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What is a Programmable
Logic Controller (PLC) ?
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
Agenda
• History
• PLC Overview
• Hardware
• Inputs/Outputs
• Operation
• Programming Software
• Communication
• What you need to know when specifying a PLC
• Glossary
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 3
• First Programmable Logic Controllers were designed and
developed by Modicon as a relay replacer for GM and Landis.
• These controllers eliminated the need for rewiring and adding
additional hardware for each new configuration of logic.
• The new system drastically increased the functionality of the
controls while reducing the cabinet space that housed the logic.
• The first PLC, model 084, was invented by Dick Morley in 1969.
• The first commercial successful PLC, the 184, was introduced
in 1973 which was designed by Michael Greenberg.
History of the PLC
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 4
What is a PLC?
Inputs Outputs
The Basic Block
A PROGRAMMABLE LOGIC CONTROLLER is a solid state control system
that continuously monitors the status of devices connected as inputs.
Based upon a user written program, stored in memory, it controls the status
of devices connected as outputs.
CPU
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 5
What is a PLC with Distributed I/O?
CPU
Inputs
Inputs
Outputs
Outputs
Remote
Getting Complex
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 6
What is a PLC with Distributed Control?
CPU
Remote
Getting Really Complex
CPU
Inputs
Inputs
InputsOutputs
Outputs
Outputs
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 7
CR
IsolationBarrierIsolation
Barrier
Central
Processor
Unit
(CPU)
MEMORY
program data
HighVoltage
HighVoltage
Low Voltage
AC Power Supply
Output
Circuits
DC Power Supplyor
CommunicationsPort
Input
Circuits
Inside a PLC
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 8
Inside a PLC
The CPU contains an “Executive” program that tells the PLC how to:
• Execute the control Instructions
• User’s Program
• Communicate with other devices
• Other PLCs, Programming devices, I/O devices, etc.
• Perform Housekeeping activities
• Diagnostics, etc
This program is stored in “nonvolatile” memory
• Meaning that the program will not be lost if power is removed
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 9
What are Inputs?
• Switches and Pushbuttons
• Sensing Devices
• Limit Switches
• Photoelectric Sensors
• Proximity Sensors
• Condition Sensors
• Pressure Switches
• Level Switches
• Temperature Switches
• Vacuum Switches
• Float Switches
• Encoders
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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What are Outputs?
• Valves
• Motor Starters
• Solenoids
• Actuators
• Control Relays
• Horns & Alarms
• Stack Lights
• Fans
• Counter/Totalizer
• Pumps
• Printers
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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PLC Operating Cycle
• Four Steps in the PLC Operations
• Input Scan
• Scan the state of the Inputs
• Program Scan
• Processes the program logic
• Output Scan
• Energize/de-energize the outputs
• Housekeeping
• This step includes communications, Internal Diagnostics, etc.
• The steps are continually repeated - processed in a loop
START
Input
Scan
Program
Scan
Housekeeping
Output
Scan
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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Programming the PLC
Types of Programming Software Languages
Ladder Logic (LL)
IEC Format
Function Block Diagram (FBD)
Structured Text (ST)
Instruction List (IL)
Ladder Diagram (LD)
Sequential Function Chart (SFC) - also known as “Grafcet”
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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Ladder Logic
Read / Conditional
Instructions
Write / Control
Instructions
| | |/|
| | |/|
| |
| | |/| ( )
| |
| |
Start (Rung #1)
End (Rung #4)
( )
( )
( )
Inputs Outputs
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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IEC - Function Block Diagram (FBD)
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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IEC - Structured Text (ST)
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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IEC - Instruction List (IL)
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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IEC - Ladder Diagram (LD)
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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IEC - Sequential Function Chart (SFC)
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 1
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Communication
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
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• Protocols
• A set of rules for data exchange (format and timing of data) in a
communications system
• Stack / profile = selected set of protocols for a communication
application
• Communication Relationships
• Master / Slave - request / response & response only
• Peer-to-Peer - client / server, publisher / subscriber
• Communication Architectures
• Star (point-to-point)
• Bus, Ring, LAN (multidrop)
Communication Basics
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
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• Ethernet
• Modbus Plus
• Modbus
• S908
• LonWorks
• Interbus
• Profi-bus
• DeviceNet
• Uni-Telway
Communication Protocols
• CAN-Open
• ASi
• Seriplex
• FIPIO/FIPway
• RIO
• HART
• DIO
• ControlNet
• ASCII
Protocols are similar to languages -
Devices must support the same protocol to understand each other
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
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• Quantity, Type and Location of I/O
• Number of Inputs and output points
• AC or DC voltage
• Analog or Discrete
• Concentrated or spread out (distributed)
• Communication Requirements
• Protocol/Network used
• Devices to communicate with (HMI, other PLCs, etc)
• Speed of Application
• Response time required (throughput) of the system
• How fast does the process change
What you need to know when specifying a PLC
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
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• Control Architecture Philosophy
• Centralized Control, Distributed Control or combination
• Redundancy - CPUs, Power Supplies, etc
• Programming Software
• IEC vs. 984
• Installed base / what is currently being used
• User Logic
• Size and complexity of Program
• Feedback control used
• etc.
What you need to know when specifying a PLC
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What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
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HMI / MMI Human Machine Interface / Man Machine Interface
I/O Input(s) and/or Output(s)
LSB / MSB Least Significant Bit / Most Significant Bit
MODICON MOdular DIgital CONtoller
PID Proportional Integral Derivative (feedback control)
RF Radio Frequency
RTU Remote Terminal Unit
SCADA Supervisory Control And Data Acquisition
TCP/IP Transmission Control Protocol / Internet Protocol
Alphabet Soup