programmable logic controller (plc) overview

Chapter

1Programmable Logic

Controller (PLC) Overview

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Objectives

• Describe the invention and the development history of programmable logic controller (PLC) systems.

• Discuss the benefits of PLCs over electromechanical relay logic systems.

• Distinguish between fixed and modular PLC devices.

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Objectives

• Discuss different types of Intel and Motorola microprocessors.

• Explain different types of random access memory (RAM) devices and their use in PLC systems.

• Explain different types of read only memory (ROM) devices.

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Objectives

• Explain the use of the address decoder circuit in a PLC system.

• List the major components of a PLC power supply.

• Describe Allen-Bradley programmable controller devices.

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Programmable Logic Controller (PLC)• All devices have:

– Discrete input ports– Discrete output ports

• Larger, more advanced PLCs have:– Variable input ports– Variable output ports

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PLC Block Diagram

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Fixed PLC

• If all units are in one fixed enclosure.

• Number of input ports cannot be expanded.

• Number of output ports cannot be expanded.

(Energy Concepts, Inc.)

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Modular PLC

• If each unit is placed in different racks.

• Input modules can be added if needed.

• Output modules can be added on modular PLC systems.

(Energy Concepts, Inc.)

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Central Processing Unit (CPU)

• Microprocessor.• Unit.• Memory.• Support chips.

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Microprocessor Unit (MPU)

• Brain of the central processing unit.

• Carries out or executes program instructions.

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Arithmetic Logic Unit (ALU)

• Carries out math and logic instructions.

• Registers:– Hold data for the

arithmetic logic unit.

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Cache

• Instruction cache:– Temporarily holds

incoming instructions.

• Data cache:– Temporarily holds

incoming data.

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Bus

• Bus width:– How many conductors comprise the address bus

and data bus. • Address bus:

– Helps the CPU to find physical locations in memory.

• Data bus:– Transfers data between CPU components, such

as between the I/O and RAM.– The wider the data bus, the more data that can

be transferred at one time.

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Instruction Set

• CISC-based microprocessor• RISC-based microprocessor

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Memory Devices

• Two types of internal memory devices for a CPU:– Random access memory (RAM).– Read only memory (ROM).

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Random Access Memory (RAM)

• Static RAM (SRAM):– Stores data bits in its internal flip-flop.

• Dynamic RAM (DRAM).– Holds binary data in the form of charged

capacitors.

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Random Access Memory (RAM) (Cont.)

• Data on the DRAM: – Must be refreshed every few

microseconds due to the discharge of the capacitors.

• Data on the SRAM:– Remains unchanged until either a new

data is written over the existing data, or power to the chip is turned off.

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Read Only Memory (ROM)

• Masked ROM, or preprogrammed ROM:– Usually programmed by the manufacturer

at the factory. – Customer must order several thousand

chips from a factory.• Programmable ROM (PROM):

– Programmed or “burned” only once by the programmer.

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Read Only Memory (ROM) (Cont.)

• Erasable programmable ROM (EPROM), or ultraviolet-erasable programmable ROM (UVEPROM):– Programmed by the programmer and can be

erased by placing it under ultraviolet light.• Electrically erasable programmable ROM (

EEPROM), or flash ROM:– Programmable ROM that is programmed and

erased by injecting an electrical current through it.

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Basic Input/Output System (BIOS)• On power up, the firmware, or the

software in ROM:– Carries out several instructions that check

the input, output, and other peripheral devices connected to the CPU.

– Serves as a communication interface between PLC hardware and the PLC program.

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Address Decoder Circuit

• MPUs use an address decoder to enable:– ROM.– RAM.– I/O port devices.– Support chips.

• When the MPU communicates with a chip, only that chip is connected to the address, data, and control lines of the CPU.

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Peripheral Chips

• Also called support chips.• Used to enable the microprocessor to

communicate with input/output devices.

• Supports the MPU with a different task.

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Discrete Input Modules

• Terminals receive input signals from switches or other input devices.

• Converts the input signal to a digital signal prior to sending it to the CPU.

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Discrete Input Modules (Cont.)

• Octal transceiver• Full-wave rectifier• Optoisolator

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Discrete Input Modules (Cont.)• Zener diode:

– Semiconductor device that can maintain a steady voltage level.

– When phototransistor becomes saturated, ensures that a steady +5 V is applied to the CPU circuit board.

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Discrete Output Modules

• Octal latch:– Output bit values remain the same until

new input bits write over them. – Two octal latches are needed with a 16

output module.

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Discrete Output Modules (Cont.)• Triac:

– Solid state relay.– Can be used for switching the output voltage on

and off in order to connect the load to AC voltage.

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Analog Input/Output Modules

• Analog-to-digital converter (ADC):– Converts analog input signals to digital

signals. • Digital-to-analog converter (DAC):

– Converts digital output signals back to analog signals.

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Allen-Bradley Programmable Controllers

• Programmable logic controllers (PLCs) are used to implement specific instructions:– Input/output (I/O) port control.– Timing.– Counting.– Data manipulation.– Reporting.– Arithmetic.– Logic.– Communication.

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Allen-Bradley Programmable Controllers (Cont.)

• Safety programmable controller:– Used for the monitoring and implementing

the safety instructions in case of faults in a control system.

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Allen-Bradley Programmable Controllers (Cont.)

• Programmable automation controller (PAC):– A personal computer (PC) with a PLC

architecture.– Can be used to perform PLC instructions

in addition to controlling electric drives and more sophisticated motion and batch control systems.

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Allen-Bradley Programmable Logic Controllers (Cont.)

• Four major types of PLCs are categorized according to:– Input/output (I/O) size.– Processor.– Speed.– Memory size.