PLC-SCADA PROJECT REPORT                                                      

                                                                  Chapter 1

  Simplification of engineering and precise control of manufacturing process can result in significant cost savings.  The most cost-effective way, which can pay big dividends in the long run, is flexible automation; a planned approach towards integrated control systems.  It requires a conscious effort on the part of plant managers to identify areas where automation can result in better deployment/utilization of human resources and savings in man-hours, down time.  Automation need not be high ended and too sophisticated; it is the phased, step-by-step effort to automate, employing control systems tailored to one’s specific requirements that achieves the most attractive results.  That is where Industrial electronics has been a breakthrough in the field of automation and control techniques.


A constant demand for better and more efficient manufacturing and process machinery has led to the requirement for higher quality and reliability in control techniques.  With the availability of intelligent, compact solid state electronic devices, it has been possible to provide control systems that can reduce maintenance, down time and improve productivity to a great extend.  By installing efficient and user friendly industrial electronics systems for manufacturing machinery or processors, one can obtain a precise, reliable and prolific means for generating quality products.
Considering the varied demand and increasing competition, one has to provide for flexible manufacturing process.  One of the latest techniques in solid state controls that offers flexible and efficient operation to the user is “PROGRAMMABLE CONTROLLERS”.  The basic idea behind these programmable controllers was to provide means to eliminate high cost associated with inflexible, conventional relay controlled systems.  Programmable controllers offer a system with computer flexibility:

1.      Suited to withstand the industrial environment

2.      Has simplicity of operation

3.      Maintenance by plant technicians and

4.      Reduce machine down time and provide expandability for future.



A Programmable controller is a solid state user programmable control system with functions to control logic, sequencing, timing, arithmetic data manipulation and counting capabilities.  It can be viewed as an industrial computer that has a central processor unit, memory, input output interface and a programming device.  The central processing unit provides the intelligence of the controller.  It accepts data, status information from various sensing devices like limit switches, proximity switches, executes the user
control program store in the memory and gives appropriate output commands to devices like solenoid valves, switches etc.

Input output interface is the communication link between field devices and the controllers; field devices are wired to the I/O interfaces.  Through these interfaces the processor can sense and measure physical quantities regarding a machine or process, such as, proximity, position, motion, level, temperature, pressure, etc.  Based on status sensed, the CPU issues command to output devices such as valves, motors, alarms, etc.
Programmer unit provides the man machine interface.  It is used to enter the application program, which often uses a simple user-friendly logic.


1.      Programmable controllers are made of solid state components and hence provide high reliability.

2.      They are flexible and changes in sequence of operation can easily be incorporated due to programmability.  They may be modular in nature and thus expandability and easy installation is possible.

3.      Use of PLC results in appreciable savings in Hardware and wiring cost.

4.      They are compact and occupy less space.

5.      Eliminate hardware items like Timers, counters and Auxiliary relays.  The presence for timers and counters has easy accessibility.

6.      PLC can control a variety of devices and eliminates the need for customized controls.

7.      Easy diagnostic facilities are provided as a part of the system.  Diagnosis of the external systems also becomes very simple.  Thus easy service/maintenance.

8.      Programming devices provide operator friendly interface with the machine. Being an outcome of the latest art of electronics technology, Programmable controllers provide higher level of performance with computers is possible.  Useful management data can be obtained and maintained.

9.      It has total protections against obsolescence and has wide scope for up gradation.

                                     Chapter 2


PLCs contain three basic sections:

1.      Central processing unit (CPU).

2. Memory: EPROM, RAM, and so on.

3. Input/output section for communication with peripherals (ADC, DAC).

A PLC is basically a black box with a number of inputs from, and a number of outputs to, the outside world.  It can make decisions, store data, do timing cycles, do simple arithmetic, convert codes, and so on.  The basic difference between this black box and a hardware logic system using IC chips or a relay controlled system, is that specific coded messages are stored in areas called program memory, which are PROM or ROM and RAM chips.  It is, however, much easier to change a program when a different process is required than to rewire the control system.  For example, it may take electricians a couple of weeks to require a pipe mill, whereas a programmer will spend only a fraction of this time to reprogram a PLC since no wires will have to be changed.  In addition, various recipes can be stored in memory and accessed when required, making the program extremely flexible.
The system operates through interaction with the processor and program memory.  When the power to the system is turned on, the processor reads the first instruction stored in memory and acts on this instruction.  When completed, it goes back to the memory for the next instruction, and so on until task is complete.  This operation is called the fetch-execute cycle.  The processor communicates with the outside world via input and output modules.


Programmable logic controllers (PLC) can be considered to have three parts:

1.      Input/output Section

The I/O section contains input modules and output modules. Functionally, the input modules are equivalent to the signal converters (i.e. Analog to Digital or high power to low power). All modern PLC input modules use optical devices to accomplish electrically isolated coupling between the input circuit and the processor electronics.
Each input device is wired to a particular input terminal on the I/O section.  Thus if the switch is closed, 5v dc appears on input terminal, converts this dc voltage to a digital 1 and sends it to the processor via programmable peripheral interface (PPI).  Conversely, if the switch is open, no dc voltage appears on input terminal.  Input section will respond to this condition by sending a digital 0 to the processor.  The other input terminals behave identically.

1.      The Processor

The processor of a PLC holds and executes the user program.  In order to carry out this job, the processor must store the most up-to-date input and output conditions.

(a)  Input  image table:

The input conditions are stored in the input image table, which is a portion of the processor’s memory.  That is, every single input module in the I/O section has assigned to it a particular location within the input image table.  That particular location is dedicated solely to the task of keeping track of the latest condition of its input terminal.  As mentioned in earlier section, if the input terminal has 5v dc power fed to it by its input device, the location within the input image table contains a binary 1(HI); if the input terminal has no 5v dc power fed to it, the location contains a binary 0(LO).
The processor needs to know the latest input conditions because the user program instructions are contingent upon those conditions.  In other words, an individual instruction may have one outcome if a particular input is HI and a different outcome if that input is LO.

(b)  Output image table:

The output conditions are stored in the output image table, which is another portion of the processor’s memory.  The output image table bears the same relation to the output interface of the I/O section that while terminals are analog inputs.  You can directly connect any analog input to the processor via these terminals.  Analog signal from these terminals is first converted to digital value via programmable peripheral interface (PPI).  The I/O section’s output modules are functionally the same as the output amplifiers.  They receive a low power digital signal from the processor and convert it into a high power signal capable of driving an industrial load.  A modern PLC output module is optically isolated, and uses a triac, power transistor or relay as the series connected load controlling device. Terminal 1 to 8 are these type of O/P terminals whereas terminal D/A is Analog output terminal from processor.  Each output device is wired to a particular output terminal on the I/O interface.  Thus, for example, if output module 1 receives a digital 1 by applying 5v dc to output terminal 1, thereby illuminating LED is extinguished.
Besides 5v dc (TTL devices), I/O module are also for interfacing to other industrial levels, including 12v dc.
The input image table bears to the input modules.  That is, every single output module has assigned to it a particular memory location is dedicated solely to the task of keeping track of the latest condition of its output module.
Of course, the output situation differs from the input situation with regard to the direction of information flow is from the output image table to the output modules, while in the input situation the information flow is from the input modules to the input image table.  The locations within the input and output image tables are identified by addresses, which refers to unique address of each terminal.

(c)        Central processing unit:

The subsection of the processor that actually performs the program execution will be called the central processing unit (CPU) with reference to input and output image table CPU executes the user program and continuously updates the output image table.
The output image table has a dual nature; its first function is to receive immediate information from the CPU and pass if on to the output modules of the I/O section; but secondly, it also must be capable of passing output information “backward” to the CPU, when the user program instruction that the CPU is working on calls for an item of output information.  The input image table does not have its dual nature.

  Its single mission is to acquire information from the input modules and pass that information “forward” to the CPU when the instruction that the CPU is working on calls for an item of input information.

(d)  User program memory:

A particular portion of the processor’s memory is used for storing the user program instructions.  We will use the name user program memory to refer to this processor subsection.
Before a PLC can begin controlling an industrial system, a human user must enter the coded instructions that make up the user program.  This procedure called programming the PLC.
As the user enters instructions, they are automatically stored at sequential locations within the user program memory.  This sequential placement of program instructions is self-regulated by the PLC, with no discretion needed by the human user.
The total number of instructions in the user program can range from a half dozen or so, for controlling a simple machine, to several thousand, for controlling a complex machine or process.
After the programming procedure is complete, the human user manually switches the PLC out to PROGRAM mode into RUN mode, which causes the CPU to start executing the program from beginning to end repeatedly.

(e)  The complete scan cycle:

As long as the PLC is left in the RUN mode, the processor executes the user program over and over again.  Figure depicts the entire repetitive series of events.  Beginning at the top of the circle representing the scan cycle, the first operation is the input scan.  During the input scan, the current status of every input module is stored in the input image table, bringing it up to date.
Following the input scan, the processor enters its user program execution.  Sometimes called “program scan”.  The program executes with reference to input and output image tables and updates output image table.
Throughout the user program execution, the processor continuously keeps its output image table up to date, as stated earlier.  However, the output modules themselves are not kept continuously up to date.  Instead, the entire output image table is transferred to the output module during the output scan following the program execution.

(f)   Data Memory:

A PLC is a computer, after all.  Therefore, it can perform arithmetic, numeric comparisons, counting, etc.  Naturally the numbers and data can change from one scan cycle to the next.  Therefore the PLC must have a section of its memory set aside for keeping track of variable data, or numbers, that are involved with the user program.  This section of memory we will call data memory.
When the CPU is executing an instruction for which a certain data value must be known, that data value is brought in from data memory.  When the CPU executes an instruction that provides a numerical result, that result is put out into data memory.  Thus, CPU can read from or write to the data memory.  Understand that this relationship is different from the relationship between the CPU and the user program memory.  When the user program is executing, the CPU can only reads from the user program memory, never write to it.

(g)  Operating System of PLC:

The function of the operating system is to present the user with the equivalent of an extended machine or virtual machine that is easier to program than the underlying hardware.

Due to this operating system, PLC is very easy to program.  It can be programmed using electrical schemes with familiar relay symbols so that a plant electrician can easily access the PLC.  Even though he does not know the assembly language or even if he may not have any familiarity with computers and electronics, he will be able to program the PLC.
The function of PLC Operating system is:

1.      Loads the user program from programming device to program memory.

2.      To read status of input devices.

3.      To execute user program.

4.      To form and update input image table.

5.      As per the status of output image table controls the output devices.

6.      To provide user-friendly functions.

This O.S. makes supervision over entire system, so O.S. programs are said to running in supervisory mode.
When the user completely enters his program in user memory, he transfers control from PROGRAM mode to RUN mode.  In RUN mode the control of the whole system is transferred to operating system.  Now operating system takes care of the whole system such that the whole system becomes automatic and appears as magic to users.



This low cost PLC system was designed to satisfy hunger of Automation of Indian Industry and also             helps beginners as well as development engineers to get into Automation field.

System consists of following main sections:

(1)   The CPU:

The CPU uses the 89c51 microcontroller, which operates at 11.0592Mhz.  It has 8k RAM, which can be used as data memory, 8k RAM that can be used as program memory as well as data memory, 8k EEPROM that can be used as program memory.

(2)   Input/output Section:

This part of system is on separate board connected to processor via cable.  It allows the processor to communicate with the outside world.  It is also called Data Acquisition System (DAS).
This part of system provides 4 digital inputs consisting of 2 dc and 2 ac, 4 digital outputs consisting of 2 dc and 2 ac each.  It also provides 8 analog inputs with following ranges:

1.      –5v to +5v (one channel).

2.      0v to 10v (one channel).

3.      4mA to 20mA (one channel).

4.      0v to 5v (five channel).

(3)   Timer/Counter:

The system has 2 timers or 2 counters or 1 timer and 1 counter.  The timer provides maximum of 255sec delay and the counter provides maximum of 255 counts.

(4)   Serial Communication:

The system uses RS-232 serial data standard.  Chip ICL232 is used as communication interface between RS-232 standard and TTL logic.

(5) Programming Device:

This system uses personal computer (PC) as programming device.  The user can write program in user friendly language.  The programming devices (PC) convert this user friendly language program into machine understandable language and transmit it to the PLC board via serial communication.

(6)  Power Supply Unit:
This system provides +12v and -12v with maximum 2amps and +5v with maximum of 1amps.
                                       Chapter 4


1.    Microcontroller:

Here we are using 89c51 microcontroller, which has one full duplex serial data receiver/transmitter, which is used for serial communication having interface with ICL232 chip.
It has also two 16 bits timer/counter namely T0 and T1 which are used for timer and counter applications.  Timer T1 is used to set baud rate for serial communication in program mode.

2.    Memory:

The system consists of four types of memory:

a.       4k of EEPROM which is internal to 89c51 microcontroller.  This memory is used to store the operating system.  It has address from 0000h to 0fffh.  It can only be accessed when the external access pin of controller is connected to +5v.  In our system this pin is permanently connected with +5v so external program memory is accessed only when the address is beyond 0fffh.

b.      8k of RAM which is used as data memory.  The CPU can read data from and write data into this memory.  This memory has address from 0000h to 1fffh.

c.       8k of RAM which is used as data memory as well as program memory.  The CPU can write program codes in and read program codes from this memory.  This memory has address from 2000h to 3fffh.

d.      8k of EEPROM, which is used as, program memory.  The subroutines, which are helpful in executing the main program, are stored here.  This memory has location from 4000h to 5fffh.

3.    Programmable Peripheral Interface(PPI):

Here two 8255 are used as PPI.  One is used to control the ADC and DAC, while other is used for Input/output interface.  The addresses for the 8255 used to control ADC and DAC are:

Port A: 6000h

Port B: 6001h

Port C: 6002h

Control Word: 6003h

The addresses for the 8255 used for Input/output interfaces are:

Port A: 8000h

Port B: 8001h

Port C: 8002h

Control Word: 8003h

4.    Analog to Digital Converter(ADC):

Here ADC0809 is used as an 8 bit ADC.  8255 whose addresses are 6xxxh is used to give control signals to this ADC.  The port pins of 8255 are connected with the control pins of ADC as shown below:

8255 Pins                    ADC Pins

PB0                             ADD0 (A)

PB1                             ADD1 (B)

PB2                             ADD2 (C)

PB3                             STC (Start of Conversion)

PB4                             OE (Output Enable)

PC7                             EOC (End of Conversion)

Note:   Here PB4 is connected to OE pin of ADC through NOT Gate.  So we have to give negative pulse by pin PB4 to pin OE to give Output Enable.
This ADC is used to convert the real world analog data into digital form.

5.    Digital to Analog Converter (DAC):

Here the only control signal is “Start of Conversion”, which is connected with PC0 of 8255 having address 6xxxh.  For converting the digital data to analog form first make PC0 low and then put digital data on port0 of 89c51.  Now make PC0 high.
This particular part of the system is idle in our application, but it is kept for future expansion.

6.    Serial Communication:

Here in-built transmitter/receiver of 89c51 is used for serial communication in conjunction with chip ICL232.  Here the transmitter/receiver is of asynchronous type (UART).  So the data is communicated byte by byte.  The UART is working in serial communication mode 1.  So the timer T1 is used to set the baud rate.  The baud rate is set to 2400.

7.    Switches and Indicators:


a.       Power ON/OFF switch.

b.      Reset Switch.

c.       Program/Run mode Switch.


a.       Power ON/OFF LED(red)

b.      Reset LED(red)

c.       Program mode LED(orange)

d.      Run mode LED(green)

e.       Fault LED(red, green, orange, yellow)

8.    Digital Input:

DC Input:

We have two digital DC inputs with following specifications:

1.      0v to 5v – LOW

2.      20v to 25v- HIGH

3.      Opt coupler Isolation.

AC Input:

We have two digital AC inputs with following specifications:

1.      0v to 10v-LOW

2.      20v to 25v-HIGH

3.      47hz to 63hz frequency.

4.      Opt coupler Isolation.

9.    Digital Outputs:

DC outputs:

We here have two DC outputs with following specification:

1.      0v t0 3v-0v

2.      3.5v to 5v-24v

3.      0.5amp output current.

4.      Opt coupler Isolation.

AC outputs:

We have two relays as AC outputs with following specification:

1.      0v to 3v- relay OFF

2.      3.5v to 5v- relay ON

3.      Opt coupler Isolation.

4.      Relay with12v, 4ohm.
                                                  Chapter 5

AC Output:

Output                                     Address

Relay 1(Normally Open)         00

Relay 2(Normally Open)         01

Relay 1(Normally Close)        20

Relay 2(Normally Close)        21

DC Output:

Output                                     Address

Out 1(Normally Open)            02

Out 2(Normally Open)            03

Out 1 (Normally Close)          22

Out 2 (Normally Close)          23

AC Input:

Input                                       Address

In 1(Normally Open)              04

In 2(Normally Open)              05

In 1(Normally Close)              24

In 2(Normally Close)              25

DC Input:

Input                                       Address

In 1(Normally Open)              06

In 2(Normally Open)             07

In 1(Normally Close)              26

In 2(Normally close)               27

Analog Input:

Input                                       Address

In 1(Normally Open)              08

In 2(Normally Open)              09

In 3(Normally Open)              0a

In 4(Normally Open)              0b

In 5(Normally Open)              0c

In 6(Normally Open)              0d

In 7(Normally Open)              0e

In 8(Normally Open)              0f

In 1(Normally Close)              28

In 2(Normally Close)              29

In 3(Normally Close)              2a

In 4(Normally Close)              2b

In 5(Normally Close)              2c

In 6(Normally Close)              2d

In 7(Normally Close)              2e

In 8(Normally Close)              2f


Counter/Timer                         Address

Counter 0/Timer 0                   00
(Normally OFF)

Counter 1/Timer 1                   01
(Normally OFF)

Counter 0/Timer 0                   02
(Normally ON)

Counter 1/Timer 1                   03
(Normally ON)

Note: The timer and counter has same address because at a time we can use only one of them i.e. either counter 0 with address 00 or timer o with address 00.  Here normally OFF means when timer/counter is running this remains low and when count of timer/counter reaches it becomes high.  Reverse is the case for normally ON.

                                           Chapter 6

                                                SOFTWARE DESCRIPTION

This chapter explains how to program the PLC.  It describes how to write a program, how the program is structured and representation of the programming language.

Writing a Program:

A control program specifies a series of operations that tell the programmable controller how it has to control a system.  For example, a control program might be the series of operations that tell the PLC how to use open loop control or close loop control for a specific system.  We must write the program in a specific programming language and according to some specific rules so that the programmable controller can understand it.

Method of Representation:

The following methods of representation are possible to program PLC.

1.      Statement List (STL):

STL represents the program as a sequence of operation mnemonics.  A statement has the following format:

A   I   04

Where A represents AND operation,

I represent INPUT and

04 represent ADDRESS of INPUT.

2.      Control System Flowchart(CSF):

CSF represents logic operations with graphics symbols.

3.      Ladder Diagram(LAD):

LAD graphically represents control functions with circuit diagram symbols.

4.      Graph 5:

Graph 5 describes the structure of sequence control systems.

In our system we are using Statement List (STL) Language.


I                       Inputs

U                     Outputs

T                      Timer

C                     Counter

A                     AND Operation

O                     OR Operation
                        =                      Equals

E                      End of Program


In our system STL supports the following operations:



AND Operation:

The AND operation scans to see if various conditions are satisfied simultaneously.

Circuit Diagram

Output U 01 is “1” when all two inputs are “1”.   The
Output is “0” if atleast one input is “0”.
I 04

I 05

U 01

Representation in Ladder Diagram                      Representation in STL

A   I  04
A   I  05
I 04   I 05    U 01                                                                    =   U  01

OR Operation:

The OR operation scans to see if one of two conditions has been satisfied.

Circuit Diagram:

                                     Output U 01 is “1” when at-least one of the
                                                  I 04         I 05
                                      Input is “1”.  Output U 01 is “0” when all
inputs are “0” simultaneously.

                                                                         U 01

Representation in STL:

O  I 04
O  I 05
=  U 01

Representation in Ladder Diagram

I 04

I 05

U 01

Representation in STL:

O  I  04
O  I  05
O  I  06
O  I  07
=  U  01

AND before OR Operation:

Output U 01 is “1” when at-least one AND condition has been satisfied.  Output U 01 is “0” when neither of the two AND conditions has been satisfied.

Representation in STL:
A  I  04
A  I  05
A  I  06
A  I  07
=  U  01

 Circuit Diagram:                                                      Ladder Diagram:

I 04

I 06

I 05

I 07

U 01

I 05

I 07

U 01

 I 04

I 06









Timer Operation:


Representation in STL:

A   I  04
A   I  05
=   T  00  10

O   I   05
O   T  00
=   U  01

If both inputs I 04 and I 05 are “1” then only timer T 00 is activated.  The count 10 after T 00 in third instruction specifies that the timer should continue till 10 seconds.  O  T  00 specifies that the output U 01 is “1” only if I 05 is  “1” or the count of timer has reached 0.  If we specify T  02 which is normally ON then the output U 01 is “1” if I 05 is “1” or during timer is running.
This is for timer 0 if we use timer 1 then T 01 will remain “0” during the timer run and normally ON T 03 will remain “1” during timer run
                                                  Chapter 7

Syntax for Program

1.      Operand must be in upper case.

A I 04        right;

Ai   04        wrong;

2.      Always leave only one space between operand.
3.      A I 04        right;

               AI 04            wrong;


   A    I 04        wrong;


   A I  04          wrong;


    A I    04       wrong;


    AI04            wrong;



4.      Clear out all the previously written instruction from the file before writing instruction for another application.


5.      Always specify hex number in lower case.



=  T 00 0a              right;


=  T 00 0A             wrong;

6.      Specify the count of counter and delay of timer in hex.

= T 00 ff                right;

= T 00 255             wrong;

7.      Do not keep any line spacing between two instructions
8.      Always end your program with ‘E’.

                                                    Chapter 8

                                                              HOW TO USE PLC

Ø  Write program in STL language in file ‘PLC’ on PC using NORTON EDITOR (Do not give any extension to the PLC file).

Ø  Switch on the PLC and confirm whether the power indicator LED ON or not.  If it is ON, switch the PLC in PROGRAM mode.  Confirm that the RED LED indicating the PROGRAM mode is ON.  Now PLC is ready for receiving the control program written on PC.

Ø  Execute the program named “PLC” on PC in TC directory (i.e. TURBO C).


Ø  Switch the PLC in RUN mode by changing the switch on RUN mode and then press RESET switch.  Confirm the GREEN LED indicating the RUN mode is ON.

Ø  If one of the four FAULT LEDs is ON then check the reason and again load whole of the program and repeat the above mentioned procedure.


Ø  If “POWER ON” LED (red) is OFF after switch ON the power, check the FUSE.  If blown then replace it.

Ø  If FAULT LED (red) is ON then “A” or “O” operand might have been missing in start of instruction.
                    i.e.           A I 04
                                    I 04

Correct it in the control program and again load the program.

Ø  If FAULT LED (green) is ON then “I” or “C” or “T” or “U” operand might have been missing in instruction.
i.e. A I 05
A 05
                    Correct it in the control program and again load the program.
Ø  If FAULT LED (saffron) is ON then “=” operand might be missing in instruction.
                 i.e.  A I 05
                       A I 04
                        U 01
Ø  If FAULT LED (yellow) is ON then the TIMER/COUNTER is specified other than ‘00’ or ‘01’.

                        i.e.       A I 04
                        A I 05
                        = T 02 0a


                                           Chapter 9

                                                     Software analysis

The higher-level language program on PC side converts the control program written in STL language into specific hex codes and transmits these codes to the PLC.  The codes are stored in RAM location in PLC.

The lower-level language program, which is stored in EEPROM internal to the controller, reads hex codes from memory location one by one.  It compares the code read from the memory location with the predefined hex code and executes the instruction with accordance to it.

As soon as we put the PLC in RUN mod, it reads the status of all the I/P. It stores the status in bit addressable memory location as ‘1’ or ‘0’ depends upon the status. Now it runs the program and performs operation on the status bit directed as user program. It stores the result in another bit addressable memory locations which are corresponds to the O/P. After this it updates all the O/P.

The Flow chart of the ASSEMBLY LANGUAGE program is given below:



¨      At present in our system we have 4 digital inputs and 4 digital outputs which are interfaced with CPU through port C of 8255 having address 8xxxh.  The port A and port B are not presently taken in use.  By using these ports we can expand digital inputs and outputs upto 8 digital inputs and outputs.

¨      Two memory slots of 8k each are not in present use.  One of these slots can be used for RAM and another one can be used for EEPROM.  Using these slots we can get more 8k of DATA memory using RAM and more 8k of PROGRAM memory using EEPROM.

¨      Interfacing of DAC is provided on the PLC board, which can be used to give an analog output.

¨      We can construct signal-conditioning circuit so we can directly connect the transducers like PT100, thermocouple, LVDT etc.

¨      We can further construct PID controller module which can be switched ON or OFF by this PLC so we can control systems using continuous controller in digital fashion.

¨      We can also design stepper motor controller, which can be interfaced with PLC.

¨      Using chip 8279 we can provide keyboard and display facility on PLC board.


 Chapter 10



In the present industrial world, a flexible system that can be controlled by user at site is preferred.  Systems, whose logic can be modified but still, used without disturbing its connection to external world, is achieved by PLC.  Utilizing the industrial sensors such as limit switches, ON-OFF switches, timer contact, counter contact etc., PLC controls the total system.  The drive to the solenoid valves, motors, indicators, enunciators, etc are controlled by the PLCs.
The above said controlling elements (normally called as inputs of PLCs) and controlled elements (called as outputs of PLCs) exist abundantly in any industry.  These inputs, outputs, timers, counters, auxiliary contacts are integral parts of all industries.  As such, it is difficult to define where a PLC cannot be used.
Proper application of a PLC begins with conversion of information into convenient parameters to save money, time and effort and hence easy operation in plants and laboratories.

The areas where PLC is used maximum are as follows:

1.      The batch processes in chemical, cement, food and paper industries which are sequential in nature, requiring time of event based decisions is controlled by PLCs.

2.      In large process plants PLCs are being increasingly used for automatic start up and shut down of critical equipment.  A PLC ensures that equipment cannot be started unless all the permissive conditions for safe start have seen established.  It also monitors the conditions necessary for safe running of the equipment and trips the equipment whenever any abnormality in the system is detected.

3.      The PLC can be programmed to function as an energy management system for boiler control for maximum efficiency and safety.

4.      In automation of blender reclaimers

5.      In automation of bulk material handling system at ports.

6.      In automation for a ship unloader.

7.      Automation for wagon loaders.

8.      For blast furnace charging controls in steel plants.

9.      In automation of brick moulding press in refractories.

10.  In automation for galvanizing unit.

11.  For chemical plants process control automation.

12.  In automation of a rock phosphate drying and grinding system.

13.  Modernization of boiler and turbogenerator set.

14.  Process visualization for mining application.

15.  Criteria display system for power station.

16.  As stored programmed automation unit for the operation of diesel generator sets.

17.  In Dairy automation and food processing.

18.  For a highly modernized pulp paper factory.

19.  In automation system for the printing industry.

20.  In automation of container transfer crane.

21.  In automation of High-speed elevators.

22.  In plastic moulding process.

23.  In automation of machine tools and transfer lines.

24.  In Mixing operations and automation of packaging plants.

25.  In compressed air plants and gas handling plants.

26.  In fuel oil processing plants and water classification plants.

27.  To control the conveyor/classifying system.

Thus PLC is ideal for application where plant machine interlock requirements are finalized at a later stage and need changes during engineering trial runs, commissioning or normal use.  It can be used extensively to replace conventional relay controls in power stations, refineries, cement, steel, fertilizer, petrochemical, chemical industries etc.
Applications can thus be extended from monitoring to supervision, control and management.


The PLC offers a compromise between advance control techniques and present day technology.  It is extremely difficult to forecast the rate and form of progress of PLCs, but there is strong evidence that development is both rapid and cumulative.  Though a PLC is not designed to replace a computer, it is useful and cost effective for medium sized control systems.  With the capability of functioning as local controllers in distributed control systems.  PLCs will retain their application in large process plants.

A further development of PLCs leads to the development of programmable function controller (PFC) is compatible to PCs and directly controls the desired functions.

In India every process industry is replacing relay control systems by PLCs and will go for PFCs in near future.  In the near future every flats and offices may possess PFCs to control room temperature, as elevator controller, maintain water tank levels, as small telephone exchange etc.




Automatic mixing system

Valve A                  Agitator

Float switch 1

Valve B
Float switch 2



     In figure when START button is pressed, solenoid valve A is energized and a batch of liquid is entered in tank. Float switch 1 detects the upper limit of liquid of liquid level and Float switch 2 detects the lower limit of liquid. As, tank begins to fill, Switch 2 closes. When the tank is full, switch 1 shuts off the solenoid valve A and start agitator to mix the liquid. The Agitator mixes the liquid for 30 seconds and shuts off. When the Agitator turn off, solenoid valve B is energized to drain the liquid. After the tank has been emptied, float switch 2 opens and solenoid B shuts off.

The Addresses of I/P and O/P are given below.

                                       DEVICE                    ADDRESS

                                              NO                    NC

Stop Switch                I 04                  I 24

Start Switch                I 05                  I 25

Float Switch 1             I 06                  I 26

Float Switch 2             I 07                  I 27

Valve A                       U 00                U 20

Valve B                       U 01                U 21

Agitator Motor            U 02                U 22

Control Relay              U 03                U

The ladder diagram of this system is given below.

I 04

I 05

O 03

O 03

I 06

O 03

I 07

O 00

O 00

T 00

30 sec

I 06

O 03

O 01




I 06

O 02

I 07


O 01

O 01


Program in STL language:

A I 04
O I 05
O U 03
= U 03
A I 26
A U 03
O I 27
O U 00
= U 00
A I 06
A U 03
A U 21
= T 00 1f
A T 02
A I 06
= U 02
A I 07
O T 00
O U 01
= U 01

Explanation of Ladder Diagram:

In first rung the stop switch is connected in series with start switch to activate the control relay. The stop switch is normally close type where as start switch is normally open type. So when the start switch is pressed control relay is activated. The start switch is push to on type so normally on contact of control relay is used to latch.
In second rung the NC contact of float switch 1 and NO contact of control relay is in series with valve A. When start switch is pressed and the liquid level is below float switch 1 the valve A is opened and it is closed when level touches the float switch 1.The NC contact of float switch 2 is latched by the NO contact of valve A so valve A does not opened as soon as the level falls below the float switch 1 level and remain close till the tank is fully emptied.
In third rung the normally open contact of float switch 1is connected in series with timer so when liquid level reaches to float switch 1, the timer is started.
In fourth rung the NC contact of timer DN bit is connected in series with NO contact of float switch 1 to Agitator motor. When liquid level is at float switch 1and the timer is running the motor is turned on.
In fifth rung The NO contact of float switch 2 is connected in series with NO DN bit of timer. So when the liquid level is above the float switch 2 and timer turned off the valve B is opened.




Chapter 11


Basically consists of data accessing feature and controlling process remotely.

Conversion of data is possible i.e. analog to digital and vice  versa.

Can communicate to any of the protocols available in market.

Completely rely on window based operating systems .                          
Distance as such doesn’t hamper SCADA operation.


n  Previously without  SCADA an industrial process was entirely controlled by PLC, CNC , PID & microcontrollers having programmed in certain languages or codes.
n  These codes were either written in assembly language or relay logic without any true animation that would explain the process running.

    In the left side you see the ladder program which is written for PLC and on the right side is the process for which the logic is written.
n  Now we can easily understand the process if it is shown with some animations rather then written codes.
n  Hence SCADA came to exist and with him he brought some exclusive features that amazed the industrial peoples.
n  SCADA related to industrial process is called industrial SCADA.

How is SCADA connected

n  SCADA is installed in the computers and through serial port it is linked to PLC .
n  All the field devices are connected to PLC and they get signals or commands from PLC.
Whatever applications we want to run can be executed either through PLC or SCADA.

*       Features of SCADA Dynamic representation
*      Database connectivity 
*      Device connectivity
*      Alarms
*      Trends
*      Scripts
*      Security
*      Recipe Management
*      Networking

*      This feature explains about the representation of various symbols of field instDYNAMIC REPRESENTATION
*      This ruments which are present in tool library which can be utilized in SCADA applications.
*      SCADA is not dedicated to any specific industry hence its library is so large that you can use it for any industries available.

*      SCADA doesn’t   its own database just like micro soft. Hence for storage it rely on databases available in the market.
*      It can be connected to VB , SQL ,EXCEL  or SAP

*      SCADA is not a 100% controller i.e. SCADA alone can’t run process. SCADA can be connected to any PLC or controller.
Hence any  DCS that is available in the market by using specific driver software

*      ALARMS
*      In the field area alarms are generated for warnings or to keep the process between
   certain limits.
 Generally these alarms are implemented by
 indicating lamps or Hooters in field but SCADA represents it with a format.
 The format consists of date , time , status ,
  priorities , many such elements which can be used for generation of reports.

n  These are also called as XY plotters or Data loggers. Basically it represents the values in wave formats .It is one of the important feature of SCADA.
n  It plots the value with reference to time.
n  Trend is subdivided into real time and historical trends.   i.e. we can see the present values of the process as well past values and can be stored and records can be maintained for the same.

v  It is the combination of logical operators which are written in a statement.
v  It is used to run the applications made or stimulate before final execution.
           Various types of scripts make project execution simpler for programmer

         Every application has to be secured from unauthorized users by different security levels .
         In SCADA this security can be given as a whole as well as individually.


          One of the finest features of any SCADA .
         It explains that we can maintain various recipes of different process and implement it on the process.
         All the recipes are stored in a single server and it can be fetched by any client server from any area to run the process.

v  It explains we can share SCADA applications on LAN or Internet as well exchange of data is possible.
v  Many Networking protocols are supported by SCADA software.
SCADA can be put on networking with other peripherals and processors with various networking topologies

v  Cost of SCADA is decided by two factors.             Number of tags and packages.
v  Packages are    DRN  and  RN    
v  DRN stands for development ,runtime &networking.
v  RN  stands for run and networking.

v  Every symbol used in software has to be specified name .
v  The logical name given to any symbol is said to be tag name.
v  Tag types define the symbol category.        It may be discrete , analog or strings.






                                        Conclusion of cost benefits analysis

                          Through total automation solution ,synthetic chemical plant achieves…..

Ø  Enhanced control over plant  operation

Ø  Better usage of utilities like boiler etc.

Ø  Higher productivity

Ø  Improved skill of workmen

Ø  Better monitoring leading to reduced maintenance

Ø  Greater safety of plant and personal







n  B. RAM.









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