Implementation Of Programmable Logic Controller (Plc) Circuits Using Digital Logic Design Concepts

view with charts and images

Implementation Of Programmable Logic Controller (Plc) Circuits Using Digital Logic Design Concepts



1.1 Introduction

Industry has begun to recognize the need for quality improvement and increase in productivity after second world war. Flexibility have also became a major concern (ability to change a process have quickly became very important in order to satisfy consumer needs).There was always a huge electrical board for system controls, and not infrequently it covered an entire wall! Within this board there was a great number of interconnected electromechanical relays to make the whole system work. By word “connected” it was understood that electrician had to connect all relays manually using wires! An engineer would design logic for a system, and electricians would receive a schematic outline of logic that they had to implement with relays. These relay schemas often contained hundreds of relays. The plan that electrician was given was called “ladder schematic”. Ladder displayed all switches, sensors, motors, valves, relays, etc. found in the system. Electrician’s job was to connect them all together. One of the problems with this type of control was that it was based on mechanical relays. Mechanical instruments were usually the weakest connection in the system due to their moveable parts that could wear out. If one relay stopped working, electrician would have to examine an entire system (system would be out until a cause of the problem was found and corrected). The other problem with this type of control was in the system’s break period when a system had to be turned off, so connections could be made on the electrical board. If a firm decided to change the order of operations.

1.2 Literature Review

Programmable Logic Controllers (PLCs) have a history that dates back to the 1960s, yet there are still many in the automation industry who’ve had little experience with them. When it comes to learning about these products, or making the jump from other areas of automation to applications that involve PLCs, the transition can sometimes be difficult. It’s hard to know where to begin, and if you’ve been charged with the task of selecting one, it can be even harder to know which manufacturer or model to choose. To make the switch to PLCs, it’s important to have a basic understanding of what they are, what they do, and which PLC is right for your application. This thesis motivates definition and history of PLC. I have collected then reference books have Training of Program on Programmable Logic Controller (PLC),from Bangladesh Industrial Technical Assistant Centre (BITAC), Programmable Logic Controllers, Fifth Ed, W. Webb & Ronald A. Reis. Digital logic and computer Design, M. Morris Mano from then I know The functional description of PLC, I also collected from reference book . How PLC Controller Work and Structure of PLC system. The Programming a PLC controller and Kinds of PLC Software. I have also collected from web site from Google “Programmable Logic Controller”.

1.3 Motivation for this thesis

The era of hard automation has come to an end; the era of soft automation has already begun, and it is increasing day by day. So, Programmable logic Controller (PLC) design which is the core of soft automation seems an interesting and challenging topic to me. Its future and scope along with PLC development opportunity motivate me to explore some little parts of ongoing technology.

1.4 Statistical tools to be used

This is experimental and statistical tools will be used Siemens Logo soft Comfort, as MS-word, and MS-power point. And the used of equipment for project.

1.5 Limitation of the Study

Limited usage environments, high temperature and harsh vibrations can disrupt electronic equipment on the PLC.

Need extra security equipment such as relay.

Programmable Logic Controller (PLC) is not considered necessary applied to industrial systems that do not need to change the wiring.

1.6 Aim of thesis

The programmable Logic Controller (PLC) is most important tool in the Industry sector and other electrical section. Because it is Flexible, Implementing Change & Correcting error, Newer Technology, Security, Documentation, Reliability & Maintainability, Ladder or Boolean programming method, Speed of Operation, Lower Cost, Visual Observation, Pilot Running, Ease of change by programming. The PLC are used to Industry sector and different electrical & electronics instruments controlling & protection. Such as Motor protection, Application of boiler, Water & Oil level indicator, Heater temperature control, Level flow indicator, Fault analysis, System protection, Boolean Algebra express, Latching process, Basic Logical Operation, Flickering lamp control, Proportional Integral Derivative (PID) Controller, etc.

Our objectives of this thesis are as follows:

? To study the what is PLC and Functional description of PLC.

? To study the how to PLC controller works and how to connection PLC to computer.

? To study the Kinds of PLC software.

? To study the programming language and data flow in PLC system.

? To study the LADDER diagram.

? To study the some example of PLC.

? To study the where are used of PLC.

? Discuss the elements of the Software LOGO! Soft Comfort.

? Discuss the application of the PLC system.

? Discuss the Advantage of PLC.

? Discuss the Some Example of PLC.


Programmable logic controllers

2.1 Programmable Logic Controller

Programmable Logic Controllers (PLC) is essential controller equipment in modem Industrial control system. Though it is totally practical equipment, it can be changed by the program of its operation that’s why it is call Programmable Logic Controllers.

Control engineering has evolved over time. In the past humans was the main method for controlling system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls. PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.

• Cost effective for controlling complex systems.

• Flexible and can be reapplied to control other systems quickly and easily.

• Computational abilities allow more sophisticated control.

• Trouble shooting aids make programming easier and reduce downtime.

• Reliable components make these likely to operate for years before failure.

2.2 Characteristic of PLC

1. Its Changeable operation.

2. Processing element is used hear.

3. Input & output signal Isolated as if PLC could not damage with electrical fault.

4. Its reliabity is high

2.3 Functional Description of PLC

PLC is actually an industrial microcontroller system where you have hardware and software specially adapted to industrial environment. Special attention needs to be given to input and output, because in these blocks you find protection needed in isolating a CPU blocks from damaging influence that industrial environment can be bring to a CPU via input lines. Program unit is usually a computer used for writing a program (often in ladder diagram).

The control program can be entered into the PLC by using a simple from of high level language like ladder diagram, instruction code etc. The input device such as switch, push buttons, sensors and output device such as motors, relays, valves, lamps, etc are connected to PLC. The operator then enters a sequence of instructions (programs) into the memory of the PLC. The controller then monitors the inputs and outputs according to these programs and carries out the control rules for which it has been programmed.

Figure 2.3 (a) : Architecture of a PLC

Figure 2.3(b) : Block Diagram of PLC

Figure 2.3(c) : Functional Description Of PLC

2.4 Power Supply

The electrical supply is used in bring electrical energy to central processing unit. Most PLC controllers work either at 24V DC or 220V AC. On some PLC controllers you’ll find electrical supply as a separate module. Those are usually bigger PLC controllers, while small and medium series already contain the supply module. User has to determine how much current to take from I/O module to ensure that electrical supply provides appropriate amount of current. Different types of module of modules use different amount of electrical current. This electrical supply is usually not used to start external inputs or outputs. User has to provide separate supplies in starting PLC controller inputs or outputs because than you can ensure so called “ pure “ supply for the PLC controller. With pure supply we mean supply where industrial environment can not affect it damagingly. Some of the smaller PLC controllers supply their inputs with voltage from a small supply source already incorporated into a PLC.

2.5 Memory

System memory (today mostly implemented in FLASH technology) is used by a PLC for an process control system. Aside from this operating system it also contains a user program translated from a ladder diagram to a binary form. FLASH memory contents can be changed only in case where user program is being changed. PLC controllers were used earlier instead of FLASH memory and have had EPROM memory instead of FLASH memory which had to be erased with UV lamp and programmed on programmers. With the use of FLASH technology this process was greatly shortened. Reprogramming a program memory is done through a serial cable in a program for application development.

User memory is divided into blocks having special functions. Some parts of a memory are used for storing input and output status. The real status of an input is stored either as “1” or as “0” in a specific memory bit. Each input or output has one corresponding bit in memory. Other parts of memory are used to store variable contents for variables used in user program. For example, timer value, or counter value would be stored in this part of the memory.

Read Only Memory (ROM)

ROM is a non-volatile memory that can be programmed only once. Its is therefore unsuitable. It is least popular as compared with other memory type.

Random Access Memory (RAM)

It stores user program and temporary buffer storage for the input /output buffer channel. The programs in the RAM can be changed by the user. However the prevent the loss of programs when the power supply is switched off., a battery is likely to be used in the PLC to maintain the RAM contents for a period of time. After a program has been developed in the ram it may be loaded into an EPROM memory chip and so made permanent.

Erasable Programmable Red only Memory(EPROM)

EPEROM holds data permanently just like ROM. It dose not require battery backup. However, Its content can be erased by exposing it to ultraviolet light. A prom writer is require to reprogram the memory.

Electrically Erasable Programmable Red only Memory(EPROM)

EEPROM combines the access flexibility of RAM and non-volatility of EPROM in one. Its contents can be erased and reprogrammed electrically, however, to a limit number of times.

2.6 Central Processing Unit (CPU)

The Central Processing Unit (CPU) is the brain of a PLC controller. CPU itself is usually one of the microcontrollers. It controls and processes all the operations with in the PLC. It is supplied with a frequency of typically between 1 and 8 MHz . This frequency determines the operating speed of PLC and provides the timing and synchronization for all elements in the system. A bus system carries information and data to and from the CPU.

Figure 2.6 : Bus System of a PLC

2.7 Address bus

Address bus is used select a certain memory location of a device. When a particular address is selected by its address being placed on the address bus, only that location is open to communications from the CPU.

2.8 Data bus

Data bus is used to transport a word to or from the CPU and the memory or the input output interface. When address bus select a specific memory location then data of that location is available on data bus.

2.9 Control bus

Control bus is used to select the specific device i.e. ROM, RAM, I/O port etc. ROM, RAM, I/O port are selected by tri state method.

2.10 PLC controller inputs

Intelligence of an automated system depends largely on the ability of a PLC controller to read signals from different types of sensors and input devices. Keys, keyboards and by functional switches are a basis for man versus machine relationship. On the other hand, in order to detect a working piece, view a mechanism in motion, check pressure or fluid level you need specific automatic devices such as proximity sensors, marginal switches, photoelectric sensors, level sensors, etc. Thus, input signals can be logical (on/off) or analogue. Smaller PLC controllers usually have only digital input lines while larger also accept analogue inputs through special units attached to PLC controller. One of the most frequent analogue signals are a current signal of 4 to 20 mA and mV range voltage signal generated by various sensors. Sensors are usually used as inputs for PLCs. You can obtain sensors for different purposes. They can sense presence of some parts, measure temperature, pressure, or some other physical dimension, etc. (ex. inductive sensors can register metal objects). Other devices also can serve as inputs to PLC controller. Intelligent devices such as robots, video systems, etc. often are capable of sending signals to PLC controller input modules (robot, for instance, can send a signal to PLC controller input as information when it has finished moving an object from one place to the other.)

PLC Input Devices

1. Push buttons

2. Switches (limit switches, level switches, etc.)

3. Sensors

4. etc

Figure 2.10 : Rocker Switch, Push Switch, Key Switch

2.11 PLC controller output

Automated system is incomplete if it is not connected with some output devices. Some of the most frequently used devices are motors, solenoids, relays, indicators, sound signalization and similar. By starting a motor, or a relay, PLC can manage or control a simple system such as system for sorting products all the way up to complex systems such as service system for positioning head of CNC machine. Output can be of analogue or digital type. Digital output signal works as a switch; it connects and disconnects line. Analogue output is used to generate the analogue signal (ex. motor whose speed is controlled by a voltage that corresponds to a desired speed).

PLC Output Devices

Relay contacts

Solenoid valves

Signal devices (such as lamps, alarms, etc.)



Figure 2.11 : Output Device (Magnetic Conductor)


Structure and working principal of PLC

3.1 Programmable controller

A PLC consists of a Central Processing Unit(CPU) containing an application program and Input and Output Interface modules, which is directly connected to the field I/O devices. The program control the PLC so that when an input signal from an input device turns ON, the appropriate response is made. The response normally involve turning On an output signal to some sort of output device.

3.2 How PLC controller works

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

Figure 3.2 : PLC Operation

3.3 Testing status

Step 1.

Testing input status. First, a PLC checks each of the inputs with intention to see which one of them has status ON or OFF. In other words, it checks whether a sensor , or a switch etc. connected with an input is activated or not. Information that processor thus obtains through this step is stored in memory in order to be used in the following step.

Step 2.

Program execution. Here a PLC executes a program, instruction by instruction. Based on a program and based on the status of that input as obtained in the preceding step, an appropriate action is taken. This reaction can be defined as activation of a certain output, or results can be put off and stored in memory to be retrieved later in the following step.

Step 3.

Checkup and correction of output status. Finally, a PLC checks up output status and adjusts it as needed. Change is performed based on the input status that had been read during the first step, and based on the results of program execution in step two. Following the execution of step 3 PLC returns to the beginning of this cycle and continually repeats these steps. Scanning time is defined by the time needed to perform these three steps, and sometimes it is an important program feature.

3.4 Connection out line of PLC

Figure 3.4(a) : Connection Outlet of PLC

PLC Input / Output Connection

Programming through standard computer

Most PLC manufacturers offer software packages that allow a standard computer to be used as a programming terminal

Figure 3.4(b): Connection Diagram PLC to computer

3.5. Structure of PLC

Figure 3.5 : Connection outlet of PLC

3.6 Data Flow in the PLC

Figure 3.6 : Data flow in PLC



4.1 Programming a PLC controller

PLC controller can be reprogrammed through a computer (usual way), but also through manual programs (consoles). This practically means that each PLC controller can programmed through a computer if you have the software needed for programming. Today’s transmission computers are ideal for reprogramming a PLC controller in factory itself. This is of great importance to industry. Once the system is corrected, it is also important to read the right program into a PLC again. It is also good to check from time to time whether program in a PLC has not changed. This helps to avoid hazardous situations in factory rooms (some automakers have established communication networks which regularly check programs in PLC controllers to ensure execution only of good programs).

Almost every program for programming a PLC controller possesses various useful options such as: forced switching on and off of the system inputs/outputs (I/O lines), program follow up in real time as well as documenting a diagram. This documenting is necessary to understand and define failures and malfunctions. Programmer can add remarks, names of input or output devices, and comments that can be useful when finding errors, or with system maintenance. Adding comments and remarks enables any technician (and not just a person who developed the system) to understand a ladder diagram right away. Comments and remarks can even quote precisely part numbers if replacements would be needed. This would speed up a repair of any problems that come up due to bad parts. The old way was such that a person who developed a system had protection on the program, so nobody aside from this person could understand how it was done. Correctly documented ladder diagram allows any technician to understand thoroughly how system functions.

4.2 Programming Language of PLC

PLC programming language refers to the method by which user communicates information to the PLC.

There are three most common languages:

Ladder Diagram Language

? The most common used by PLC language.

Boolean language

? The statements refers to the basic AND, OR and NOT logic gate function.

Function chart system

? It is a method of programming a control system that uses a more structured


4.3 Classification of program

All functional elements need to execute a certain control process are called as a program. A program is stored in the RAM mounted on a CPU module or flash memory of an external memory module. The following table shows the classification of the program.

Program type Description
Scan program The scan program is executed regularly in every scan, the CPU cannot execute not only the scan program but also other program.
The time driven interrupt program (TDI) The TDI programs are executed with a constant time interval specified with the parameter setting.
Process driven interrupt program (PDI) The PDI programs are executed only external interrupt input is applied and the corresponding interrupt routine is enable by EI instruction.
Subroutine program The subroutine programs are executed when they are called by scan program with a CALL instruction.

4.4 Processing Method

The following diagram shows that the CPU module process program when the CPU module is powered on or switch to run mode.

Figure 4.4 : Program processing method


Ladder diagram can be understood by simplify considering an electrical circuit in figure:1. The diagram shows the circuit for switching on or off an electric motor. There is a dc voltage source heaving positive and negative terminal, a switch and motor. From the positive terminal current start to flow and finished it at negative terminal via the switch and the motor. Thus the circuit is completed. Now just redrawing the circuit using two vertical lines to represent the positive terminal (Left line) and negative terminal (Right line) of the source. Now the switch and motor are connect to a horizontal line in figure: Both circuit have the switch in series with the motor and supplied with electrical power when the switch is closed. The vertical lines are known as power rails and the horizontal component known as the rung. The shown in figure: are termed as ladder diagram.

Figure 4.5 : Comparison between ladder diagram and electrical diagram

4.6 PLC LADDER Programming

A popular method of PLC programming is LADDER diagram. Ladder diagram is equivalent to drawing a switching circuit. It consists of two vertical lines representing power rails and the circuit is connect to the horizontal line.

4.7 Standard Symbol For Ladder Diagram

Finger 4.7 : Standard Symbol For Ladder Diagram

The input symbol is just like switch is used for input. Output are represent by just one symbol, regardless of the device connect to the output.


Types of Software &Application of PLC

5.1 Kinds of PLC Software

We also discuss the software PLC. Some name of brand is given are below.

? Siemens Logo soft comfort

? LG

? Saia Burgess – PCD1

? Mitsubishi

Figure 5.1(a): Saia Burgess – PCD1

Figure 5.1(b) : Siemens PL

5.2 The Software LOGO!Soft Comfort

Create ladder and function block diagrams simply by selecting, dragging and dropping the relevant functions and your connections. Make use of fully offline simulation of the entire switching program on the PC as well as online testing during operation.

Fogure 5.2 : SIEMENS PLC programming function

5.3 Modules for Logo

Figure 5.3 : Expansion modules for Logo Comfort

Computer desktop page of Siemens LogoSoft

Figure 5.4 : Siemens Logo Soft Functional Block Diagram

That isn’t all function but is enough to start fun with Logo.

5.5 Application of PLC system

? In Industry, there are many production tasks which are of highly repetitive

nature. Although repetitive & monotonous, each stage needs careful attention

of operator to ensure good quality of final product.

? Many times, a close supervision of processes cause high fatigue on operator

resulting in loss of track of process control.

? Under all such conditions we can use PLCs effectively in totally eliminating

the possibilities of human error.


PLC are fully solid state & hence extremely compact as compared to hard – wired controller where in electro – mechanical devices are used.

? Easy to Program.

? The interfacing for inputs and outputs in inside the controller.

? Does not suffer from fatigue problem.

? Can be checked without field device.

? Can perform complex logic operation.

? Faster system response.

? Monitoring facilities available.

? High reliability.

? Easy to maintenance.

5.7 Disadvantage of PLC system

In this panel we can observer the following points

? There are too many wiring work in the panel.

? Modification can be quite difficult.

? Troubleshooting can be quite troublesome as you many require a skillful person.

? Power consumption can be quite high as the coil consumes power.

? Machine downtime is usually long when problems occur, as it takes a longer time to troubleshoot the control panel.

? Drawings are not updated over the years due to changes. Its causes longer downtime in maintenance and modification.

5.8 Populate of PLC

? Realiabity.

? Easy used friendly.

? Programming facilities.

? Process monitoring facilities.


Some Example using by PLC

6.1 Basic logical Operation

The logical operation is useful to solve many complex control situations. A complex program is build up by many small logic gates.

OR Implementation

An OR with input I1 and I2 gives an output 1 when I1 or I2 is 1.

I1 I2 Output
0 0 0
0 1 1
1 0 1
1 1 1

Figure 6.1(a-1): Truth table for OR Implementation

Figure 6.1(a-2): Compare the Electrical base to PLC base of an OR Implementation


I1 & I2 is the make contract,

M is the motor.

When any input (I1 or I2) is high then the Motor goes on. When every input is low then the Motor goes off. Shown in truth table.

Figure 6.1(a-3) : PLC Ladder diagram of an OR Implementation


I1 & I2 is the make contract,

Q1 is the Relay coil.

AND Implementation

The AND gates is used where an outputs is not energized unless two, normally open switch are both close. Suppose there are two liquid tanks and a motor. The condition is that the motor will start when both tanks will fill liquid. Shown in truth table.

I1 I2 Output
0 0 0
0 1 0
1 0 0
1 1 1

Figure 6.1 (b-1): Truth table for AND Implementation

Figure 6.1(b-2): Compare the Electrical base to PLC base of an AND Implementation


I1 & I2 is the make contract,

M is motor, Q1 is the Relay coil.

When every input (I1 & I2) is on this time the motor goes on, otherwise every condition the motor goes off.

Figure 6.1(b-3): PLC Ladder diagram of an AND Implementation


I1 & I2 is the make contract Q1 is the Relay coil.

NAND Implementation

The condition of NAND gates is that there is no output when both A & B have an input. But when one of the inputs is energized the output goes high. Shown in truth table.

I1 I2 Output
0 0 1
0 1 1
1 0 1
1 1 0

Figure 6.1(c-1): Truth table for NAND Implementation

Figure 6.1(c-2): Compare the Electrical base to PLC base of an NAND Implementation


I1 & I2 is the Brake contract,

M is the motor.

It is opposite of the AND Implementation. When every input is high this time the output is low, otherwise every condition the output is high.

Figure 6.1(C-3) : PLC Ladder diagram of an NAND Implementation


I1 & I2 is the Brake contract,

Q1 is the Relay coil.

NOR Implementation

From the truth table we can see that there is only single case the output will high only if the both input is low. But if one of the input or both of the input is high then the output will low. Shown in truth table.

I1 I2 Output
0 0 1
0 1 0
1 0 0
1 1 0

Figure 6.1(D-1) : Truth table for NOR Implementation

Figure 6.1(D-2) : Compare the Electrical base to PLC base of an NOR Implementation


I1 & I2 is the Brake contract.

M is the Motor.

It is opposite of the OR Implementation. When every input is low then output is high, otherwise every condition the output is low.

Figure 6.1(D-3) : PLC Ladder diagram of an NOR Implementation


I1 & I2 is the Brake contract,

Q1 is the Relay coil.

Exclusive OR Implementation

The Exclusive OR gate gives the output 1 when either of the inputs is 1 but not when both are 1. When input I1 is activated and I2 is not activated then outout Q1 is activated. Also when the input I1 is not activated and I2 is activated the output Q1 is activated. But when I1 & I2 both are activated at a time then the output Q1 is not activated. Shown in truth table.

I1 I2 Output
0 0 0
0 1 1
1 0 1
1 1 0

Figure 6.1(e-1) : Truth table for Exclusive OR Implementation

Figure 6.1(e-2) : Compare the Electrical base to PLC base of an Exclusive OR Implementation


I1 is the make & brake contract.

I2 is the make & brake contract.

M is the motor.

When every input is high or low then the output is low, otherwise any condition output is high.

Figure 6.1(e-3) : PLC Ladder diagram of an Ex-OR Implementation


I1 is the make & brake contract.

I2 is the make & brake contract.

Q1 is the Relay coil.

6.2 Basic Multiplexer operation

A Multiplexer Combines more than one input in to Sign ale output. It is an Electronics device. The multiplexer or MUX is Combines several electrical into a signal.

A 4-line to 1-line multiplexer is shown figure. Each of the four input lines I0 to I3 is to applied to one input of an AND gate. Selection lines S1 and S0 are decoded to select a particular AND gate. The function table in the figure lists the input to output path for each possible bit combination of selection line. When this MSI function is used in the design of the digital system. It is represented in block diagram from as show in figure. To demonstrate the circuit operation consider the case when S1 S0 =10. The AND gate associated to I2 has two of its input equal to 1 and the third input connection to I2 . The other three AND gates have at least one input equal to 0, which makes their output equal to 0 .the OR-gate output is now equal to the value of 0, thus providing a path from the selected input to the output. A multiplexer is also called a data selector since it selects one of many input and steers the binary information to the output line.

The AND gates and invertors in the multiplexer resemble a decoder circuit and indeed they decode the input selection lines. In general, a 2n-to-1 line multiplexer is constructed from an n-to-2n decoder by eddied to a single OR gate to provide the 1-line output. The size of a multiplexer is specified by the number 2n of its input lines and the single output line. It is then implied that it also contains n selection lines. A multiplexer is often ad deviated as MUX.

There are different types of multiplexer for analog or digital.

Input Output
S0 S1 S2 Y
0 0 0 I0
0 0 1 I1
0 1 0 I2
0 1 1 I3
1 0 0 I4
1 0 1 I5
1 1 0 I6
1 1 1 I7

8to1-line multiplexer

Logical diagram

Figure 6.2(a) : A (4X1) lines Multiplexer

Figure- Truth Table

Figure 6.2(b) : PLC Ladder diagram of an Multiplexer

6.3 Basic De-multiplexer operation

A De-multiplexer is a Combines one then more input into signal. It is a electronics devise. The de-multiplexer or DMUX is combine several electrical in to a signal.

A decoder with an enable input can function as a de-multiplexer. A de-multiplexer is a circuit that receives information on a single line and transmits this information on one of 2n possible output lines. The selection of specific output line is controlled by the bit values of n selection line. The decoder function as a de-multiplexer if the E line is taken as a data input line and line A and B are taken as the selection lines. The single input variable E has a path to all four output but the input information is directed to one of the output lines as specified by the binary value of the two selection lines A and B. this can be verified from the truth table of this circuit showing the figure. From example if the selection lines AB=10,output D2 will be the same as the input value E which all other output are maintained at 1. Because decoder and de-multiplexer operations are obtained from the same circuit, a decoder with an enable input is referred to as a decoder / de-multiplexer. It is the enable input that makes the circuit a de-multiplexer.

There are different types of multiplexer for analog or digital.


Input Output
I S0 S1 S2 D0 D1 D2 D3 D4 D5 D6 D7
1 0 0 0 1 0 0 0 0 0 0 0
1 0 1 1 0 1 0 0 0 0 0 0
1 0 0 0 0 0 1 0 0 0 0 0
1 0 1 1 0 0 0 1 0 0 0 0
1 1 0 0 0 0 0 0 1 0 0 0
1 1 1 1 0 0 0 0 0 1 0 0
1 1 0 0 0 0 0 0 0 0 1 0
1 1 1 1 0 0 0 0 0 0 0 1

Logical diagram

Figure 6.3(a) :(1X4) Lines De-Multiplexer Figure- Truth Table

Figure 6.3(b) : PLC Ladder diagram of an De-multiplexer

6.4 Basic Encoder operation

A encoder is quantities of information are represented in digital system with binary codes.

A encoder is a digital function that produces a reverse operation from that of decoder. An encoder has 2n input lines n output lines. The output lines generate the binary code for the 2n input variables. An example of an encoder. The opal-to-binary encoder consists of eight inputs, one for each of the eight digits, and three outputs that generate the corresponding binary number. It is constructed with or gates whose input can be determined from the truth table given. The low order output bit z is 1 if the output octal digit is odd. Output y is 1 for octal digit 2,3,6 or 7. Output is a 1 for octal digits 4,5,6 or 7. Notes the D0 is not connected to any OR gate. The binary output must all 0,s. in this case. An all 0,s output is also obtained when all inputs are all 0,s. this discrepancy can be resolved by providing one more output to indicate the fact that the all inputs are not 0,s.

The encoder assumes that only one input line can be equal to 1 at any time, otherwise the circuit has no meaning. Note that the circuit has eight input and could 28 = 256 possible input combinations. Only eight of these combinations have any meaning. The other input combinations are don’t-care conditions.

Encoders of the type are not available in IC packages, since they can be easily constructed with OR gates. The type of encoder available in IC from is called a priority encoder. These encoders establish an input priority to ensure that only the highest-priority input line is encoder. Thus, in priority is given to an input with a higher subscript number over one with lower subscript number, then if both D2 and D5 are logic-1 simultaneously the output will be 101 because D5 has a higher priority over D2 . of course the truth table of a priority encoder is different from the one.

Input Output
D0 D1 D2 D3 D4 D5 D6 D7 X Y Z
1 0 0 0 0 0 0 0 0 0 0
0 1 0 0 0 0 0 0 0 0 1
0 0 1 0 0 0 0 0 0 1 0
0 0 0 1 0 0 0 0 0 1 1
0 0 0 0 1 0 0 0 1 0 0
0 0 0 0 0 1 0 0 1 0 1
0 0 0 0 0 0 1 0 1 1 0
0 0 0 0 0 0 0 1 1 1 1

8X3 Encoder

Figure 6.4(a) : A 8X3 Encoder Figure: Truth table

Figure 6.4(b): PLC Ladder diagram of an Encoder

6.5 Basic Decoder operation

A decoder is quantities of information are represented in digital system with codes.

A decoder is a combinational circuit that convents binary information form N input line to a maximum of 2n unique output lines. If the n-bit decoder information has unused or don’t care combination, the decoder output will have less then 2n output. The decoder presented here called n-to-m liens decoder where m ?2n. their purpose is to generate the 2n (or less) minters of n input variables. The name decoder is also used in conjuration with some cod converters such as a BCD-to-seven-segment decoder. As an example consider of the decoder the 3-to-8 lines decoder circuits. The three input are decoder into eight output. Each output representing one of the minters of the 3-input variables. The three inverters provide the complement of the input, and each one of the eight AND gate generates one of the minters. A particular application of this decoder would be a binary to octal conversion. The input variables may represent a binary number and the output will then represent the eight digit in the octal number system. However a 3-to-8-bit line decoder can be used for decoding any 3-bit code to provide eight output one for each element of the code.The operation of the decoder may be further clarified from its input-output relationship listed in table. Observer that the output variables are mutually exclusive because only one output can be equal to 1 at any one time. The output line whose value is equal to 1 represent the midterms equivalent of the binary number presently available in the output lines.

The element of information in this case are the ten decimal digits represented by the BCD code. The code itself has for bits. Therefore the decoder should have four input to accept the coded digit and ten output one four each decimal digit. This will give a 4-lines to 10-lines BCD-to-decimal decoder.

3X8-Line Decoder:

Input Output
A B C D0 D1 D2 D3 D4 D5 D6 D7
0 0 0 1 0 0 0 0 0 0 0
0 0 1 0 1 0 0 0 0 0 0
0 1 0 0 0 1 0 0 0 0 0
0 1 1 0 0 0 1 0 0 0 0
1 0 0 0 0 0 0 1 0 0 0
1 0 1 0 0 0 0 0 1 0 0
1 1 0 0 0 0 0 0 0 1 0
1 1 1 0 0 0 0 0 0 0 1

Figure 6.5(a) : 3*8 lines decoder

Figure: Truth table

Figure 6.5(b) : PLC Ladder diagram of an Decoder



7.1 Conclusion and Future work

Every Equipment is being conducted through Technology in the present modern world. At present every kinds of machine is being controlled automatically. PLC is one kinds of automatic controlled Instrument. In which machine, motor and various instrument are controlled. PLC is a modern invention of a technology.

We know This Thesis Programmable Logical Controller (PLC) we used to Industry sector and different electrical & electronics instruments controlling & protection. We prepare my dissertation paper helping from Bangladesh Industrial Technical Assistance Center (BITAC). Short Term Technical Training Programme on Programmable Logical Controller (PLC) and some book, web site and my dissertation teacher. Actually, we prepare my dissertation paper is study base. To draw the figure, we am using Computer soft program SIEMENCE logo Soft Comfort & Power point. As we can implement DLD (Digital Logic Design) design using PLC (Programmable Logic Controller) design efficiently and skillfully, it is better to explore and enhance the quality of PLC design instead of DLD design. we can implement PLC timers not only for the control of Industrial Process but also we can implement by using the PLC we control the alarm of drill replacement, conveyer control system, Flickering lamp, Motor protection, Heater temperature control, Level flow indicator, Fault analysis, System protection, Boolean Algebra express, Latching process, Basic Logical Operation, Multiplexer, De-multiplexer, Encoder, Decoder etc.

I am doing my thesis with new equipment of technology. I have tried a lot through my thesis the good conception of PLC and its uses. I will try to make same thing good through PLC in future which contribute in modern world and in the era of technology advancement. I will take a better concept with all types of instrument which is used in PLC. Furthermore I will try at my level How to use PLC in different instruments. I will make a Electronic Vote Counting in future in which vote counting can be done authentically and which will control through PLC. Through Electronic Voting Marching is used at present. But my machine will be ultra-modern one.

7.2 Bibliography

[1] John W. Webb, Northcentral Technical College, Wausau,Wisconsin & Ronald A.

Reis,Los Angeles Valley College, Valle Glen, California. Fifth Ed. ‘‘Programmable

Logic Controllers’’ 2003,

[2] M.Morris Mano,Professor of Engineering, California State University, Los

Angeles. ‘‘Digital Logic and Computer Design’’ 2011-2012.

[3] B L. Theraja, Professor of Head, Electrical Engineering Department Visvesvaraya

Regional College of Engineering, Nagpur. ‘‘A Text Book of Electrical


[4] Md.masud Rana, B.Sc Engg.(ME),Assistant Engineer, Bangladesh Industrial

Technical Assistant Center (BITAC), ‘‘Training Course on Programmable Logic

Controller (PLC)’’ 2011.

[5] Hugh Jack, ‘‘Automating Manufacturing Systems with PLCs’’ Version 5.1.

[6] M. Motherul Hossain, ‘‘Microprocessors & Microcomputer’’ 2007.

[7] M. Rafiquzzaman, V Professor, California State Polytechnic University.

‘‘Microprocessors & Microcomputer-Based system design’’1995.

[8] john wiley & Sons, ‘‘Microprocessors and Logic Design’’ 1980.

[9] M. Petriu, ‘‘Digital Logic Pocket Data Book’’ 2007.

[10] M. Rifekuzzaman, ‘‘Digital Electronic and Microprocessors’’ 2004.

[11] D.Gritzalis, editor‘‘Secure Electronic Voting’’ Springer-Verlag,

BerlinnGermany, 2003.

[12] MEYER, B. ‘‘Applying design by contract’’. IEEE Computer (Oct.1992)

[13] D.Geer, K. Soo Hoo, A. Jaquith. ‘‘Information Security’’: Why the Future

Belongs to


The information contains in this web-site is prepared for educational purpose. This site may be used by the students, faculties, independent learners and the learned advocates of all over the world. Researchers all over the world have the access to upload their writes up in this site. In consideration of the people’s participation in the Web Page, the individual, group, organization, business, spectator, or other, does hereby release and forever discharge the Lawyers & Jurists, and its officers, board, and employees, jointly and severally from any and all actions, causes of actions, claims and demands for, upon or by reason of any damage, loss or injury, which hereafter may be sustained by participating their work in the Web Page. This release extends and applies to, and also covers and includes, all unknown, unforeseen, unanticipated and unsuspected injuries, damages, loss and liability and the consequences thereof, as well as those now disclosed and known to exist.  The provisions of any state’s law providing substance that releases shall not extend to claims, demands, injuries, or damages which are known or unsuspected to exist at this time, to the person executing such release, are hereby expressly waived. However the Lawyers & Jurists makes no warranty expressed or implied or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product process or service by trade name, trade mark, manufacturer or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favouring by the Lawyers & Jurists. The views and opinions of the authors expressed in the Web site do not necessarily state or reflect those of the Lawyers & Jurists. Above all, if there is any complaint drop by any independent user to the admin for any contents of this site, the Lawyers & Jurists would remove this immediately from its site.