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INTRODUCTION TO PROGRAMMABLE CONTROL

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DEFINITION

•Programmable logic controllers, also called programmable controllers or PLCs, are solid-state members of the computer family, using integrated circuits instead of electromechanical devices to implement control functions.

•They are capable of storing instructions, such as sequencing, timing, counting, arithmetic, data manipulation, and communication, to control industrial machines and processes.

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Small modular PLC

mounted on DIN-rail, 24V supplycheaper (€5000) not water-proof, no ventilatorextensible by a parallel bus (flat cable or rail)

courtesy ABBcourtesy Backmann

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Simple PLCnetwor

k

binary inputs

binary outputs

analog inputs / outputs

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PLC in a cabinetCPU1

redundant field bus connection

CPU2

inputs/outputs

serial connections

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Global players

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• The control system had to be price competitive.

• It had to be capable of sustaining an industrial environment.

• The I/O interfaces had to be easily replaceable.

• The controller had to be designed in modular form, so that subassemblies could be removed easily for replacement or repair.

• It needed the capability to pass data to a central system.

• The system had to be reusable.

• The method used to program it had to be simple.

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TODAY’S PROGRAMMABLE CONTROLLERS

• Faster scan times.

• Small, low-cost.

• High-density input/output (I/O) systems.

• Intelligent, microprocessor-based.

• Mechanical design improvements.

• Special interfaces.

•Peripheral equipment has improved operator interface techniques.

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Programmable controller block diagram

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Block diagram of major CPU components

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Graphic Representation of a Scan

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Input/Output Interface

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Personal Computer Used as a Programming Devices

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Typical programmable controller applications

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PLC PRODUCT APPLICATION RANGES

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LADDER DIAGRAMS AND THE PLC

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Typical programmable controller features and benefits.

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Programmable controller I/O connection diagram showing no physical connections between the inputs and outputs.

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Remote I/O station installation

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EASE OF MAINTENANCE AND TROUBLESHOOTING

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Failures in a PLC-based system

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A programming device being used to monitor inputs and outputs, with highlighted contacts indicating an

ON condition.

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NUMBER SYSTEMSAND CODES

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DECIMAL NUMBER SYSTEM

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BINARY NUMBER SYSTEM

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ASCII Coderepresentation of the character Z

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Binary Coded Decimal (BCD)

(Thumbwheel switch)

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Gray codeIt is one of a series of cyclic codes known as reflected codes and is suited primarily for position transducers

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LOGIC CONCEPTS

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Binary Concept Using Positive Logic

Binary Concept Using Negative Logic

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AND Gate

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OR Gate

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NOT Function

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Hardwired relay logic circuit

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ADDRESSES USED IN PLCS

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CONTACT SYMBOLS USED IN PLCS

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Multiple contactsfrom a PLC output coil