2. Contents
❖ What is PLC?
❖ History of PLC
❖ Major components of PLC
❖ Operational sequence of PLC
❖ Ladder logic
❖ Example of starting and stopping of a motor
❖ Advantages
❖ Disadvantages
❖ Application
Prof.
P.
R.
Doke
3. What is PLC?
•PLC is a digital computer designed for multiple
inputs and output arrangements, extended
temperature ranges, immunity to electrical noise,
and resistance to vibration and impact. A PLC is an
example of a real timesystem.
Prof.
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Doke
4. What is a PLC ?
Inputs Outputs
A PROGRAMMABLE LOGIC CONTROLLER is a solid state
control system that continuously monitors the status of devices
connected as inputs. Based upon a user written program, stored in
memory, it controls the status of devices connected as outputs.
The Basic Block
CPU
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5. Definition of PLC
• A digital electronic device that uses a programmable memory to store
instructions and to implement specific functions such as logic, sequence,
timing, counting and arithmetic to control machines and process.
• It uses a programmable memory to store the instructions and specific
functions that include On/Off control, timing counting, sequencing,
arithmetic and data handling.
• A PLC is a computer designed to work in an industrial environment.
Prof.
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R.
Doke
6. What is Control?
“ CONTROL is the process in a system in which
one or several input variables influence other
variables. “
A Simple View of a Control System
C
O
N
T
R
O
L
S
Y
S
T
E
M
P
L
A
N
T
INFORMATION
COMMANDS ACTUATORS
SENSORS
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Doke
7. Need for PLC
Hardwired panels were very time consuming to time, debug and change.
The following requirements for computer controllers to replace hardwired
panels.
1.Solid-state not mechanical.
2.Easy to modify input and output devices.
3.Easily programmed and maintained by plant electricians. 4.Be able to
function in an industrial environment.
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Doke
8. .
.
Hardwire System and PLC
L
N
S1
S2
K1
S1 S2
PLC
K1
0 V
Hardwire PLC
24 VDC
K1
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9. .
➢ PLC was introduced in late 1960’s
➢ First commercial & successful Programmable Logic Controllers was
designed and developed by Modicon as a relay replacer for General
Motors.
➢ Earlier, it was a machine with thousands of electronic parts.
➢ Later ,in late 1970’s,the microprocessor became reality & greatly
enhanced the role of PLC permitting it to evolve form simply relay to
the sophisticated system as it is today.
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10. Major Components of a Common
PLC
.
5
PROCESSOR
POWER
SUPPL
Y
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
PROGRAMMING
DEVICE
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
Prof.
P.
R.
Doke
11. Major Components of a Common PLC
➢ POWER SUPPLY
• Provides the voltage needed to run the primary PLC components. The power supply gives the voltage
required for electronics module (I/O Logic signals, CPU, memory unit and peripheral devices) of the PLC
from the line supply.
• The power supply provides isolation necessary to protect the solid state devices from most high voltage
line spikes.
• As I/O is expanded, some PLC may require additional power supplies in order to maintain proper power
levels.
➢ I/O MODULES
• Provides signal conversion and isolation between the internal logic- level signals inside the PLC and
the field’s high level signal. These modules act as interface between real-time status of process
variable and the CPU.
• Analog input module : Typical input to these modules is 4-20 mA, 0-10 V
• Ex : Pressure, Flow, Level Tx, RTD (Ohm), Thermocouple (mV)
• Digital input module : Typical input to these modules is 24 V DC, 115 V AC, 230 V AC
• Ex. : Switches, Pushbuttons, Relays, pump valve on off status
Prof.
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R.
Doke
12. Major Components of a Common
PLC
➢ OUTPUT MODULES
• These modules act as link between the CPU and the output devices in the field.
• Analog output module : Typical output from these modules is 4-20 mA, 0-10 V
• Ex : Control Valve, Speed, Vibration
• Digital output module : Typical output from these modules is 24 V DC, 115 V AC, 230 V AC
• Ex. : Solenoid Valves, lamps, Actuators, dampers, Pump valve on off control
➢ BUS SYSTEM
• It is path for the transmission of the signal . Bus system is responsible for the signal exchange
between processor and I/O modules
• The bus system comprise of several single line ie wires / tracks
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13. Major Components of a Common
PLC
➢ Central Processing Unit/ Processor
• Provides intelligence to command and govern the activities of the entire PLC systems. It
is a micro-controller based circuitary. The CPU consists of following blocks :
Arithmatic Logic Unit (ALU), Program memory Process image memory (Internal
memory of CPU) Internal timers and counters Flags
• CPU performs the task necessary to fulfill the PLC funtions. These tasks include Scanning,
I/O bus traffic control, Program execution, Peripheral and External device
communication, special functions or data handling execution and self diagnistics.
➢ PROGRAMMING DEVICE
Used to enter the desired program that will determine the sequence of operation and
control of process equipmentor driven machine.
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15. PLC operation sequence
1) Self test: Testing of its own hardware and
software for faults.
2) Input scan: If there are no problems, PLC will copy
all the inputs and copy theirvalues into memory.
3) Logic solve/scan: Using inputs, the ladder logic
program is solved once and outputsare updated.
4) Output scan: While solving logic the output values
are updated only in memory when ladder scan is
done, the outputs will be updated
using temporary values in memory.
Self test
Input scan
Logicscan
Output
scan
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16. PLC SignalFlow
Programming Terminal
O:0/7
O:0/7
O:1/5
Output Devices
Output Modules
Input Module Processor Memory
Input Devices
Ladder Program
O:0/7
O:1/5
I:0/6
I:1/4
O:1/5
I:0/6
I:1/4
I:0/6
I:1/4
Data
Input
Image Table
Output
Image Table
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17. TYPES OF PLCS
The general classification of PLC based upon the number of inputs and outputs is
•Fixed type PLC
•Modular type PLC
•Rack type PLC
Fixed type PLC: In this type of PLC all the components of the PLC are as a single unit. The
number of I/O supported by the PLC is decided by the manufacturer and cannot be
changed. This type of PLC can support a small number of I/Os.
Classification: Restricted Page 17
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Doke
18. Programmable Logic Controller
• Modular Type PLC: In modular type PLC the number of I/Os can be increased by the addition of modules
to the existing PLC. In modular type PLC the number of I/O supported can be increased to few
hundreds by adding I/O modules.
• Rack Type PLC: In rack type PLC all the components of the PLC are as separate modules and are
assembled to form one unit by mounting the individual components on a rack. This PLC can support
upto thousands of I/Os.
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Doke
19. Companies OF PLC
•There are lot of PLC manufacturers, some of the commonly known brands of
PLC are,
•Allen Bradley(AB)
•Siemens
•ASEA Brown Boveri
•Omoron
•GE Fanuc
•DELTA
•Mitsubishi
•Messung
•Keyence
•In all the brands there may be the different types of PLCs available.
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22. L1 L2
P. B SWITCH
I:2
0
I= Input
Module
slot #in rack
Allen-Bradley 1746-1A16
INPUT MODULE
WIRING DIAGRAM
Module
Terminal #
Address I:2.0/0
LADDER PROGRAM
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23. Normally Open Pushbutton
Normally Closed Pushbutton
Normally Open switch
Normally Closed switch
Normally Open contact
Normally closed contact
Discrete Input
A discrete input also referred as digital input is an input that is either ON or OFF
are connected to the PLC digital input. In the ON condition it is referred to as logic
1 or a logic high and in the OFF condition maybe referred to as logic o or logic
low.
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Doke
25. Analog Input:
An analog input is an input signal that has a continuous signal. Typical inputs may vary
from 0 to 20mA, 4 to 20mA or 0 to10V. Below, a level transmitter monitors the level of
liquid in the tank. Depending on the level Tx, the signal to the PLC can either increase or
decrease as the level increases or decreases.
IN
PLC
Analog
Input
Module
T
ank
Level Transmitter
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26. OUT
PLC
Lamp
Digital
Output
Module
Discrete Output Module:
A discrete output is either in an ON or OFF condition. Solenoids, contactors coils, lamps
are example of devices connected to the Discrete or digital outputs. Below, the lamp can be
turned ON or OFF by the PLC output it is connected to.
Prof.
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Doke
27. OUT
Analog Output
An analog output is an output signal that has a continuous signal. Typical
outputs may vary from 0 to 20mA, 4 to 20mA or 0 to10V.
Electric to pneumatic transducer
PLC
Analog
Output
Module
E
P
Pneumatic control valve
Supply air
0 to 10V
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R.
Doke
28. Memory Map Organization
SYSTEM
• System memory includes an area called the EXECUTIVE, composed of
permanently-stored programs that direct all system activities, such as
execution of the users control program, communication with peripheral
devices, and other system activities.
• The system memory also contains the routines that implement the PLC’s
instruction set, which is composed of specific control functions such as logic,
sequencing,timing, counting, and arithmetic.
• System memory is generally built from read-only memory devices.
APPLICATION
• The application memory is divided into the data table area and user
program area.
• The data table stores any data associated with the user’s control program,
such as system input and output status data, and any stored constants,
variables, or preset values. The data table is where data is monitored,
manipulated, and changed for control purposes.
• The user program area is where the programmed instructions entered by
the user are stored as an application control program.
•Data Table
•User Program
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29. Memory Designs
VOLATILE.
A volatile memory is one that loses its stored information when power is removed.
Even momentary losses of power will erase any information stored or programmed on a
volatile memory chip.
Common Type of Volatile Memory
RAM- Random Access Memory(Read/Write)
Read/write indicates that the information stored in the memory can be retrieved or read,
while write indicates that the user can program or write information into the memory.
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30. Memory Designs
The words random access refer to the ability of any location (address) in the
memory to be accessed or used. Ram memory is used for both the user memory
(ladder diagrams) and storage memory in many PLC’s.
RAM memory must have battery backup to retain or protect the stored program.
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31. Memory Designs
Several Types of RAM Memory:
1.MOS
2.HMOS
3.CMOS
The CMOS-RAM (Complimentary Metal Oxide Semiconductor) is probably one of
the most popular. CMOS-RAM is popular because it has a very low current drain
when not being accessed (15microamps.), and the information stored in memory
can be retained by as little as 2Vdc.
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32. Memory Designs
NON-VOLATILE
Has the ability to retain stored information when power is removed, accidentally or
intentionally. These memories do not require battery back-up.
Common Type of Non-Volatile Memory
ROM, Read Only Memory
Read only indicates that the information stored in memory can be read only and cannot
be changed. Information in ROM is placed there by the manufacturer for the internal use
and operation of the PLC.
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Doke
33. Memory Designs
Other Types of Non-Volatile Memory
PROM, Programmable Read Only Memory
Allows initial and/or additional information to be written into the chip.
PROM may be written into only once after being received from the PLC manufacturer;
programming is accomplish by pulses of current.
The current melts the fusible links in the device, preventing it from being reprogrammed. This type
of memory is used to prevent unauthorized program changes.
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34. Memory Designs
EEPROM, Electrically Erasable Programmable Read Memory
Also referred to as E2PROM, is a chip that can be programmed using a standard
programming device and can be erased by the proper signal being applied to the erase
pin.
EEPROM is used primarily as a non-volatile backup for the normal RAM memory. If
the program in RAM is lost or erased, a copy of the program stored on an EEPROM
chip can be down loaded into the RAM.
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