chapter 1 programmable logic controller (plc) overview
TRANSCRIPT
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.