General PLC Architecture

8/17/2019 General PLC Architecture

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Plc + programmable logical

circuit, digitall! operated electronic euipment

which uses a programmable memor! .or internalstorage o. instruction b! implementing specic

.unction through analog or digital modules/ specic instruction such as

0ogical seuence

Timing

Counting

,rithmetic


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• , P0C 1i/e/ Programmable 0ogic Controller2 is a de%icethat was in%ented to replace the necessar! seuential

rela! circuits .or machine control/

Ma3or Component o. P0C

The three basic element o. P0C are Central Processing unit

'nput Modules

Output Modules


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Central Processing unit1CPU2 The CPU recei%es i4p signals .rom

the %arious i4p modules and based onthe programs stored in the memor!5decided on the appropirate signals5which it transmits to the respecti%e

output module/


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Memor!

VOLATILE/

• , %olatile memor! is one that loses its stored in.ormationwhen power is remo%ed/

• E%en momentar! losses o. power will erase an! in.ormationstored or programmed on a %olatile memor! chip/

• Common T!pe o. 6olatile Memor!

• RAM. Random Access Memor!1Read)7rite2• Read)write indicates that the in.ormation stored in the

memor! can be retrie%ed or read5 while write indicates thatthe user can program or write in.ormation into the memor!/


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• The words random access re.er to the abilit! o. an!location 1address2 in the memor! to be accessed orused/ Ram memor! is used .or both the user memor!1ladder diagrams2 and storage memor! in man! P0C8s/

• R,M memor! must ha%e batter! backup to retain orprotect the stored program/


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• NON-VOLATILE

9as the abilit! to retain stored in.ormation when poweris remo%ed5 accidentall! or intentionall!/ Thesememories do not reuire batter! back-up/

• Common T!pe o. :on-6olatile Memor!

• ROM, Read Onl! Memor!

• Read onl! indicates that the in.ormation stored inmemor! can be read onl! and cannot be changed/'n.ormation in ROM is placed there b! the manu.acturer.or the internal use and operation o. the P0C/


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• PROM, Programmable Read Onl! Memor!

,llows initial and)or additional in.ormation to bewritten into the chip/

• PROM ma! be written into onl! once a.ter beingrecei%ed .rom the P0C manu.acturer; programming isaccomplish b! pulses o. current/

• The current melts the .usible links in the de%ice5pre%enting it .rom being reprogrammed/ This t!pe o.memor! is used to pre%ent unauthori


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• EPROM, Erasable Programmable Read Onl! Memor!

• 'deall! suited when program storage is to be semi-permanent or additional securit! is needed to pre%ent

unauthori


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EEPROM5 Electricall! Erasable Programmable ReadOnl! Memor!

• ,lso re.erred to as E#PROM5 is a chip that can be

programmed using a standard programming de%iceand can be erased b! the proper signal being appliedto the erase pin/

• EEPROM is used primaril! as a non-%olatile backup.or the normal R,M memor!/ '. the program in R,M islost or erased5 a cop! o. the program stored on anEEPROM chip can be down loaded into the R,M/


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':PUT)OUTPUT MO&U0E

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• The main purpose of the I/O interface is to condition the various signals

received from or sent to the external input and output devices.

• Input modules converts signals from discrete or analog input devices to logic

levels acceptable to PLCs processor.

• Output modules converts signal from the processor to levels capable of driving

the connected discrete or analog output devices.


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I/O Module

DC INPUT MODULE

OPTO-

ISOLATOR

I! "##$#$ TO%• Prevent voltage

transients from

damaging the

processor.•&elps reduce the

effects of electrical

noise

Current

Limiting

'esistor

FROM

INPUTDEVICE

(!# TO

$'OP T

)OLT*G#

TO LOGIC

L#)#L

+uffer,

-ilter,

hsteresisCircuits

TO

PROCESSOR


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14

I/O Module

AC INPUT MODULE

OPTO-ISOLATOR

I! "##$#$ TO%• Prevent voltage

transients from

damaging the

processor.•

&elps reduce theeffects of electrical

noise

'ectifier,

'esistor

"etor0

FROM

INPUTDEVICE

CO")#'T! T *C

I"P(T TO $C *"$

$'OP! T )OLT*G#

TO LOGIC L#)#L

+uffer,

-ilter,

&steresisCircuits

TOPROCESSOR


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Programming the P0C

P0C is programmed b! means o. a programming

de%ice/

Programming language o. P0C are=

0adder diagram0ow le%el language based on booleane(pression

>unctional ?lock

9igh le%el languages

Most o. the programming method used toda! .orplcs are based on ladder logic diagram


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0adder &iagram

0adder logic uses graphic s!mbols similar to

rela! schematic circuit diagrams/

0adder diagram consists o. two %ertical lines

representing the power rails/

Circuits are connected as hori


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0adder builder diagram.eatures

*/Power fows .rom le.t to right/

#/Otput on right side can not be connected directl! with le.t side/

$/ Contact can not be placed on the right o. output/

@/Each rung contains one output at least/ A/ Each ouutput can be used onl! once in the program/

B/, particular input a)o output can appear in more than one rung o. a

ladder/

/The inputs a)o outputs are all identied b! their addresses5 the

notation used depending on the P0C manu.acturer/


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LADDER DIAGRAM FORMAT

•, ladder rung is TRUE when it has logic continuit!/

•0ogic continuit! e(ists when power fows through

the rung .rom le.t to right/

• The e(ecution o. logic e%ents that enable theoutput pro%ide this continuit!/

*D


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*


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There are %e program elements)operations commonl!used in P0C ladder diagram the! are

*/ P0C ?it logic operations

#/ Timer Operations$/ Counter operations

@/ Comparison operations

A/ ,rithmetic operations/


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E(ample

*/&ouble acting c!linder is used toper.orm machinng operation/Pneumatic c!linder is ad%anced b!

pressing two push buttonssimultaneousl!/ '. an! one o. thepush button is released5 c!linder

comes back to start position/ &rawthe pneumatic circuit5 P0C wiringdiagram and ladder diagram to

implement this task/


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,s shown in the P0C wiring diaram 5 The pushbuttons

P?* and P?# are connected at memor! address '* and

'#/ '* and '# are connected in series in ladder diagram

to relase this ,:& logic .untion/ 7hen the push

buttons P?* and P?# are pressed simultaneousl!5 the

addresses '* and ' # turn to state * .rom state F 5 as a

result power fows thorugh the coil and there will be

output at coil F*/ Output at the coil F* operated the

solenoid coil and c!linder mo%es .oraward to do the

reuired operation/ '. an! one o. P?* and P?# is

pressed5 then corresponding bit addresses turns to F5


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>unctional block

The term .unction block diagram 1>?&2 is used .or P0Cprograms described in terms o. graphical blocks/ 't isdescribed as being a graphical language .or depictingsignal and data fows through blocks5 these being

reusable so.tware elements/ , .unction block is aprogram instruction unit which5 when e(ecuted5 !ields

one or more output %alues/


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Enhanced func!ona" #"oc$ forma


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Func!ona" #"oc$ !nsruc!ons

1a2 one enable line and one output1b2 one enable line5 a start timing command5 andtwo outputs/

#


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P0C operation

?asic >unction o. a T!pical P0C

• Read all eld input de%ices %ia the inputinter.aces5 e(ecute the user program stored inapplication memor!5 then5 based on whate%ercontrol scheme has been programmed b! theuser5 turn the eld output de%ices on or o5 orper.orm whate%er control is necessar! .or theprocess application/

• This process o. seuentiall! reading the inputs5e(ecuting the program in memor!5 andupdating the outputs is known as scanning/


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P0C operation

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PHASE 2Progr!

E"e#u$%o&

PHASE 3

D%g&o'$%#'/

Co!!PHASE 4

Ou$(u$

S#&

PHASE 1

Red I&(u$'

S#&


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3)

PHASE 1 * I&(u$ S$$u' '#&

•

, P0C scan c!cle begins with the CPU reading the statuso. its inputs/

PHASE 2* Log%# Sol+e/Progr! E"e#u$%o&

•

The application program is e(ecuted using the status o.the inputs

PHASE 3* Log%# Sol+e/Progr! E"e#u$%o&

• Once the program is executed, the CP( performs diagnostics and

communication tas0s


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PHASE 4 , Ou$(u$ S$$u' S#&

• *n output status scan is then performed, hereb the stored output values aresent to actuators and other field output devices. The ccle ends b updating the

outputs.


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P0C widel! used in industries areadue to .ollowing reasons

• Cost o. P0C automation is less and P0C is %er!%ersatile

• P0C can be commissioned and installed easil!

• Programming o. P0C is uite simple/ 0adderprogramming is fe(ible

• The! are not hard wired control/ The! can beprogrammed and reprogrammed toaccommodate .reuent changes in program

• Monitoring o. on line work process is eas!5there.ore trouble shooting and maintenance o.P0C is not a diGcult task/

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General PLC Architecture