What is a Programmable Logic Controller (PLC) ?
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What is a Programmable Logic Controller (PLC) ?. Agenda. History PLC Overview Hardware Inputs/Outputs Operation Programming Software Communication What you need to know when specifying a PLC Glossary. History of the PLC.

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What is a programmable logic controller plc

What is a Programmable Logic Controller (PLC) ?


What is a programmable logic controller plc

Agenda

  • History

  • PLC Overview

  • Hardware

  • Inputs/Outputs

  • Operation

  • Programming Software

  • Communication

  • What you need to know when specifying a PLC

  • Glossary


What is a programmable logic controller plc

History of the PLC

  • 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.


What is a programmable logic controller plc

Inputs

Outputs

What is a PLC?

The Basic Block

CPU

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.


What is a programmable logic controller plc

Inputs

Outputs

Remote

Outputs

Inputs

What is a PLC with Distributed I/O?

Getting Complex

CPU


What is a programmable logic controller plc

CPU

CPU

Outputs

Inputs

Inputs

Outputs

Outputs

Inputs

What is a PLC with Distributed Control?

Getting Really Complex

Remote


What is a programmable logic controller plc

MEMORY

data

program

Inside a PLC

Communications

Port

Circuits

Circuits

Central

Processor

Unit

(CPU)

Input

Output

CR

High

Voltage

High

Voltage

Isolation

Barrier

Isolation

Barrier

Low Voltage

or

DC Power Supply

AC Power Supply


What is a programmable logic controller plc

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


What is a programmable logic controller plc

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


What is a programmable logic controller plc

What are Outputs?

  • Valves

  • Motor Starters

  • Solenoids

  • Actuators

  • Control Relays

  • Horns & Alarms

  • Stack Lights

  • Fans

  • Counter/Totalizer

  • Pumps

  • Printers


What is a programmable logic controller plc

START

Housekeeping

Input

Scan

Output

Scan

Program

Scan

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


What is a programmable logic controller plc

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”


What is a programmable logic controller plc

Ladder Logic

Inputs

Outputs

Read / Conditional

Instructions

Write / Control

Instructions

( )

| |

Start (Rung #1)

( )

| |

|/|

| |

( )

| |

|/|

( )

| |

|/|

End (Rung #4)

| |


What is a programmable logic controller plc

IEC - Function Block Diagram (FBD)


What is a programmable logic controller plc

IEC - Structured Text (ST)


What is a programmable logic controller plc

IEC - Instruction List (IL)


What is a programmable logic controller plc

IEC - Ladder Diagram (LD)


What is a programmable logic controller plc

IEC - Sequential Function Chart (SFC)


What is a programmable logic controller plc

Communication


What is a programmable logic controller plc

Communication Basics

  • 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)


What is a programmable logic controller plc

Communication Protocols

  • Ethernet

  • Modbus Plus

  • Modbus

  • S908

  • LonWorks

  • Interbus

  • Profi-bus

  • DeviceNet

  • Uni-Telway

  • 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


What is a programmable logic controller plc

What you need to know when specifying a PLC

  • 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 is a programmable logic controller plc

What you need to know when specifying a PLC

  • 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 is a programmable logic controller plc

Alphabet Soup

HMI / MMIHuman Machine Interface / Man Machine Interface

I/OInput(s) and/or Output(s)

LSB / MSBLeast Significant Bit / Most Significant Bit

MODICONMOdular DIgital CONtoller

PIDProportional Integral Derivative (feedback control)

RFRadio Frequency

RTURemote Terminal Unit

SCADA Supervisory Control And Data Acquisition

TCP/IPTransmission Control Protocol / Internet Protocol


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