3.2.

2 Characteristics of Programmable Logic Controller (PLC)

" It is a digital electronic device which uses programmable mnemory to store the
instruction.
.The characteristics of PLC:
o PLC is field programmable by the user which allows the user to write or edit the
programs in the field without the use of rewiring the unit
o PLCcontains logic counting, timing and the memory based functions that allows the
user toaccess some type of programming language
o It scans the mnemory and IVO in the proper manner

o They allow proper checking and the diagonosis of the device

o PLC is not designed for a specific application but they can handle variety of the tasks
assigned to them.
 L3.8 PLC Selection
selection is basically based on the controllers input, output and their characteristics.
PLC
type, mini type, micro type
The first choice is tochoose whether the PLC should be rack
any requirement based on the software.
O1. Input output slze and type
are used to convert the signal
" Nowadays, PLC arc available in plug-in module type and
of the PLC. Based on the
level to the level which is compatible with the architecture
small process
manufacturer design, the input output module has 1, 4, 8 or 16 points. For
Similarly, for
control applications, the IO module comes out with 20 to 256 /O.
medium sized up prince with 256 - 1024 input-output module. For larger system, 1024
VOand above module are used.
02. Input output remoting equlpment
The overall wiring cost of the PLC is reduced by multiplexing the I/Oports of PLC. If
IO rack are centralized, the wiring requirement of the plant can be reduced. The cost of
the wiring between the tacks and the CPU is reduced by using twisted pairs of wires or
signal cable without compromising the accuracy or capability.
Remote IO is classified into two types : The integral type (upto 4.5 km) and the receiver
type (unlimited transmission). The main disadvantage of this remote IO system is that if
there is any failure in the bus, effects of the failure is unpredictable.
03. Memory slze and type
Depending on the size of the controller, the size and the type of PLC memory can be
used. Almost all the PLC comes was fixed RAM. Large and mid-sized PLCs g0ves a
variety of options, for the type of memory to be used.
04. Programmer unlt
" There are two programming tools provided by the manufacturer for midsize PLC.
a) Hand held programmer which enables the operator to enter the
program at any
instant of time. They are widely used because of rugged
construction and easy
operation.
b) CRTprogrammers provide visual picture of the program and ladder diagram drawn
on the screen.This type of programmer reduces the
troubleshooting time.
 Os. Electrical requlrements
When defining the electrical necesities of asystem, we need to consider three things:
o Incoming power
o Input device voltage
o Output voltage and cñrent.
O6. Application requirements
" We ncedto define which input and output devices the system requires after defining the
task of the system.
o Define the type and the feature of the device that need to be used
o Type of logics that to be used
o Listthe essential and sophisticated features in addition to a discrete logic.
O7. Speed of operation
" The CPUmust be capable of handling multiple VO and collecting the data based on the
application. It should have sufficient memory to make the system work faster. Another
important factor is the scan time, which is the timne taken by the CPU to complete one
cycle in collecting the information,running the PLC and producing the desired output.
 enhancements. output input interface
and computer aids,
cperational
programming
aids, such
as types many ofdevices
are Peripheral devices:Peripberal C.
back.
restored be
program
can user execution
the upon and fail break or supply power the
when memory theretain tendency
to hasa PLC the user, the written by program is the
Whenprogram. ladder PLC troubleshoot the used
to also isPLC. It the program
of the
execution
of thduring
e monitored and edited becan memory
it or progam program,
the
defined loading
the for used programmable
isunit The Programmer
uni: t b.
program. defined user contains
the memory user the while
calculation mathematical
the required
for data the store toused memory is Data system. operating the and
rogram application driver, contains
the memory ROM the whiprogram
le user-defined
the stores RAM memories.
The ROM and RAM includes memory The unit: Memory a.
output.appropriate
producing
the and input accepting
the processing
by data operations
and computation
the all doesprocessor microprocessor.
The octagonal hexagonal
or be can and PLC
the brai
ofn the memory. is It the processor
and central the has module CPU module- CPU
modules: threefollowing the hasystem
s PlLC Every
Architecture PLC 3.2.3
ys. meter, panel are
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ofs of The
The box. lamp solve enoid
valor
uts discrete example
of
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aresignals output three The .
mal Coded Binary innuts (BCD).
is reoister example of The time. same th e PLC
beconditions
toare
at the to sent
process the when used are registers,
that the input
als. digital examples is oftype other Ihe
of the buttons
are contacts,
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relay, the while
used, becan gauge strain meter. flow liake device is frequency the variable measuring
fcouple,
or RTD, from signals aresignals analogue example
of common The value.
ance off current
or voltage varving controller programmable
as the gitovennormally
signals non-deterministic
The signal. deterministic or frequency, low frequency
are or
high signal, digital analog
or becan signals input The module: (/O) output Input
volts. 24on
and DC 0V
operate
on They devices. peripheral other and unimemory
t theProcessing
Unit,
entral devices,
the output input the to given isunit supply power components.The state
solid the needed
by are thatvoltages those voltages
to linepower convert
the They voltage.
high from device state solid
protecting necessary
for is
thisolation
at the provide needed to
are Theymodules.input/output CPU, the power
to supply They module: supply Power .
Architecture
PLC of 3.2.2: Fig.
buttons starters
pushSwitches, motorSolenoids,
device Output device Output
modules system /O
Bus /O
Memory
CPU supply Power
device
Programming
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Logic (3-5) Automation Industrial
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 Graphical
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Digital Closed Open
program
User connected
the to Diagram
Task Task Task Task left program
or logical
another Block logical logical
sequentiallyfunctional Flg.
segments Y and more output
output symbols (LD):
3.4.2:Programming to more contact
block. the output. Diagram contact
at
connectedwrittern chart functional lines functions logic
as+1 It if if like: program A
(SFC) leaving is (FBD) O the the
-2
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is is It is
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 Symbol Function
Normally The contact closes when the associated
open contact Boolean object becomesTRUE
Normally The contact closes when the associated
closed Boolean object becomes FALSE
contact

Flank Rising edge: The contact is closed only in t

detecting scan during which the associated Boolean
contacts object changes state from 0 to 1
Falling edge : The contact is closed in the so
where the associated Boolean object change
state from 1to 0
Horizontal Toconnect elements in series
Vertical Toconnect elements in paralle
6 3.2.5 PLC Scan Cycle .Q.9
" The time taken by the PLC to complete one cycle, like gathering the inputs, running the
program and then updating the output makes the one scan cycle.
there is
1. Input scan : PLCscan start with the CPUreading the status of.the input. When
no error, the PLCwill copy the input and their values into the memory.
2. Program scan : In this step application program is executed. Ladder logic program is
designed and the outputs are updated.
3. Output scan : When the ladder program is over, the output values are updated in the
memory and the outputs are temporarily stored in the memory.
TLCHNICAL PUBLICATIONSon up thrust for knowledge
Dndustrial Automation (3 - 8) Programmable Logic Controllers

The status of
Internal checks
INPOT external inputs
on memory, speed START is written to the
and operation. SCAN input image table
Service any· HOUSE (file or register)
communication KEEPING
requests

Each ladder rung

is scanned and solved
OUTPUT using the date in the
SCAN input file. The resuting
logic is written to the
The output image output image table
date is transferred (file or register)
to the externaloutput
circuits, turning the
output device
ON or OFF

Fig. 3.2.3 : PLC scan cycle

" The entire process is repeated as long as the PLC is in the running condition.
 Alphanumeric type iymtbol
Start of logic End of bg
12 01

Check for ON Check for OFF Energlze output M

the IF-THEN rule
Isvalid
Loglc branch

| Input
O Output

Flg. 3.5.2:Gonorlc form of laddor dlagram

" The important parts of ladder diagram of PLC are :
o Rungs
o Branches

o Inp uts and outputs for programming for PLC

o Addressing inputs and outputs
o Instructions sets.
Rungs : In ladder diagram, rungs are the horizontal lines. Based on the project
requirements, we can have as thany number of rungs as desired. Vertical lines represent the
power supply or flow.
+ ve Power rall -ve Power rall
Rung (R)
R
R
Rung Input
H Rg Rung output
HH
R,
Power flow direction

Rung and rail in PLC

Flg.3.3.3
 Automation
"Industrial
3. 2.
in different
rungs.Nested 1.
Branches
connected
Different
inputs
series
Parallel Series

and HE branch HE H branch

branch Basically :
parallel Number
parallel
inputs Inthe of
Fig. Flg. :
Flg. branch : : Here
HH It Input Number there
is3.5.5: 3.5.4
3.5.6 the the
or HHE are
Representation
combination the
Representatlon : of input
tlo. : inputs three
output or
connected the types
are output 26)(3-
of connected of
Output
by series of in
branches
the the HE of series are
of Q, the
nestconnected In
Different
outputs latching
consider
HH andparallel connected
serles outputs
parallel in
HHE
Numberthe of parallelly series
anch parallel
and process as
input branch branch Controllers
Logic
inProgrammable
parallel
branches series.
branch

in
the
same

or