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PLC programming

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What is a PLC ?

A PLC works by looking at its inputs and depending on their

state, and the user entered program, turns on/off outputs.

A PLC can be thought of as:

•Industrial Computers with specially designed architecture in both their central

units (the PLC itself) and their interfacing circuitry to field devices (input /

output connections to the real world).

• Special form of micro-processor-based controller that uses a programmable

memory to store instructions and to implement functions such as logic,

sequencing, timing, counting and arthimatic to control machine and process.

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What is a PLC ?

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Commercial And Industrial Computers

CommercialComputer

IndustrialComputer

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PLC Hardware

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PLC Hardware

The processor unit or central processing unit (CPU) This unit interprets the input signals and carries out the

control actions according to the program stored in its memory,communicating the decisions as action signals to the outputs.

The power supply unit

Convert the mains AC voltage to the low DC voltage necessaryfor the processor and circuits in the input and output interfacemodules.

The programming device

Used to enter the required program into the memory of theprocessor.

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PLC Hardware

The memory unit It is where the program containing the control actions to be

exercised by the microprocessor is stored and where the datais stored from the input for processing and for the output.

The input and output sections

It are where the processor receives information from externaldevices and communicates information to external devices.

The communications interface

It is used to receive and transmit data on communicationnetworks from or to other remote PLCs.

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Basic PLC Schema

CPU

Power Supply

Memory

Input Module

Output Module

Programming devices

Communications Expansion Connections

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Internal Architecture

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Input Scan

Program ScanOutput Scan

Housekeeping

START

Each ladder rung is scannedusing the data in the Inputfile. The resulting status(Logic being solved) iswritten to the Output file(“Output Image”).

The status of external inputs(terminal block voltage) iswritten to the Input image(“Input file”).

The Output Image datais transferred to theexternal outputcircuits, turning theoutput devices ON orOFF.

Internal checks onmemory, speed andoperation. Service anycommunicationrequests, etc.

PLC Operating Cycle Cont’d.

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Scan Time

The scan time is the total time the PLC takes to

complete the program and I/O update scans

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Scan Time Cont’d

The program scan time generally depends on two factors:

the amount of memory taken by the control program

the type of instructions used in the program (which

affects the time needed to execute the instructions)

The time required to make a single scan can vary from a

few tenths of a millisecond to 50 milliseconds.

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An AC/DC Input

Block diagram

Circuit

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Input Devices

Pushbuttons

Selector Switches Limit Switches

Level Switches

Photoelectric Sensors

Proximity Sensors

Motor Starter Contacts

Relay Contacts

Thumbwheel Switches

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An AC Output

Circuit

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DC Output

As in DC inputs, DC output modules may have eithersinking or sourcing configurations. If a module has a

sinking configuration, current flows from the load into

the module’s terminal, switching the negative (return or

common) voltage to the load. The positive current flows

from the load to the common via the module’s powertransistor.

In a sourcing module configuration, current flows from

the module into the load, switching the positive voltage

to the load

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An Example of a Relay Output Card

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Outputs

Typical output voltages are listed below,

5 Volts DC TTL level

24 Volts AC/DC

48 Volts AC/DC

110 Volts AC/DC

220 Volts AC/DC

WARNING: Always check rated voltages and currents for PLCs

and never exceed.

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Output Devices

Valves

Motor Starters Solenoids

Control Relays Alarms

Lights

Fans

Horns

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Programming Techniques Ladder Diagrams (LAD)

Instruction List (IL)

Sequential Function Charts (SFC)

Structure Text (ST)

Function Block Diagram (FBD)

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Ladder Diagram

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Logic Function

It is a group of conditions that is evaluated and theoutput of the function give the result of this evaluation

AND

OR

NOT

NAND

NOR

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AND Logic Function

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OR

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NOT Logic Function

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Exclusive OR (XOR)

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Latching Circuit

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Time diagram

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Time diagram

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Time diagram

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Time diagram

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Programming Language Instruction List (IL)

It is programming method gives the programs as a series of instructions, with each instruction

on a new line. Each instruction consists of an operator followed by one or more operands, thatis, the subjects of the operator.

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Programming Language Instruction List (IL)