respond to 3 things: INPUT- It took a certain amount of time for the brain to notice the input signal from the eyes. EXECUTION- It took a certain amount of time to process the information received from the eyes. Consider the program to be: If the eyes see an ugly picture then output appropriate words to the mouth. OUTPUT- The mouth receives a signal from the brain and eventually spits (no pun intended) out the words "Gee, that's a really ugly picture!"

Response Time Concerns

Now that we know about response time, here's what it really means to the application. The PLC can only see an input turn on/off when it's looking. In other words, it only looks at its inputs during the check input status part of the scan.

In the diagram, input 1 is not seen until scan 2. This is because when input 1 turned on, scan 1 had already finished looking at the inputs.

Input 2 is not seen until scan 3. This is also because when the input turned on scan 2 had already finished looking at the inputs.

Input 3 is never seen. This is because when scan 3 was looking at the inputs, signal 3 was not on yet. It turns off before scan 4 looks at the inputs. Therefore signal 3 is never seen by the plc.

To avoid this we say that the input should be on for at least 1 input delay time + one scan time.

But what if it was not possible for the input to be on this long? Then the plc doesn't see the input turn on. Therefore it becomes a paper weight! Not true... of course there must be a way to get around this. Actually there are 2 ways.

Pulse stretch function. This function extends the length of the input signal until the plc looks at the inputs during the next scan.( i.e. it stretches the duration of the pulse.)

Interrupt function. This function interrupts the scan to process a special routine that you have written. i.e. As soon as the input turns on, regardless of where the scan currently is, the plc immediately stops what its doing and executes an interrupt routine. (A routine can be thought of as a mini program outside of the main program.) After its done executing the interrupt routine, it goes back to the point it left off at and continues on with the normal scan process.

Now let's consider the longest time for an output to actually turn on. Let's assume that when a switch turns on we need to turn on a load connected to the plc output. 

The diagram below shows the longest delay (worst case because the input is not seen until scan 2) for the output to turn on after the input has turned on. 

The maximum delay is thus 2 scan cycles - 1 input delay time.

It's not so difficult, now is it ?

Relays

Now that we understand how the PLC processes inputs, outputs, and the actual program we are almost ready to start writing a program. But first lets see how a relay actually works. After all, the main purpose of a plc is to replace "real-world" relays.

We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a magnetic field is generated. This magnetic field sucks the contacts of the relay in, causing them to make a connection. These contacts can be considered to be a switch. They allow current to flow between 2 points thereby closing the circuit.

Let's consider the following example. Here we simply turn on a bell (Lunch time!) whenever a switch is closed. We have 3 real-world parts. A switch, a relay and a bell. Whenever the switch closes we apply a current to a bell causing it to sound.

Notice in the picture that we have 2 separate circuits. The bottom(blue) indicates the DC part. The top(red) indicates the AC part.

Here we are using a dc relay to control an AC circuit. That's the fun of relays! When the switch is open no current can flow through the coil of the relay.

 As soon as the switch is closed, however, current runs through the coil causing a magnetic field to build up. This magnetic field causes the contacts of the relay to close. Now AC current flows through the bell and we hear it. Lunch time!

A typical industrial relay

Replacing Relays

Next, lets use a plc in place of the relay. (Note that this might not be very cost effective for this application but it does demonstrate the basics we need.) 

The first thing that's necessary is to create what's called a ladder diagram. After seeing a few of these it will become obvious why its called a ladder diagram. We have to create one of these because, unfortunately, a plc doesn't understand a schematic diagram. It only recognizes code. Fortunately most PLCs have software which convert ladder diagrams into code. This shields us from actually learning the plc's code.

First step- We have to translate all of the items we're using into symbols the plc understands. The plc doesn't understand terms like switch, relay, bell, etc. It prefers input, output, coil, contact, etc. It doesn't care what the actual input or output device actually is. It only cares that its an input or an output.

First we replace the battery with a symbol. This symbol is common to all ladder diagrams. We draw what are called bus bars. These simply look like two vertical bars. One on each side of the diagram. Think of the left one as being + voltage and the right one as being ground. Further think of the current (logic) flow as being from left to right. 

Next we give the inputs a symbol. In this basic example we have one real world input. (i.e. the switch) We give the input that the switch will be connected to, to the symbol shown below. This symbol can also be used as the contact of a relay.

A contact symbol

Next we give the outputs a symbol. In this example we use one output (i.e. the bell). We give the output that the bell will be physically connected to the symbol shown below. This symbol is used as the coil of a relay.

A coil symbol

The AC supply is an external supply so we don't put it in our ladder. The plc only cares about which output it turns on and not what's physically connected to it.

Second step- We must tell the plc where everything is located. In other words we have to give all the devices an address. Where is the switch going to be physically connected to the plc? How about the bell? We start with a blank road map in the PLCs town and give each item an address. Could you find your friends if you didn't know their address? You know they live in the same town but which house? The plc town has a lot of houses (inputs and outputs) but we have to figure out who lives where (what device is connected where). We'll get further into the addressing scheme later. The plc manufacturers each do it a different way! For now let's say that our input will be called "0000". The output will be called "500".

Final step- We have to convert the schematic into a logical sequence of events. This is much easier than it sounds. The program we're going to write tells the plc what to do when certain events take place. In our example we have to tell the plc what to do when the operator turns on the switch. Obviously we want the bell to sound but the plc doesn't know that. It's a pretty stupid device, isn't it!

The picture above is the final converted diagram. Notice that we eliminated the real world relay from needing a symbol. It's actually "inferred" from the diagram. Huh? Don't worry, you'll see what we mean as we do more examples.

Basic Instructions

Now let's examine some of the basic instructions is greater detail to see more about what each one does.

Load

The load (LD) instruction is a normally open contact. It is sometimes also called examine if on.(XIO) (as in examine the input to see if its physically on) The symbol for a load instruction is shown below.

 A LoaD (contact) symbol

This is used when an input signal is needed to be present for the symbol to turn on. When the physical input is on we can say that the instruction is True. We examine the input for an on signal. If the input is physically on then the symbol is on. An on condition is also referred to as a logic 1 state.

This symbol normally can be used for internal inputs, external inputs and external output contacts. Remember that internal relays don't physically exist. They are simulated (software) relays.

LoadBar

The LoaDBar instruction is a normally closed contact. It is sometimes also called LoaDNot or examine if closed. (XIC) (as in examine the input to see if its physically closed) The symbol for a loadbar instruction is shown below.

A LoaDNot (normally closed contact) symbol

This is used when an input signal does not need to be present for the symbol to turn on. When the physical input is off we can say that the instruction is True. We examine the input for an off signal. If the input is physically off then the symbol is on. An off condition is also referred to as a logic 0 state.

This symbol normally can be used for internal inputs, external inputs and sometimes, external output contacts. Remember again that internal relays don't physically exist. They are simulated (software) relays. It is the exact opposite of the Load instruction.

*NOTE- With most PLCs this instruction (Load or Loadbar) MUST be the first symbol on the left of the ladder.

Logic State	Load	LoadBar
0	False	True
1	True	False

Out

The Out instruction is sometimes also called an OutputEnergize instruction. The output instruction is like a relay coil. Its symbol looks as shown below.

An OUT (coil) symbol

When there is a path of True instructions preceding this on the ladder rung, it will also be True. When the instruction is True it is physically On. We can think of this instruction as a normally open output. This instruction can be used for internal coils and external outputs.

Outbar

The Outbar instruction is sometimes also called an OutNot instruction. Some vendors don't have this instruction. The outbar instruction is like a normally closed relay coil. Its symbol looks like that shown below.

An OUTBar (normally closed coil) symbol

When there is a path of False instructions preceding this on the ladder rung, it will be True. When the instruction is True it is physically On. We can think of this instruction as a normally closed output. This instruction can be used for internal coils and external outputs.

 It is the exact opposite of the Out instruction.

Logic State	Out	OutBar
0	False	True
1	True	False

A Simple Example

Now let's compare a simple ladder diagram with its real world external physically connected relay circuit and SEE the differences.

In the above circuit, the coil will be energized when there is a closed loop between the + and - terminals of the battery. We can simulate this same circuit with a ladder diagram. A ladder diagram consists of individual rungs just like on a real ladder. Each rung must contain one or more inputs and one or more outputs. The first instruction on a rung must always be an input instruction and the last instruction on a rung should always be an output (or its equivalent).

Notice in this simple one rung ladder diagram we have recreated the external circuit above with a ladder diagram. Here we used the Load and Out instructions. Some manufacturers require that every ladder diagram include an END instruction on the last rung. Some PLCs also require an ENDH instruction on the rung after the END rung.

Next we'll trace the registers. 

Registers? Let's see...

PLC Registers

We'll now take the previous example and change switch 2 (SW2) to a normally closed symbol (loadbar instruction). SW1 will be physically OFF and SW2 will be physically ON initially. The ladder diagram now looks like this:

Notice also that we now gave each symbol (or instruction) an address. This address sets aside a certain storage area in the PLCs data files so that the status of the instruction (i.e. true/false) can be stored. Many PLCs use 16 slot or bit storage locations. In the example above we are using two different storage locations or registers.

REGISTER 00
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00
														1	0

REGISTER 05
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00
															0

In the tables above we can see that in register 00, bit 00 (i.e. input 0000) was a logic 0 and bit 01 (i.e. input 0001) was a logic 1. Register 05 shows that bit 00 (i.e. output 0500) was a logic 0. The logic 0 or 1 indicates whether an instruction is False or True.

 *Although most of the items in the register tables above are empty, they should each contain a 0. They were left blank to emphasize the locations we were concerned with.

LOGICAL CONDITION OF SYMBOL
LOGIC BITS	LD	LDB	OUT
Logic 0	False	True	False
Logic 1	True	False	True

The plc will only energize an output when all conditions on the rung are TRUE. So, looking at the table above, we see that in the previous example SW1 has to be logic 1 and SW2 must be logic 0. Then and ONLY then will the coil be true (i.e. energized). If any of the instructions on the rung before the output (coil) are false then the output (coil) will be false (not energized).

Let's now look at a truth table of our previous program to further illustrate this important point. Our truth table will show ALL possible combinations of the status of the two inputs.

Inputs		Outputs	Register Logic Bits
SW1(LD)	SW2(LDB)	COIL(OUT)	SW1(LD)	SW2(LDB)	COIL(OUT)
False	True	False	0	0	0
False	False	False	0	1	0
True	True	True	1	0	1
True	False	False	1	1	0

Notice from the chart that as the inputs change their states over time, so will the output. The output is only true (energized) when all preceding instructions on the rung are true.

A Level Application

Now that we've seen how registers work, let's process a program like PLCs do to enhance our understanding of 

how the program gets scanned.

Let's consider the following application: 

We are controlling lubricating oil being dispensed from a tank.

 This is possible by using two sensors. We put one near the bottom and one near the top, as shown in the picture below.

Here, we want the fill motor to pump lubricating oil into the tank until the high level sensor turns on. At that point we want to turn off the motor until the level falls below the low level sensor. Then we should turn on the fill motor and repeat the process.

Here we have a need for 3 I/O (i.e. Inputs/Outputs). 2 are inputs (the sensors) and 1 is an output (the fill motor). Both of our inputs will be NC (normally closed) fiber-optic level sensors. When they are NOT immersed in liquid they will be ON. When they are immersed in liquid they will be OFF.

We will give each input and output device an address. This lets the plc know where they are physically connected. 

The addresses are shown in the following tables:

Inputs	Address		Output	Address		Internal Utility Relay
Low	0000		Motor	0500		1000
High	0001

Below is what the ladder diagram will actually look like. Notice that we are using an internal utility relay in this example. You can use the contacts of these relays as many times as required. Here they are used twice to simulate a relay with 2 sets of contacts. Remember, these relays DO NOT physically exist in the plc but rather they are bits in a register that you can use to SIMULATE a relay.

We should always remember that the most common reason for using PLCs in our applications is for replacing real-world relays. The internal utility relays make this action possible. It's impossible to indicate how many internal relays are included with each brand of plc. Some include 100's while other include 1000's while still others include 10's of 1000's! Typically, plc size (not physical size but rather I/O size) is the deciding factor. If we are using a micro-plc with a few I/O we don't need many internal relays. If however, we are using a large plc with 100's or 1000's of I/O we'll certainly need many more internal relays.

If ever there is a question as to whether or not the manufacturer supplies enough internal relays, consult their specification sheets. In all but the largest of large applications, the supplied amount should be MORE than enough.

The Program Scan

Let's watch what happens in this program scan by scan.

Initially the tank is empty. Therefore, input 0000 is TRUE and input 0001 is also TRUE.

Scan 1

Scan 2-100

Gradually the tank fills because 500(fill motor) is on.

After 100 scans the oil level rises above the low level sensor and it becomes open. (i.e. FALSE)

Scan 101-1000

Notice that even when the low level sensor is false there is still a path of true logic from left to right. This is why we used an internal relay. Relay 1000 is latching the output (500) on. It will stay this way until there is no true logic path from left to right.(i.e. when 0001 becomes false)

After 1000 scans the oil level rises above the high level sensor at it also becomes open (i.e. false)

Scan 1001

Scan 1002

Since there is no more true logic path, output 500 is no longer energized (true) and therefore the motor turns off.

After 1050 scans the oil level falls below the high level sensor and it will become true again.

Scan 1050

Notice that even though the high level sensor became true there still is NO continuous true logic path and therefore coil 1000 remains false!

After 2000 scans the oil level falls below the low level sensor and it will also become true again. At this point the logic will appear the same as SCAN 1 above and the logic will repeat as illustrated above.

Latch Instructions

Now that we understand how inputs and outputs are processed by the plc, let's look at a variation of our regular outputs. Regular output coils are of course an essential part of our programs but we must remember that they are only TRUE when ALL INSTRUCTIONS before them on the rung are also TRUE. What happens if they are not? Then of course, the output will become false.(turn off)

Think back to the lunch bell example we did a few chapters ago. What would've happened if we couldn't find a "push on/push off" switch? Then we would've had to keep pressing the button for as long as we wanted the bell to sound. (A momentary switch) The latching instructions let us use momentary switches and program the plc so that when we push one the output turns on and when we push another the output turns off.

Maybe now you're saying to yourself "What the heck is he talking about?". (It's also what I'm thinking!) So let's do a real world example. 

Picture the remote control for your TV. It has a button for ON and another for OFF. (mine does, anyway) When I push the ON button the TV turns on. When I push the OFF button the TV turns off. I don't have to keep pushing the ON button to keep the TV on. This would be the function of a latching instruction.

The latch instruction is often called a SET or OTL (output latch). The unlatch instruction is often called a RES (reset), OUT (output unlatch) or RST (reset).

 The diagram below shows how to use them in a program.

Here we are using 2 momentary push button switches. One is physically connected to input 0000 while the other is physically connected to input 0001. When the operator pushes switch 0000 the instruction "set 0500" will become true and output 0500 physically turns on. Even after the operator stops pushing the switch, the output (0500) will remain on. It is latched on. The only way to turn off output 0500 is turn on input 0001. This will cause the instruction "res 0500" to become true thereby unlatching or resetting output 0500.

 

Here's something to think about. What would happen if input 0000 and 0001 both turn on at the exact same time.

Will output 0500 be latched or unlatched?

To answer this question we have to think about the scanning sequence. The ladder is always scanned from top to bottom, left to right. The first thing in the scan is to physically look at the inputs. 0000 and 0001 are both physically on. Next the plc executes the program. Starting from the top left, input 0000 is true therefore it should set 0500. Next it goes to the next rung and since input 0001 is true it should reset 0500. The last thing it said was to reset 0500. Therefore on the last part of the scan when it updates the outputs it will keep 0500 off. (i.e. reset 0500).

Makes better sense now, doesn't it?