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A digitally operating electronic system, designedfor use in an industrial environment, which uses aprogrammable memory for the internal storage ofuser-oriented instructions for implanting specific

functions such as logic, sequencing, timing, countingand arithmetic, to control through digital or analoginputs and outputs, various types of machines and

processes.

PLC’s operate with a program written by theuser in the form of laid down instructions which are

stored in a memory. The control function therefore isimplemented through software.

HPC’s are conventional control systemsincorporating programs as permanent wiring to the

individual component. The control function isimplemented through hardware.

1. Simple to install and quick modification of programs2. Small proportion of hardware for universal applications3. Easy fault finding or simple troubleshooting4. Automatic documentation5. Programming without hardware6. Extension using modules7. High reliability and processing speed8. Low space requirement and low power consumption9. No moving parts hence no wearing of parts

1. Serial processing2. Programming unit required3. High initial cost for simple processes4. Sensitive to dust, high temperature and high

humidity5. Repair must be made by qualified personnel6. No uniform programming language

1. Parallel processing2. Small cost for small systems3. Advantage to small scale operation

1. Difficult to change2. Large proportion of hardware3. Complicated fault finding4. Larger performance requirement5. Construction only possible after parts

are available6. Extensions not easily made

The INPUT MODULES accepts a variety of digital or analog signalsfrom various field devices (sensors) and converts them into a logic

signal that can be used by the CPU.

The CPU makes decisions and executes control instructions based onprogram instructions in memory.

The OUTPUT MODULES convert control instructions from the CPUinto a digital or analog signal that can be used to control various field

devices (actuators).

A PROGRAMMING DEVICE is used to input the desired instructions.These instructions determine what the PLC will do for a specific input.

An OPERATOR MODULE device allows process information to bedisplayed and new control parameters to be entered.

1. HARDWAREIt pertains to all electronic and mechanical

components of a computer system which includes:

Central Control Unit ( CCU ) Central Processor Unit System memory Power Supply

Peripherals Input and Output Peripherals Connecting cables Personal Computer

Depending on the CPU model, the PLCs can be connected topower supply voltage levels of 24 VDC or 120/230 VAC.

Note that the internal circuitry of the PLC is much like thatof a computer and therefore only operates on the sameinternal computer voltage levels of 5 volts DC and 12

volts DC.

The internal circuitry of the PLC is much like that of acomputer and therefore only operates on the same internal

computer voltage levels of 5 volts DC and 12 volts DC.

Much like the input module’s conversion of voltage level,the output module has the same task of transforming

the internal voltage level of the PLC back to the original24 VDC output voltage level.

2. SOFTWAREIt pertains to programs which specify the conditions

under which the components in the installation are to betriggered.

The software program, running on a PC, may beused to create a program for the PLC and is typicallyspecific to one PLC or a family of PLCs.

The programming languages in PLC includes:

a. Instruction List ( IL )b. Structured Text ( ST )c. Ladder Diagram ( LD )d. Function Block Diagram ( FBD )e. Sequential Function Chart ( SFC )

3. SENSORSThey are electronic devices that report to the PLC

the status of the system to be controlled and ifnecessary, non-electrical signals are converted toelectrical ones in order that they may be accepted bythe input module. Sensor may include:

a. Limit switchesb. Cylinder switchesc. Inductive sensorsd. Capacitive sensorse. Reflective sensorsf. Others

4. ACTUATORSActuators are the components that make devices

move or start working.

These actuators may be:

• Motor Starters for Electrical Motors• Solenoid Valves for Cylinders• Encoder for Motor Controls

5. PROGRAMMERIt refers to the programming devices that are used

to write the PLC programs and translate the programsinto machine codes for the CCU until it is transferred tothe program memory for testing. Programmers include:

a. Handheld Programming Consoleb. Personal Computer

1. INSTRUCTION LISTA text-based Low-level type of programming

language which is based on how the processor worksand consists of control instruction. Every instructionconsists of an operator and an operand.

A I0.0AN I0.1= Q0.0

EP

Operator Operand

Bit Address

Byte Address

2. STRUCTURED TEXTIt is a so-called "High level language" which is

based on the programming language PASCAL.Structured Text is eventually used for more complexproblems and data structures.

Example of a small ST-Program:PROGRAM HELLO_ST (* Start and name of POU *)VAR (* Declaration section *)

OUTPUT AT %Q0.0 : BOOL;INPUT AT %I0.0 : BOOL;Enable_signal : BOOL;

END_VARIF INPUT=TRUE THEN (* Instruction section *)

Enable_signal:=TRUEELSE

OUTPUT:=0;END_IF

END_PROGRAM (* End of program *)WinSPS Help 3.2 Copyright (c) 1996-2002 Bosch Rexroth AG

3. LADDER DIAGRAMIt is a graphical programming language which has

its origins in the circuit diagrams of direct wired relaycontrols.

I0.0 I0.1 Q0.0

EP

OperandField

Connections/LinksInput

Elements

ElementField

Contacts

OutputElements

Contact Rail

4. FUNCTION BLOCK DIAGRAM

It is a graphical programming language which hasits origins in the logic plans of electronic circuits.

ConnectingLine

Input Pin

& =I0.0I0.1 Q0.0

OperandField

ElementField

5. SEQUENTIAL FUNCTION CHARTIt is a flowchart-type of programming language that

is used to structure controls programs to be processedsequentially in the form of cascaded sequence. It is mainlyused for programming sequence-orientated controlsystems.

&

Start

End

M0.1

R

I0.0

1N1

Logical One

Q0.0

I0.1

M0.1

S1N2

Logical One

Q0.0

1. INPUT ( I )I0.0 to I0.7I1.0 to I1.7Up to I23.7 = 192 Inputs

2. OUTPUT ( Q)Q0.0 to Q0.7Q1.0 to Q1.7Up to Q15.7 = 128 Outputs

3. FLAGS / MARKERS ( M )M0.0 to M0.7M1.0 to M1.7Up to M191.7 = 1536 Markers ( Internal Relays )

4. TIMERS ( T )T0 to T127 = 128 Timers

5. COUNTERS ( C )C0 to C63 = 64 Counters

FIELDOUTPUTS

FIELDINPUTS

CONTROLMEASURE

PROCESSOR

MACHINE

PROGRAMMABLELOGIC

CONTROLLER

24V DCPowerSupply

PROGRAMMABLE

LOGIC

CONTROLLERI 0.0

24V DCOutput

Q 0.0

24V Power SupplyUnit

Power-ON Switch

Output Terminals+24V Output

Programmable LogicController

Input Terminals

Built-in InputSwitches

Ground

I0.0

PLC StatusIndicators

Output LEDIndicators

Input LEDIndicators

+24V Input

PLC CONTACTSTATUS

PLC CONTACTTYPE

INPUT CONTACTSTATUS

INPUT CONTACTSTATUS

PLC CONTACTSTATUS

PLC CONTACTTYPE

A I0.0A I0.1

= Q0.0

AN I0.0A I0.1

= Q0.0

A I0.0O I0.1

= Q0.0

A I0.0ON I0.1

= Q0.0

A I0.0A I0.1O I0.2

= Q0.0

A(A I0.0O I0.2)AN I0.1= Q0.0

A I0.0O I0.1AN I0.2

= Q0.0

A(A I0.0ON I0.2O I0.5)A(A I0.1O I0.3AN I0.4)

= Q0.0

A I0.0A I0.1O(A I0.2O I0.4)A I0.3= Q0.0

BREAK TIME

A(A I0.3O M0.1)AN I0.4= M0.1

A M0.1AN M0.2= Q0.1

A(A I0.2O M0.2)AN M0.3= M0.2

A M0.2= Q0.2

A I0.1A M0.2= M0.3

EPLEGEND:

SYMBOLS DESIGNATION

S1 I0.1S2 I0.2START I0.3STOP I0.4

Y1 Q0.1Y2 Q0.2

K1 M0.1K2 M0.2K3 M0.3

(A I0.5O M0.1)AN I0.6= M0.1

(A M0.1O M0.2)AN M0.4= M0.2

(A I0.2O M0.3)AN M0.5= M0.3

(A I0.4O M0.4)AN M0.6= M0.4

A I0.1A M0.4= M0.5

A I0.3A M0.5= M0.6

A M0.2= Q0.1

AN M0.2= Q0.2

A M0.3= Q0.3

AN M0.3= Q0.4

EP

LEGEND:

SYMBOLSDESIGNATION

S1 I0.1S2 I0.2S3 I0.3S4 I0.4START I0.5STOP I0.6

Y1 Q0.1Y2 Q0.2Y3 Q0.3Y4 Q0.4

K1 M0.1K2 M0.2K3 M0.3K4 M0.4K5 M0.5K6 M0.6

THANK YOU