Transcript Programmable Logic Controllers - G-W

Chapter
1
Programmable Logic
Controller (PLC) Overview
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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Permission granted to reproduce for educational use only.
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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Permission granted to reproduce for educational use only.
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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Permission granted to reproduce for educational use only.
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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Permission granted to reproduce for educational use only.
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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Permission granted to reproduce for educational use only.
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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Permission granted to reproduce for educational use only.
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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Permission granted to reproduce for educational use only.
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.
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Permission granted to reproduce for educational use only.