what is a plc presentation

What is a Programmable

Logic Controller (PLC) ?

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


• 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


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


What is a PLC with Distributed I/O?

CPU

Inputs

Inputs

Outputs

Outputs

Remote

Getting Complex


What is a PLC with Distributed Control?

CPU

Remote

Getting Really Complex

CPU

Inputs

Inputs

InputsOutputs

Outputs

Outputs


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


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 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


0

What are Outputs?

• Valves

• Motor Starters

• Solenoids

• Actuators

• Control Relays

• Horns & Alarms

• Stack Lights

• Fans

• Counter/Totalizer

• Pumps

• Printers


1

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


2

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”


3

Ladder Logic

Read / Conditional

Instructions

Write / Control

Instructions

| | |/|

| | |/|

| |

| | |/| ( )

| |

| |

Start (Rung #1)

End (Rung #4)

( )

( )

( )

Inputs Outputs


4

IEC - Function Block Diagram (FBD)


5

IEC - Structured Text (ST)


6

IEC - Instruction List (IL)


7

IEC - Ladder Diagram (LD)


8

IEC - Sequential Function Chart (SFC)


9

Communication


0

• 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


1

• 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


2

• 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


3

• 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


4

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

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