plc programmable logic controllers

Programmable ControllersTheory and Implementation

Second Edition

L.A. BryanE.A. Bryan

THEORY AND IMPLEMENTATION

PROGRAMMABLECONTROLLERS

An Industrial Text Company PublicationAtlanta Georgia USA

Second Edition

L. A. BryanE. A. Bryan

1988, 1997 by Industrial Text CompanyPublished by Industrial Text CompanyAll rights reservedFirst edition 1988. Second edition 1997Printed and bound in the United States of America03 02 01 00 99 98 97 10 9 8 7 6 5 4 3 2| | | | | | | | | | | || | | | | | | | | | | | | | |

| | | | | | | | | | | | | | || | | | | | | | | | | | | | |

Reproduction or translation of any part of this work beyondthat permitted by Sections 107 and 108 of the 1976 UnitedStates Copyright act are unlawful.Requests for permission, accompanying workbooks, orfurther information should be addressed to:Industrial Text and Video Company1950 Spectrum CircleTower A-First FloorMarietta, Georgia 30067(770) 240-2200(800) PLC-TEXT

Library of Congress Cataloging-in-Publication DataBryan, L.A.

Programmable controllers: theory and implementation/L.A. Bryan, E.A. Bryan.2nd ed. p. cm. Includes index. ISBN 0-944107-32-X 1. Programmable controllers. I. Bryan, E.A. II. Title. TJ223.P76B795 1997 629.8'9dc21 96-49350 CIP

Due to the nature of this publication and because of the different applications ofprogrammable controllers, the readers or users and those responsible for applying theinformation herein contained must satisfy themselves to the acceptability of eachapplication and the use of equipment therein mentioned. In no event shall the publisherand others involved in this publication be liable for direct, indirect, or consequentialdamages resulting from the use of any technique or equipment herein mentioned.The illustrations, charts, and examples in this book are intended solely to illustrate themethods used in each application example. The publisher and others involved in thispublication cannot assume responsibility or liability for actual use based on theillustrative uses and applications.No patent liability is assumed with respect to use of information, circuits, illustrations,equipment, or software described in this text.

Industrial Text & Video Company 1-800-752-8398www.industrialtext.com

iii

Contents


CONTENTS

Preface ..................................................................................................... ixAbout the Authors .................................................................................... xHow to Use this Book ............................................................................. xi

SECTION 1 INTRODUCTORY CONCEPTSChapter 1 Introduction to Programmable Controllers

1-1 Definition ................................................................................................. 41-2 A Historical Background .......................................................................... 51-3 Principles of Operation ........................................................................... 101-4 PLCs Versus Other Types of Controls ................................................... 131-5 PLC Product Application Ranges .......................................................... 221-6 Ladder Diagrams and the PLC ............................................................... 241-7 Advantages of PLCs ............................................................................... 26

Chapter 2 Number Systems and Codes2-1 Number Systems .................................................................................... 342-2 Number Conversions .............................................................................. 412-3 Ones and Twos Complement ............................................................... 432-4 Binary Codes .......................................................................................... 462-5 Register Word Formats .......................................................................... 50

Chapter 3 Logic Concepts3-1 The Binary Concept ............................................................................... 563-2 Logic Functions ...................................................................................... 573-3 Principles of Boolean Algebra and Logic .............................................. 643-4 PLC Circuits and Logic Contact Symbology ......................................... 68

SECTION 2 COMPONENTS AND SYSTEMSChapter 4 Processors, the Power Supply, and Programming Devices

4-1 Introduction ............................................................................................ 824-2 Processors ............................................................................................... 844-3 Processor Scan........................................................................................ 864-4 Error Checking and Diagnostics ............................................................ 924-5 The System Power Supply ..................................................................... 984-6 Programming Devices .......................................................................... 104

Chapter 5 The Memory System and I/O Interaction5-1 Memory Overview ............................................................................... 1105-2 Memory Types ..................................................................................... 1115-3 Memory Structure and Capacity .......................................................... 1155-4 Memory Organization and I/O Interaction ........................................... 119

iv

Contents


5-5 Configuring the PLC MemoryI/O Addressing ................................. 1275-6 Summary of Memory, Scanning, and I/O Interaction .......................... 1325-7 Memory Considerations ....................................................................... 133

Chapter 6 The Discrete Input/Output System6-1 Introduction to Discrete I/O Systems ................................................... 1386-2 I/O Rack Enclosures and Table Mapping ............................................ 1396-3 Remote I/O Systems ............................................................................. 1466-4 PLC Instructions for Discrete Inputs .................................................... 1476-5 Types of Discrete Inputs ...................................................................... 1506-6 PLC Instructions for Discrete Outputs ................................................. 1626-7 Discrete Outputs ................................................................................... 1656-8 Discrete Bypass/Control Stations ......................................................... 1776-9 Interpreting I/O Specifications ............................................................. 178

6-10 Summary of Discrete I/O ..................................................................... 182

Chapter 7 The Analog Input/Output System7-1 Overview of Analog Input Signals ....................................................... 1867-2 Instructions for Analog Input Modules ................................................ 1877-3 Analog Input Data Representation ....................................................... 1897-4 Analog Input Data Handling ................................................................ 1967-5 Analog Input Connections .................................................................... 1997-6 Overview of Analog Output Signals .................................................... 2017-7 Instructions for Analog Output Modules ............................................. 2017-8 Analog Output Data Representation .................................................... 2037-9 Analog Output Data Handling .............................................................. 207

7-10 Analog Output Connections ................................................................. 2137-11 Analog Output Bypass/Control Stations .............................................. 214

Chapter 8 Special Function I/O and Serial Communication Interfacing8-1 Introduction to Special I/O Modules .................................................... 2188-2 Special Discrete Interfaces ................................................................... 2208-3 Special Analog, Temperature, and PID Interfaces ............................... 2248-4 Positioning Interfaces ........................................................................... 2338-5 ASCII, Computer, and Network Interfaces .......................................... 2488-6 Fuzzy Logic Interfaces ......................................................................... 2558-7 Peripheral Interfacing ........................................................................... 260

SECTION 3 PLC PROGRAMMINGChapter 9 Programming Languages

9-1 Introduction to Programming Languages ............................................. 2769-2 Types of PLC Languages ..................................................................... 2769-3 Ladder Diagram Format ....................................................................... 2829-4 Ladder Relay Instructions .................................................................... 2899-5 Ladder Relay Programming ................................................................. 2989-6 Timers and Counters ............................................................................ 3069-7 Timer Instructions ................................................................................ 308

vContents


9-8 Counter Instructions ............................................................................. 3129-9 Program/Flow Control Instructions ...................................................... 317

9-10 Arithmetic Instructions ......................................................................... 3229-11 Data Manipulation Instructions ............................................................ 3349-12 Data Transfer Instructions .................................................................... 3489-13 Special Function Instructions ............................................................... 3589-14 Network Communication Instructions ................................................. 3639-15 Boolean Mnemonics ............................................................................. 369

Chapter 10 The IEC 1131 Standard and Programming Language10-1 Introduction to the IEC 1131 ................................................................ 37410-2 IEC 1131-3 Programming Languages .................................................. 38010-3 Sequential Function Chart Programming ............................................. 40310-4 Types of Step Actions .......................................................................... 41910-5 IEC 1131-3 Software Systems ............................................................. 42910-6 Summary .............................................................................................. 439

Chapter 11 System Programming and Implementation11-1 Control Task Definition ....................................................................... 44411-2 Control Strategy ................................................................................... 44411-3 Implementation Guidelines .................................................................. 44511-4 Programming Organization and Implementation ................................. 44611-5 Discrete I/O Control Programming ...................................................... 46511-6 Analog I/O Control Programming........................................................ 49211-7 Short Programming Examples ............................................................. 521

Chapter 12 PLC System Documentation12-1 Introduction to Documentation ............................................................ 53612-2 Steps for Documentation ...................................................................... 53712-3 PLC Documentation Systems............................................................... 54712-4 Conclusion ............................................................................................ 549

SECTION 4 PLC PROCESS APPLICATIONSChapter 13 Data Measurements and Transducers

13-1 Basic Measurement Concepts .............................................................. 55413-2 Interpreting Errors in Measurements .................................................... 56013-3 Transducer Measurements .................................................................... 56513-4 Thermal Transducers ............................................................................ 57213-5 Displacement Transducers ................................................................... 58613-6 Pressure Transducers ............................................................................ 58813-7 Flow Transducers ................................................................................. 59113-8 Vibration Transducers .......................................................................... 59913-9 Summary .............................................................................................. 608

Chapter 14 Process Responses and Transfer Functions14-1 Process Control Basics ......................................................................... 61014-2 Control System Parameters .................................................................. 614

vi

Contents


14-3 Process Dynamics ................................................................................ 62314-4 Laplace Transform Basics .................................................................... 63214-5 Dead Time Responses in Laplace Form ............................................... 64414-6 Lag Responses in Laplace Form .......................................................... 64514-7 Types of Second-Order Responses ...................................................... 65314-8 Summary .............................................................................................. 665

Chapter 15 Process Controllers and Loop Tuning15-1 Introduction .......................................................................................... 67015-2 Controller Actions ................................................................................ 67115-3 Discrete-Mode Controllers ................................................................... 67615-4 Continuous-Mode Controllers .............................................................. 69015-5 Proportional Controllers (P Mode) ....................................................... 69215-6 Integral Controllers (I Mode) ............................................................... 70615-7 Proportional-Integral Controllers (PI Mode) ........................................ 71515-8 Derivative Controllers (D Mode) ......................................................... 72515-9 Proportional-Derivative Controllers (PD Mode) .................................. 729

15-10 Proportional-Integral-Derivative Controllers (PID Mode) .................. 73615-11 Advanced Control Systems .................................................................. 74415-12 Controller Loop Tuning ....................................................................... 74715-13 Summary .............................................................................................. 766

SECTION 5 ADVANCED PLC TOPICS AND NETWORKSChapter 16 Artificial Intelligence and PLC Systems

16-1 Introduction to AI Systems .................................................................. 77416-2 Types of AI Systems ............................................................................ 77416-3 Organizational Structure of an AI System ........................................... 77616-4 Knowledge Representation .................................................................. 77816-5 Knowledge Inference ........................................................................... 78116-6 AI Fault Diagnostics Application ......................................................... 788

Chapter 17 Fuzzy Logic17-1 Introduction to Fuzzy Logic ................................................................. 79817-2 History of Fuzzy Logic ........................................................................ 80117-3 Fuzzy Logic Operation ......................................................................... 80217-4 Fuzzy Logic Control Components ....................................................... 80517-5 Fuzzy Logic Control Example ............................................................. 82817-6 Fuzzy Logic Design Guidelines ........................................................... 835

Chapter 18 Local Area Networks18-1 History of Local Area Networks .......................................................... 84818-2 Principles of Local Area Networks ...................................................... 84818-3 Network Topologies ............................................................................. 85118-4 Network Access Methods ..................................................................... 85718-5 Communication Media ......................................................................... 86018-6 Understanding Network Specifications ................................................ 862

vii

Contents


18-7 Network Protocols ................................................................................ 86618-8 Network Testing and Troubleshooting ................................................. 87418-9 Network Comparison and Selection Criteria ....................................... 875

Chapter 19 I/O Bus Networks19-1 Introduction to I/O Bus Networks ........................................................ 88019-2 Types of I/O Bus Networks .................................................................. 88319-3 Advantages of I/O Bus Networks ......................................................... 88519-4 Device Bus Networks ........................................................................... 88619-5 Process Bus Networks .......................................................................... 89919-6 I/O Bus Installation and Wiring Connections ...................................... 91019-7 Summary of I/O Bus Networks ............................................................ 916

SECTION 6 INSTALLATION AND START-UPChapter 20 PLC Start-Up and Maintenance

20-1 PLC System Layout ............................................................................. 92220-2 Power Requirements and Safety Circuitry ........................................... 93120-3 Noise, Heat, and Voltage Considerations ............................................. 93520-4 I/O Installation, Wiring, and Precautions ............................................. 94220-5 PLC Start-Up and Checking Procedures .............................................. 94820-6 PLC System Maintenance .................................................................... 95220-7 Troubleshooting the PLC System ........................................................ 954

Chapter 21 System Selection Guidelines21-1 Introduction to PLC System Selection ................................................. 96221-2 PLC Sizes and Scopes of Applications ................................................ 96221-3 Process Control System Definition ...................................................... 96921-4 Other Considerations ............................................................................ 98121-5 Summary .............................................................................................. 982

APPENDICES

Appendix A Logic Symbols, Truth Tables, and Equivalent Ladder/Logic Diagrams ..... 987Appendix B ASCII Reference .................................................................................. 989Appendix C Electrical Relay Diagram Symbols ...................................................... 991Appendix D P&ID Symbols ..................................................................................... 993Appendix E Equation of a Line and Number Tables ............................................... 995Appendix F Abbreviations and Acronyms ............................................................... 997Appendix G Voltage-Current Laplace Transfer Function Relationships ................. 999

Glossary .............................................................................................. 1001Index ................................................................................................... 1025

This page intentionally left blank.

ix

Preface


PREFACESince the first edition of this book in 1988, the capabilities of programmablelogic controllers have grown by leaps and bounds. Likewise, the applicationsof PLCs have grown with them. In fact, in todays increasingly computer-controlled environment, it is almost impossible to find a technical industrythat does not use programmable controllers in one form or another. Torespond to these phenomenal changes, we introduce the second edition ofProgrammable Controllers: Theory and Implementation.

This second edition, like the first, provides a comprehensive theoretical, yetpractical, look at all aspects of PLCs and their associated devices and systems.However, this version goes one step further with new chapters on advancedPLC topics, such as I/O bus networks, fuzzy logic, the IEC 1131-3 program-ming standard, process control, and PID algorithms. This new edition alsopresents revised, up-to-date information about existing topics, with expandedgraphics and new, hands-on examples. Furthermore, the new layout of thebookwith features like two-tone graphics, key terms lists, well-definedheadings and sections, callout icons, and a revised, expanded glossarymakes the information presented even easier to understand.

This new edition has been a labor-intensive learning experience for all thoseinvolved. As with any task so large, we could never have done it alone.Therefore, we would like to thank the following companies for their help inbringing this book to press: Allen-Bradley CompanyIndustrial ComputerGroup, ASI-USA, B & R Industrial Automation, Bailey Controls Company,DeviceNet Vendors Association, ExperTune Software, Fieldbus Foundation,Hoffman Engineering Company, HoneywellMicroSwitch Division,LANcityCable Modem Division of Bay Networks, Mitsubishi Electronics,Omron Electronics, Phoenix Contact, PLC Direct, PMC/BETA LP, ProfibusTrade Organization, Schaevitz Engineering Company, Siemens Automation,Square D Company, Thermometrics, and WAGO.

We hope that you will find this book to be a valuable learning and referencetool. We have tried to present a variety of programmable control operations;however, with the unlimited variations in control systems, we certainly havenot been able to provide an exhaustive list of PLC applications. Only you,armed with the knowledge gained through this book, can explore the truelimits of programmable logic controllers.

Stephanie PhilippoEditor

xAbout the Authors


ABOUT THE AUTHORS

LUIS BRYAN

Luis Bryan holds a Bachelor of Science in Electrical Engineering degree anda Master of Science in Electrical Engineering degree, both from the Univer-sity of Tennessee. His major areas of expertise are digital systems, electron-ics, and computer engineering. During his graduate studies, Luis was in-volved in several projects with national and international governmentalagencies.

Luis has extensive experience in the field of programmable controllers. Hewas involved in international marketing activities, as well as PLC applica-tions development, for a major programmable controller manufacturer. Healso worked for a consulting firm, providing market studies and company-specific consultations about PLCs. Furthermore, Luis has given lectures andseminars in Canada, Mexico, and South America about the uses of program-mable controllers. He continues to teach seminars to industry and governmententities, including the National Aeronautics and Space Administration(NASA).Luis is an active member of several professional organizations, including theInstitute of Electrical and Electronics Engineers (IEEE) and the IEEEsinstrument and computer societies. He is a senior member of the InstrumentSociety of America, as well as a member of Phi Kappa Phi honor society andEta Kappa Nu electrical engineering honor society. Luis has coauthoredseveral other books about programmable controllers.

ERIC BRYAN

Eric Bryan graduated from the University of Tennessee with a Bachelor ofScience in Electrical Engineering degree, concentrating in digital design andcomputer architecture. He received a Master of Science in Engineeringdegree from the Georgia Institute of Technology, where he participated in aspecial computer-integrated manufacturing (CIM) program. Erics special-ties are industrial automation methods, flexible manufacturing systems(FMS), and artificial intelligence. He is an advocate of artificial intelligenceimplementation and its application in industrial automation.

Eric worked for a leading automatic laser inspection systems company, aswell as a programmable controller consulting firm. His industrial experienceincludes designing and implementing large inspection systems, along withdeveloping PLC-based systems. Eric has coauthored other publications aboutPLCs and is a member of several professional and technical societies.

xi

How to Use this Book


HOW TO USE THIS BOOKWelcome to Programmable Controllers: Theory and Implementation. Be-fore you begin reading, please review the following strategies for using thisbook. By following these study strategies, you will more thoroughly under-stand the information presented in the text and, thus, be better able to applythis knowledge in real-life situations.

BEFORE YOU BEGIN READING

Look through the book to familiarize yourself with its structure. Read the table of contents to review the subjects you will be studying. Familiarize yourself with the icons used throughout the text:

Chapter Highlights

Key Terms

Look at the appendices to see what reference materials have been provided.

AS YOU STUDY EACH CHAPTER

Before you start a chapter, read the Chapter Highlights paragraph at thebeginning of the chapters text. This paragraph will give you an overviewof what youll learn, as well as explain how the information presented inthe chapter fits into what youve already learned and what you will learn.

Read the chapter, paying special attention to the bolded items. These arekey terms that indicate important topics that you should understand afterfinishing the chapter.

When you encounter an exercise, try to solve the problem yourself beforelooking at the solution. This way, you'll determine which topics youunderstand and which topics you should study further.

WHEN YOU FINISH EACH CHAPTER

At the end of each chapter, look over the list of key terms to ensure thatyou understand all of the important subjects presented in the chapter. Ifyoure not sure about a term, review it in the text.

Review the exercises to ensure that you understand the logic and equa-tions involved in each problem. Also, review the workbook and studyguide, making sure that you can work all of the problems correctly.

When youre sure that you thoroughly understand the information that hasbeen presented, youre ready to move on to the next chapter.

INTRODUCTORYCONCEPTS

SECTION ONE

Introduction to Programmable Controllers Number Systems and Codes Logic Concepts

Industrial Text and Video Company 1-800-752-8398www.industrialtext.com

This page intentionally left blank.


INTRODUCTION TOPROGRAMMABLE CONTROLLERS

CHAPTERONE

I find the great thing in this world is not somuch where we stand as in what direction weare moving.

Oliver Wendell Holmes


SECTION1

IntroductoryConcepts

4Industrial Text and Video Company 1-800-752-8398www.industrialtext.com

CHAPTER1

Introduction toProgrammable Controllers

Figure 1-1. PLC conceptual application diagram.

CHAPTERHIGHLIGHTS

Programmable controllers have many definitions. However, PLCs can bethought of in simple terms as industrial computers with specially designedarchitecture in both their central units (the PLC itself) and their interfacingcircuitry to field devices (input/output connections to the real world).

Every aspect of industryfrom power generation to automobile painting tofood packaginguses programmable controllers to expand and enhanceproduction. In this book, you will learn about all aspects of these powerful andversatile tools. This chapter will introduce you to the basics of programmablecontrollersfrom their operation to their vast range of applications. In it, wewill give you an inside look at the design philosophy behind their creation,along with a brief history of their evolution. We will also compare program-mable controllers to other types of controls to highlight the benefits anddrawbacks of each, as well as pinpoint situations where PLCs work best.When you finish this chapter, you will understand the fundamentals ofprogrammable controllers and be ready to explore the number systemsassociated with them.

1-1 DEFINITION

Programmable logic controllers, also called programmable controllers orPLCs, are solid-state members of the computer family, using integratedcircuits instead of electromechanical devices to implement control functions.They are capable of storing instructions, such as sequencing, timing,counting, arithmetic, data manipulation, and communication, to controlindustrial machines and processes. Figure 1-1 illustrates a conceptualdiagram of a PLC application.

ProgrammableControllerField

InputsField

Outputs

Measure Control

Processor

Machine

5CHAPTER1



SECTION1


The Hydramatic Division of the General Motors Corporation specified thedesign criteria for the first programmable controller in 1968. Their primarygoal was to eliminate the high costs associated with inflexible, relay-controlled systems. The specifications required a solid-state system withcomputer flexibility able to (1) survive in an industrial environment, (2) beeasily programmed and maintained by plant engineers and technicians, and(3) be reusable. Such a control system would reduce machine downtime andprovide expandability for the future. Some of the initial specificationsincluded the following:

The new control system had to be price competitive with the use ofrelay systems.

The system had to be capable of sustaining an industrial environment.

The input and output interfaces had to be easily replaceable.

The controller had to be designed in modular form, so that subassem-blies could be removed easily for replacement or repair.

The control system needed the capability to pass data collection to acentral system.

The system had to be reusable.

The method used to program the controller had to be simple, so thatit could be easily understood by plant personnel.

As you will see throughout this book, programmable logic controllers aremature industrial controllers with their design roots based on the principles ofsimplicity and practical application.

The product implementation to satisfy Hydramatics specifications wasunderway in 1968; and by 1969, the programmable controller had its firstproduct offsprings. These early controllers met the original specifications andopened the door to the development of a new control technology.

The first PLCs offered relay functionality, thus replacing the originalhardwired relay logic, which used electrically operated devices to mechani-cally switch electrical circuits. They met the requirements of modularity,expandability, programmability, and ease of use in an industrial environment.These controllers were easily installed, used less space, and were reusable.The controller programming, although a little tedious, had a recognizableplant standard: the ladder diagram format.

1-2 A HISTORICAL BACKGROUND

THE FIRST PROGRAMMABLE CONTROLLER

SECTION1



CHAPTER1


In a short period, programmable controller use started to spread to otherindustries. By 1971, PLCs were being used to provide relay replacement asthe first steps toward control automation in other industries, such as food andbeverage, metals, manufacturing, and pulp and paper.

THE CONCEPTUAL DESIGN OF THE PLC

The first programmable controllers were more or less just relay replacers.Their primary function was to perform the sequential operations that werepreviously implemented with relays. These operations included ON/OFFcontrol of machines and processes that required repetitive operations, such astransfer lines and grinding and boring machines. However, theseprogrammable controllers were a vast improvement over relays. They wereeasily installed, used considerably less space and energy, had diagnosticindicators that aided troubleshooting, and unlike relays, were reusable if aproject was scrapped.Programmable controllers can be considered newcomers when they arecompared to their elder predecessors in traditional control equipmenttechnology, such as old hardwired relay systems, analog instrumentation,and other types of early solid-state logic. Although PLC functions, such asspeed of operation, types of interfaces, and data-processing capabilities, haveimproved throughout the years, their specifications still hold to thedesigners original intentionsthey are simple to use and maintain.

TODAYS PROGRAMMABLE CONTROLLERS

Many technological advances in the programmable controller industrycontinue today. These advances not only affect programmable controllerdesign, but also the philosophical approach to control system architecture.Changes include both hardware (physical components) and software (con-trol program) upgrades. The following list describes some recent PLChardware enhancements:

Faster scan times are being achieved using new, advanced micro-processor and electronic technology.

Small, low-cost PLCs (see Figure 1-2), which can replace four to tenrelays, now have more power than their predecessor, the simple relayreplacer.

High-density input/output (I/O) systems (see Figure 1-3) providespace-efficient interfaces at low cost.

Intelligent, microprocessor-based I/O interfaces have expanded dis-tributed processing. Typical interfaces include PID (proportional-

7CHAPTER1



SECTION1


integral-derivative), network, CANbus, fieldbus, ASCII communica-tion, positioning, host computer, and language modules (e.g., BASIC,Pascal).

Mechanical design improvements have included rugged input/outputenclosures and input/output systems that have made the terminal anintegral unit.

Special interfaces have allowed certain devices to be connecteddirectly to the controller. Typical interfaces include thermocouples,strain gauges, and fast-response inputs.

Peripheral equipment has improved operator interface techniques,and system documentation is now a standard part of the system.

Figure 1-3. PLC systemwith high-density I/O(64-point modules).

Figure 1-2. Small PLC with built-inI/O and detachable, handheldprogramming unit.

All of these hardware enhancements have led to the development ofprogrammable controller families like the one shown in Figure 1-4. Thesefamilies consist of a product line that ranges from very smallmicrocontrollers, with as few as 10 I/O points, to very large and

Cou

rtes

y of

Mits

ubis

hi E

lect

roni

cs, M

ount

Pro

spec

t, IL

Cou

rtes

y of

Mits

ubis

hi E

lect

roni

cs, M

ount

Pro

spec

t, IL

SECTION1



CHAPTER1


sophisticated PLCs, with as many as 8,000 I/O points and 128,000 words ofmemory. These family members, using common I/O systems andprogramming peripherals, can interface to a local communication network.The family concept is an important cost-saving development for users.

Figure 1-4. Allen-Bradleys programmable controller family concept with several PLCs.

Like hardware advances, software advances, such as the ones listed below,have led to more powerful PLCs:

PLCs have incorporated object-oriented programming tools andmultiple languages based on the IEC 1131-3 standard.

Small PLCs have been provided with powerful instructions, whichextend the area of application for these small controllers.

High-level languages, such as BASIC and C, have been implementedin some controllers modules to provide greater programming flex-ibility when communicating with peripheral devices and manipulat-ing data.

Advanced functional block instructions have been implemented forladder diagram instruction sets to provide enhanced software capabil-ity using simple programming commands.

Diagnostics and fault detection have been expanded from simplesystem diagnostics, which diagnose controller malfunctions, toinclude machine diagnostics, which diagnose failures ormalfunctions of the controlled machine or process.

Floating-point math has made it possible to perform complex calcu-lations in control applications that require gauging, balancing, andstatistical computation.

Cou

rtes

y of

Alle

n-B

rad

ley,

Hig

hlan

d, H

eig

hts,

OH

9CHAPTER1



SECTION1


Data handling and manipulation instructions have been improved andsimplified to accommodate complex control and data acquisitionapplications that involve storage, tracking, and retrieval of largeamounts of data.

Programmable controllers are now mature control systems offering manymore capabilities than were ever anticipated. They are capable ofcommunicating with other control systems, providing production reports,scheduling production, and diagnosing their own failures and those of themachine or process. These enhancements have made programmablecontrollers important contributors in meeting todays demands for higherquality and productivity. Despite the fact that programmable controllers havebecome much more sophisticated, they still retain the simplicity and ease ofoperation that was intended in their original design.

PROGRAMMABLE CONTROLLERS AND THE FUTURE

The future of programmable controllers relies not only on the continuation ofnew product developments, but also on the integration of PLCs with othercontrol and factory management equipment. PLCs are being incorporated,through networks, into computer-integrated manufacturing (CIM) systems,combining their power and resources with numerical controls, robots, CAD/CAM systems, personal computers, management information systems, andhierarchical computer-based systems. There is no doubt that programmablecontrollers will play a substantial role in the factory of the future.

New advances in PLC technology include features such as better operatorinterfaces, graphic user interfaces (GUIs), and more human-oriented man/machine interfaces (such as voice modules). They also include thedevelopment of interfaces that allow communication with equipment,hardware, and software that supports artificial intelligence, such as fuzzylogic I/O systems.

Software advances provide better connections between different types ofequipment, using communication standards through widely used networks.New PLC instructions are developed out of the need to add intelligence to acontroller. Knowledge-based and process learningtype instructions may beintroduced to enhance the capabilities of a system.

The users concept of the flexible manufacturing system (FMS) will deter-mine the control philosophy of the future. The future will almost certainlycontinue to cast programmable controllers as an important player in thefactory. Control strategies will be distributed with intelligence instead ofbeing centralized. Super PLCs will be used in applications requiring complexcalculations, network communication, and supervision of smaller PLCs andmachine controllers.

SECTION1



CHAPTER1


Figure 1-5. Programmable controller block diagram.

Figure 1-6. Block diagram of major CPU components.

Processor

PowerSupply

Memory

The central processing unit (CPU) governs all PLC activities. The followingthree components, shown in Figure 1-6, form the CPU:

the processor

the memory system

the system power supply

OUTPUTS

INPUTS

CentralProcessing

Unit

1-3 PRINCIPLES OF OPERATION

A programmable controller, as illustrated in Figure 1-5, consists of two basicsections:

the central processing unit

the input/output interface system

11

CHAPTER1



SECTION1


Figure 1-7. Illustration of a scan.

The operation of a programmable controller is relatively simple. The input/output (I/O) system is physically connected to the field devices that areencountered in the machine or that are used in the control of a process. Thesefield devices may be discrete or analog input/output devices, such as limitswitches, pressure transducers, push buttons, motor starters, solenoids, etc.The I/O interfaces provide the connection between the CPU and the informa-tion providers (inputs) and controllable devices (outputs).During its operation, the CPU completes three processes: (1) it reads, oraccepts, the input data from the field devices via the input interfaces, (2) itexecutes, or performs, the control program stored in the memory system, and(3) it writes, or updates, the output devices via the output interfaces. Thisprocess of sequentially reading the inputs, executing the program in memory,and updating the outputs is known as scanning. Figure 1-7 illustrates agraphic representation of a scan.

The input/output system forms the interface by which field devices areconnected to the controller (see Figure 1-8). The main purpose of the interfaceis to condition the various signals received from or sent to external fielddevices. Incoming signals from sensors (e.g., push buttons, limit switches,analog sensors, selector switches, and thumbwheel switches) are wired toterminals on the input interfaces. Devices that will be controlled, like motorstarters, solenoid valves, pilot lights, and position valves, are connected tothe terminals of the output interfaces. The system power supply providesall the voltages required for the proper operation of the various centralprocessing unit sections.

(1)

(2)

(3)

SCAN

READ

EXECUTE

WRITE

SECTION1



CHAPTER1


Figure 1-9. (a) Personal computer used as a programming device and (b) a mini-programmer unit.

Chapters 4 and 5 will present a more detailed discussion of the centralprocessing unit and how it interacts with memory and input/output interfaces.Chapters 6, 7, and 8 discuss the input/output system.

Although not generally considered a part of the controller, the programmingdevice, usually a personal computer or a manufacturers miniprogrammerunit, is required to enter the control program into memory (see Figure 1-9).The programming device must be connected to the controller when enteringor monitoring the control program.

Figure 1-8. Input/output interface.

01234567

01234567

01234567

01234567

01234567

01234567

0 1 2 301234567

01234567

Processorand

Power Supply

I/O Interfaces

0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

INPUTMODULE

OUTPUTMODULE

(a)

Cou

rtes

y of

Om

ron

Ele

ctro

nics

, Sch

aum

bur

g, I

L

Cou

rtes

y of

Mits

ubis

hi E

lect

roni

cs, M

ount

Pro

spec

t, IL

(b)

13

CHAPTER1



SECTION1


1-4 PLCS VERSUS OTHER TYPES OF CONTROLS

PLCS VERSUS RELAY CONTROL

For years, the question many engineers, plant managers, and originalequipment manufacturers (OEMs) asked was, Should I be using aprogrammable controller? At one time, much of a systems engineers timewas spent trying to determine the cost-effectiveness of a PLC over relaycontrol. Even today, many control system designers still think that they arefaced with this decision. One thing, however, is certaintodays demand forhigh quality and productivity can hardly be fulfilled economically withoutelectronic control equipment. With rapid technology developments andincreasing competition, the cost of programmable controls has been drivendown to the point where a PLC-versus-relay cost study is no longer necessaryor valid. Programmable controller applications can now be evaluated on theirown merits.

When deciding whether to use a PLC-based system or a hardwired relaysystem, the designer must ask several questions. Some of these questions are:

Is there a need for flexibility in control logic changes?

Is there a need for high reliability?

Are space requirements important?

Are increased capability and output required?

Are there data collection requirements?

Will there be frequent control logic changes?

Will there be a need for rapid modification?

Must similar control logic be used on different machines?

Is there a need for future growth?

What are the overall costs?

The merits of PLC systems make them especially suitable for applications inwhich the requirements listed above are particularly important for theeconomic viability of the machine or process operation. A case which speaksfor itself, the system shown in Figure 1-10, shows why programmablecontrollers are easily favored over relays. The implementation of this systemusing electromechanical standard and timing relays would have made thiscontrol panel a maze of large bundles of wires and interconnections.

SECTION1



CHAPTER1


If system requirements call for flexibility or future growth, a programmablecontroller brings returns that outweigh any initial cost advantage of a relaycontrol system. Even in a case where no flexibility or future expansion isrequired, a large system can benefit tremendously from the troubleshootingand maintenance aids provided by a PLC. The extremely short cycle (scan)time of a PLC allows the productivity of machines that were previously underelectromechanical control to increase considerably. Also, although relaycontrol may cost less initially, this advantage is lost if production downtimedue to failures is high.

PLCS VERSUS COMPUTER CONTROLS

Figure 1-10. The uncluttered control panel of an installed PLC system.

The architecture of a PLCs CPU is basically the same as that of a generalpurpose computer; however, some important characteristics set them apart.First, unlike computers, PLCs are specifically designed to survive the harshconditions of the industrial environment. A well-designed PLC can be placedin an area with substantial amounts of electrical noise, electromagneticinterference, mechanical vibration, and noncondensing humidity.

Cou

rtes

y of

Om

ron

Ele

ctro

nics

, Sch

aum

bur

g, I

L

15

CHAPTER1



SECTION1


A second distinction of PLCs is that their hardware and software are designedfor easy use by plant electricians and technicians. The hardware interfaces forconnecting field devices are actually part of the PLC itself and are easilyconnected. The modular and self-diagnosing interface circuits are able topinpoint malfunctions and, moreover, are easily removed and replaced. Also,the software programming uses conventional relay ladder symbols, or othereasily learned languages, which are familiar to plant personnel.

Whereas computers are complex computing machines capable of executingseveral programs or tasks simultaneously and in any order, the standard PLCexecutes a single program in an orderly, sequential fashion from first to lastinstruction. Bear in mind, however, that PLCs as a system continue to becomemore intelligent. Complex PLC systems now provide multiprocessor andmultitasking capabilities, where one PLC may control several programs in asingle CPU enclosure with several processors (see Figure 1-11).

Figure 1-11. PLC system with multiprocessing and multitasking capabilities.

PLCS VERSUS PERSONAL COMPUTERS

With the proliferation of the personal computer (PC), many engineers havefound that the personal computer is not a direct competitor of the PLC incontrol applications. Rather, it is an ally in the implementation of the controlsolution. The personal computer and the PLC possess similar CPU architec-ture; however, they distinctively differ in the way they connect field devices.

While new, rugged, industrial personal computers can sometimes sustainmidrange industrial environments, their interconnection to field devices stillpresents difficulties. These computers must communicate with I/O interfacesnot necessarily designed for them, and their programming languages may notmeet the standards of ladder diagram programming. This presents a problemto people familiar with the ladder diagram standard when troubleshooting andmaking changes to the system.

Cou

rtes

y of

Gid

din

gs

& L

ewis

, Fon

d d

u La

c, W

I

SECTION1



CHAPTER1


The personal computer is, however, being used as the programming device ofchoice for PLCs in the market, where PLC manufacturers and third-partyPLC support developers come up with programming and documentationsystems for their PLC product lines. Personal computers are also beingemployed to gather process data from PLCs and to display informationabout the process or machine (i.e., they are being used as graphic userinterfaces, or GUIs). Because of their number-crunching capabilities,personal computers are also well suited to complement programmablecontrollers and to bridge the communication gap, through a network, betweena PLC system and other mainframe computers (see Figure 1-12).

Figure 1-12. A personal computer used as a bridge between a PLC system and amain computer system.

Some control software manufacturers, however, utilize PCs as CPUhardware to implement a PLC-like environment. The language they use isbased on the International Electrotechnical Commission (IEC) 1131-3standard, which is a graphic representation language (sequential functioncharts) that includes ladder diagrams, functional blocks, instruction lists, andstructured text. These software manufacturers generally do not provide I/Ohardware interfaces; but with the use of internal PC communication cards,these systems can communicate with other PLC manufacturers I/O hardwaremodules. Chapter 10 explains the IEC 1131-3 standard.

PLC

PersonalComputer

MainComputerSystem

17

CHAPTER1



SECTION1


Since its inception, the PLC has been successfully applied in virtuallyevery segment of industry, including steel mills, paper plants, food-process-ing plants, chemical plants, and power plants. PLCs perform a great varietyof control tasks, from repetitive ON/OFF control of simple machines tosophisticated manufacturing and process control. Table 1-1 lists a few ofthe major industries that use programmable controllers, as well as some oftheir typical applications.

Table 1-1. Typical programmable controller applications.

TYPICAL AREAS OF PLC APPLICATIONS

C LACIMEH P/ LACIMEHCORTE M GNIRUTCAFUNA M/ GNINIHCAssecorphctaB senihcamylbmessA

gnildnahtcudorpdehsiniF gniroBgnildnahslairetaM senarC

gnixiM dnamedygrenEgnillirderohs-ffO gnidnirGlortnocenilepiP gnidlomwolb/noitcejnI

tnemtaertetsaw/retaW sroyevnoclairetaMgnitsaclateM

G SSAL F/ MLI gnilliMgnihgiewtelluC gnitniaP

gnihsiniF gnitalPgnimroF sdnatstseT

lortnocrheL ehtalrecarTgnigakcaP gnidleWgnissecorP

M SLATEF DOO B/ EGAREVE lortnocecanruftsalB

sroyevnocgnitalumuccA gnitsacsuounitnoCgnidnelB sllimgnilloR

gniwerB tipgnikaoSgnildnahreniatnoC

gnillitsiD M GNINIgnilliF sroyevnoclairetamkluB

gnimrofdaoL gnidaolnu/gnidaoLgnidaolnu/gnidaolgnimroflateM gnissecorperO

gnizitellaP tnemeganametsaw/retaWgnildnahtcudorPsroyevnocgnitroS P REWO

laveirter/egarotsesuoheraW lortnocrenruBgnihgieW gnildnahlaoC

gnissecorphtgnel-ot-tuCL REBMU P/ PLU P/ REPA lortnoceulF

sretsegidhctaB gniddehsdaoLgnildnahpihC gnitroS

gnitaoC gnissecorp/gnidniWgnipmats/gnipparW gnikrowdooW

SECTION1



CHAPTER1


Table 1-2. Examples of PLC applications.

Because the applications of programmable controllers are extensive, it isimpossible to list them all in this book. However, Table 1-2 provides a smallsample of how PLCs are being used in industry.

A EVITOMOTU.gnirotinoMenignEnoitsubmoClanretnI morfdedroceratadseriuqcaCLPA

edulcninekatstnemerusaeM.enignenoitsubmoclanretniehttadetacolsrosneslio,erutarepmettsuahxe,euqrot,sMPR,erutarepmetlio,erutarepmetretaw

.gnimitdna,erusserpdlofinam,erusserp

.gnitseTnoitcudorProterubraC evitomotuafosisylanaenil-noedivorpsCLPtsetehtecuderyltnacifingissmetsysehT.enilylbmessanoitcudorpanisroterubrac,muucav,erusserP.sroterubracytilauqrettebdnadleiyretaerggnidivorpelihw,emit

.detsetselbairavehtfoemoserawolfriadnaleufdna

.senihcaMnoitcudorPevitomotuAgnirotinoM ,straplatotsrotinommetsysehT.ycneiciffeenihcamdna,emitelcycenihcam,decudorpstrap,strapdetcejer

.tfihshcaeretfaroemitynarotarepoehtotelbaliavasiatadlacitsitatS

.gnitseTdnaylbmessAevlaVgnireetSrewoP aslortnocmetsysCLPehTgninrutthgirdnatfelezimixamotdnasevlavehtfoecnalabreporperusneotenihcam

.soitar

C LACIMEH DNA P LACIMEHCORTE

.gnissecorPenelyhtEdnaainommA dnarotinomsrellortnocelbammargorPehT.gnirutcafunamenelyhtednaainommagniruddesusrosserpmocegrallortnocrosserpmoc,stekcopecnaraelcfonoitarepo,serutarepmetgniraebsrotinomCLPdna,erusserp,serutarepmetegrahcsid,noitarbiv,noitpmusnocrewop,deeps

.wolfnoitcus

.seyD yehT.yrtsudnielitxetehtnidesugnissecorpeydehtlortnocdnarotinomsCLP.seulavdenimretederpotsrolocdnelbdnahctam

.gnihctaBlacimehC slairetameromroowtfooitargnihctabehtslortnocCLPehThcaefoegrahcsidfoetarehtsenimretedmetsysehT.ssecorpsuounitnocanidnadeggolebnacsepicerhctablareveS.sdroceryrotnevnispeekdnalairetam

.rotarepoehtmorfdnammocnoroyllacitamotuadevierter

.lortnoCnaF lacimehcanisesagcixotfoslevelnodesabsnaflortnocsCLPlevelteserpanehwsesagsevomerylevitceffemetsyssihT.tnemnorivnenoitcudorp

dna,gnilcyc,pots/tratsnafehtslortnocCLPehT.dehcaersinoitanimatnocfo.deziminimsinoitpmusnocygreneelihwdeniatniameraslevelytefastahtos,sdeeps

.noitubirtsiDdnanoissimsnarTsaG dnarotinomsrellortnocelbammargorPsiataD.smetsysnoitubirtsiddnanoissimsnartsagfoswolfdnaserusserpetaluger

.metsysCLPehtotdettimsnartdnadleifehtniderusaemdnaderehtag

.lortnoCnoitatSpmuPenilepiP rofspmupretsoobdnaenilniamlortnocsCLPhgih/wolknatdna,egrahcsid,noitcus,wolferusaemyehT.noitubirtsidlioedurcataDdnalortnoCyrosivrepuS(ADACShtiwnoitacinummocelbissoP.stimil

.enilepipehtfonoisivrepuslatotedivorpnacsmetsys)noitsiuqcA

.sdleiFliO ottnenitrepatadfognissecorpdnagnirehtagetis-noedivorpsCLPdnaslortnocCLPehT.sgirgnillirdfoytisneddnahtpedsahcusscitsiretcarahc.snoitcnuflamelbissopynaforotarepoehtstreladnanoitarepogirlatotehtsrotinom

19

CHAPTER1



SECTION1


Table 1-2 continued.

G SSAL P GNISSECOR.lortnoCrheLgnilaennA ssertslanretniehtevomerotdesurhelehtlortnocsCLP

gnilaennaehtgniwollofybnoitarepoehtslortnocmetsysehT.stcudorpssalgmorfgniloocdipardna,gniniarts,gnilaenna,gnitaeherehtgnirudevrucerutarepmetniedamerastnemevorpmI.senozgniloocdnagnitaehtnereffidhguorhtsessecorp

.noitazilituygrenedna,tsocrobalninoitcuder,parcsotssalgdoogfooitareht

.gnihctaBssalG ssalgderotsotgnidroccametsysgnihgiewhctabehtlortnocsCLPdnaotdeefnirofsredeefcitengamortceleehtslortnocoslametsysehT.salumrof

.tnempiuqerehtodna,setagffo-tuhslaunam,sreppohhgiewehtmorfdeeftuo

.gnihgieWtelluC tellucyrotarbivehtgnillortnocybmetsystellucehttceridsCLPdnanoitarepofosecneuqesllA.royevnocelttuhsdna,elacstleb-thgiew,redeef

.esuerutufrofCLPehtybtpekeradehgiewseititnauqfoyrotnevni

.tropsnarThctaB tlebelbisrevergnidulcni,metsystropsnarthctabehtlortnocsCLPelttuhs,sreppohgnidloh,esuohtellucehtotsroyevnocrefsnart,sroyevnocegrahcsidehtretfanoitcasekatrellortnocehT.srotarapescitengamdna,sroyevnocsitierehw,elttuhsecanrufehtothctabdeximehtsrefsnartdnareximehtmorf

.reppohdeefecanrufehtfohtgnelllufehtotdegrahcsid

M GNIRUTCAFUNA M/ GNINIHCA.senihcaMnoitcudorP noitcudorpcitamotuasrotinomdnaslortnocCLPehT

dnanoitcudorptnuoc-eceipsrotinomoslatI.setarycneiciffehgihtasenihcamastcetedCLPehtfiyletaidemminekatebnacnoitcaevitcerroC.sutatsenihcam

.eruliaf

.senihcaMeniLrefsnarT gninihcamenilrefsnartllalortnocdnarotinomsCLPseviecermetsysehT.noitatshcaeneewtebgnikcolretniehtdnasnoitareponoitatsdetnuom-enilehtnosnoitidnocgnitarepoehtkcehcotrotarepoehtmorfstupnienihcamretaergsedivorptnemegnarrasihT.snoitcnuflamynastroperdnaslortnoc

.slevelparcsrewoldna,stcudorpytilauqrehgih,ycneiciffe

.enihcaMeriW -eriwafoselcycFFO/NOdnaemitehtsrotinomrellortnocehTfonoitazinorhcnysdnalortnocgnipmarsedivorpmetsysehT.enihcamgniward

ehtniatbootdnamednodetroperdnadedrocereraselcycllA.sevirdrotomcirtcele.CLPehtybdetaluclacsaycneiciffes'enihcam

.gnignahClooT lareveshtiwenihcamgnittuclatemsuonorhcysaslortnocCLPehTdesab,decalperebdluohsloothcaenehwfokcartspeekmetsysehT.spuorglootstnemecalperdnatnuocehtsyalpsidoslatI.serutcafunamtistrapforebmunehtno

.spuorglootehtllafo

.gniyarpStniaP ehT.gnirutcafunamotuanisecneuqesgnitniapehtlortnocsCLPtrapehtskcartdnanoitamrofnirolocdnaelytssretneretupmoctsoharorotarepo

ehtsedocedrellortnocehT.htoobyarpsehtsehcaertilitnuroyevnocehthguorhtnugyarpsehT.trapehttniapotsnugyarpsehtslortnocnehtdnanoitamrofnitrap

.tuphguorhttrapesaercnidnatniapevresnocotdezimitpositnemevom

M SLAIRETA H GNILDNA.eniLgnitalPcitamotuA ,tsiohdetamotuaehtrofnrettaptesaslortnocCLPehT

.snoitulosgnitalpsuoiravehthguorhtnwoddna,pu,thgir,tfelesrevartnachcihw.semitllatasitsiohehterehwswonkmetsysehT

SECTION1



CHAPTER1



.smetsySlaveirteRdnaegarotS nimehtyrracdnastrapdaolotdesusiCLPAenalaekilnoitamrofniskcartrellortnocehT.metsyslaveirterdnaegarotsehtnisetotralucitrapanistrapfoytitnauqehtdna,senalcificepsotdengissastrapeht,srebmunrodedaolstrapfosutatsehtnisegnahcdiparswollatnemegnarraCLPsihT.enaldnastuotnirpyrotnevnisedivorposlarellortnocehT.metsysehtmorfdedaolnu

.snoitcnuflamynaforotarepoehtsmrofni

.smetsySroyevnoC ,smrala,snoitarepolaitneuqesehtfollaslortnocmetsysehTtI.royevnocenilniamanostrapetalucricdnadaolotyrassecencigolytefasdna

ezimitpootgnitrosenaleludehcsnacdnasenaltcerrocriehtotstcudorpstrososladeniatboebnacstcejerotstrapdoogfooitarehtgniliatedsdroceR.ytudrezitellap

.tfihshcaefodneehtta

.gnisuoheraWdetamotuA fotnemevomehtsezimitpodnaslortnocCLPehT,detamotuananistseuqerslairetamfodnuoranruthgihsedivorpdnasenarcgnikcatsesacdnasroyevnocelsiaslortnocoslaCLPehT.esuoherawlacitrev,ebuc-hgihserugiflortnocyrotnevnI.stnemeriuqerrewopnamecuderyltnacifingisotsrezitellap

.tseuqernodedivorpebnacdnadeniatniamera

M SLATE

.gnikaMleetS nilatemecudorpotsecanrufsetarepodnaslortnocCLPehTnegyxosetaluclacoslarellortnocehT.snoitacificepsteserphtiwecnadrocca

.stnemeriuqerrewopdna,snoitiddayolla,stnemeriuqer

.syollAfognidaolnUdnagnidaoL gnidaoldnagnihgiewetaruccahguorhTdna,eronori,laocfoytitnauqehtsrotinomdnaslortnocmetsyseht,secneuqesaotleetsehtfoecneuqesgnidaolnuehtlortnocoslanactI.detlemebotenotsemil

.racodeprot

.gnitsaCsuounitnoC -suounitnocehtoteldaltropsnartleetsnetlomehttceridsCLP.noitacifidilosrofdlomdelooc-retawaotnideruopsileetsehterehw,enihcamgnitsac

.gnilloRdloC dehsinifotnistcudorpdehsinifimesfonoisrevnocehtlortnocsCLPtcerrocniatbootdeepsrotomslortnocmetsysehT.sllimgnillor-dlochguorhtsdoog

.lairetamdellorehtfogniguagetauqedaedivorpdnanoisnet

.gnikaMmunimulA eraseitirupmihcihwni,ssecorpgniniferehtrotinomsrellortnoCeroehtseximdnasdnirgmetsysehT.slacimehcdnataehybetixuabmorfdevomererayehterehw,sreniatnocerusserpotnimehtspmupnehtdnaslacimehchtiw

.slacimehceromhtiwdenibmocdna,deretlif,detaeh

P REWO

.metsySrewoPtnalP noitubirtsidreporpehtsetalugerrellortnocelbammargorpehTesuohrewopsrotinomCLPeht,noitiddanI.maetsro,sag,yticirtceleelbaliavafoehT.stropernoitubirtsidsetarenegdna,ygrenefonoitubirtsidseludehcs,seitilicafdaolcitamotuaehtsallewsa,tnalpehtfonoitarepognirudsdaolehtslortnocCLP

.segatuorewopgnirudgnirotserrogniddehs

.tnemeganaMygrenE eht,serutarepmetedistuodnaedisnifognidaerehthguorhTmetsysCLPehT.tnalpgnirutcafunamanistinugniloocdnagnitaehslortnocCLPfokcartgnipeekdnaselcycdenimretederpgnirudmehtgnilcyc,sdaolehtslortnocsedivorpmetsysehT.emitelcycehtgnirudfforonoebdluohshcaegnolwoh.stinugniloocdnagnitaehehtybdesuygrenefotnuomaehtnostroperdeludehcs

21

CHAPTER1



SECTION1



.gnissecorPnoitazidiulFlaoC siygrenehcumwohsrotinomrellortnocehTgniximdnagnihsurclaocehtsetalugerdnalaocfotnuomanevigamorfdetarenegserutarepmet,setargninrubslortnocdnasrotinomCLPehT.enotsemildehsurchtiw

.sevlavtejfolortnocgolanadna,sevlavfognicneuqes,detareneg

.lortnoCycneiciffErosserpmoC lacipytatasrosserpmoclareveslortnocsCLPnwodtuhs/putrats,skcolretniytefasseldnahmetsysehT.noitatsrosserpmoctagninnursrosserpmocpeeksCLPehT.gnilcycrosserpmocdna,secneuqes

.srosserpmocehtfosevrucraenilnonehtgnisuycneiciffemumixam

P PLU DNA P REPA

.gnidnelBhctaBpluP tneidergni,noitarepoecneuqesslortnocCLPehTswollametsysehT.ssecorpgnidnelbehtrofegarotsepicerdna,tnemerusaemsedivorpdna,yrassecenfi,ytitnauqhcaefoseirtnehctabyfidomotsrotarepo

.desustneidergnifognitnuoccarofdnalortnocyrotnevnirofstuotnirpypocdrah

.gnissecorPgnikaM-repaProfnoitaraperPhctaB folortnocedulcnisnoitacilppAhcaerofsepiceR.gnirutcafunamrepaprofmetsysnoitaraperpkcotsetelpmocehtkcabdeeflortnocnacsCLP.seirtnerotarepoaivdetsujdadnadetceleseraknathctabehttA.slangistnemerusaemlevelknatnodesabnoitiddalacimehcrofcigolslairetamnostropertnemeganamsedivorpmetsysCLPeht,tfihshcaefonoitelpmoc

.esu

.retsegiDlliMrepaP doowmorfpluprepapgnikamfossecorpehtlortnocsCLPdnaytisnednodesabspihcfotnuomaehtslortnocdnasetaluclacmetsysehT.spihcdnadetaluclacsisrouqilgnikoocderiuqerfotnecrepeht,nehT.emulovretsegidgnikoocehtsdlohdnaspmarCLPehT.ecneuqesehtotdeddaerastnuomaeseht

.detelpmocsignikoocehtlitnuerutarepmet

.noitcudorPlliMrepaP dnathgiewsisabegarevaehtsetalugerrellortnocehT,sevlavwolfmaetsehtsetalupinammetsysehT.edargrepaprofelbairaverutsiom

.wolflatotslortnocdnasrotinomdna,thgiewetalugerotsevlavkcotsehtstsujdaR REBBU DNA P CITSAL

.gnirotinoMsserPgniruC-eriT rofgnirotinomsserplaudividnismrofrepCLPehTehtstrelametsysehT.elcycsserphcaegniruderutarepmetdna,erusserp,emitderotssisutatsenihcamgninrecnocnoitamrofnI.snoitcnuflamsserpynaforotarepoyrammusaedulcnitfihshcaerofstuotnirpnoitarenegtropeR.esuretalrofselbatni

.snoitcnuflamoteudemitnwodsserpdnaserucdoogfo

.gnirutcafunaMeriT smetsyseruc/sserperitrofdesuerasrellortnocelbammargorPehtroftiferitaotnieritwarasmrofsnarttahtstnevefognicneuqesehtlortnocotniatbootrebburehtgnirucdnanrettapdaertehtgnidlomsedulcnilortnocsihT.daorecapsehtsecuderyllaitnatsbusnoitacilppaCLPsihT.scitsiretcarahctnatsiser-daor

.tcudorpehtfoytilauqehtdnametsysehtfoytilibailersesaercnidnaderiuqer

.noitcudorPrebbuR dna,snoitcnufcigolrexim,lortnocelacsetaruccaedivorpsCLPnoitcudorpehtnidesutnemgipdna,lio,kcalbnobracfonoitarepoalumrofelpitlumnoitcudorpgnirudslootenihcamfonoitazilitusezimixammetsysehT.rebburfoderiuqerlennosrepdnaemitsecuderdna,seirotnevnissecorp-niskcart,seludehcs

.stroperdne-tfihsehtdnaytivitcanoitcudorpehtesivrepusot

.gnidloMnoitcejnIcitsalP erutarepmetsahcus,selbairavslortnocmetsysCLPAmetsysehT.ssecorpgnidlomnoitcejniehtezimitpootdesuerahcihw,erusserpdnaotdemmargorpebnacslevelyticolevlareveserehw,noitcejnipool-desolcsedivorp

.emitelcycnetrohsdna,stcefedecafrusecuder,gnilliftnetsisnocniatniam

SECTION1



CHAPTER1


Figure 1-13. PLC product ranges.

Micro PLCs are used in applications controlling up to 32 input and outputdevices, 20 or less I/O being the norm. The micros are followed by the smallPLC category, which controls 32 to 128 I/O. The medium (64 to 1024 I/O),large (512 to 4096 I/O), and very large (2048 to 8192 I/O) PLCs completethe segmentation. Figure 1-14 shows several PLCs that fall into thiscategory classification.

The A, B, and C overlapping areas in Figure 1-13 reflect enhancements, byadding options, of the standard features of the PLCs within a particularsegment. These options allow a product to be closely matched to theapplication without having to purchase the next larger unit. Chapter 20

A

B

C

1

2

3

4

5

Com

plex

ity a

nd C

ost

32 819240961024 204851212864

I/O Count

1-5 PLC PRODUCT APPLICATION RANGES

Figure 1-13 graphically illustrates programmable controller product ranges.This chart is not definitive, but for practical purposes, it is valid. The PLCmarket can be segmented into five groups:

1. micro PLCs

2. small PLCs

3. medium PLCs

4. large PLCs

5. very large PLCs

23

CHAPTER1



SECTION1


covers, in detail, the differences between PLCs in overlapping areas. Thesedifferences include I/O count, memory size, programming language, soft-ware functions, and other factors. An understanding of the PLC productranges and their characteristics will allow the user to properly identify thecontroller that will satisfy a particular application.

Figure 1-14. (a) Mitsubishis smallest print size PLC (14 I/O), (b) PLC Direct DL105 with 18I/O and a capacity of 6 amps per output channel, (c) Giddings & Lewis PIC90capable of handling 128 I/O with motion control capabilities, (d) Allen-BradleysPLC 5/15 (512 I/O), (e) Omrons C200H PLC (1392 I/O), and (f) Allen-BradleysPLC 5/80 (3072 I/O).

(a)

Cou

rtes

y of

Mits

ubis

hi E

lect

roni

cs, M

ount

Pro

spec

t, IL

Cou

rtes

y of

Gid

din

gs

& L

ewis

, Fon

d d

u La

c, W

I

(c)

Cou

rtes

y of

Om

ron

Ele

ctro

nics

, Sch

aum

bur

g, I

L

(e)

Cou

rtes

y of

PLC

Dire

ct, C

umm

ing

, GA

(b)

Cou

rtes

y of

Alle

n-B

rad

ley,

Hig

hlan

d H

eig

hts,

OH

(f)

Cou

rtes

y of

Alle

n-B

rad

ley,

Hig

hlan

d H

eig

hts,

OH

(d)

SECTION1



CHAPTER1


1-6 LADDER DIAGRAMS AND THE PLC

The ladder diagram has and continues to be the traditional way of represent-ing electrical sequences of operations. These diagrams represent the inter-connection of field devices in such a way that the activation, or turningON, of one device will turn ON another device according to a predeterminedsequence of events. Figure 1-15 illustrates a simple electrical ladder diagram.

The original ladder diagrams were established to represent hardwired logiccircuits used to control machines or equipment. Due to wide industry use,they became a standard way of communicating control information fromthe designers to the users of equipment. As programmable controllers wereintroduced, this type of circuit representation was also desirable because itwas easy to use and interpret and was widely accepted in industry.

Programmable controllers can implement all of the old ladder diagramconditions and much more. Their purpose is to perform these controloperations in a more reliable manner at a lower cost. A PLC implements, inits CPU, all of the old hardwired interconnections using its software instruc-tions. This is accomplished using familiar ladder diagrams in a manner thatis transparent to the engineer or programmer. As you will see throughout thisbook, a knowledge of PLC operation, scanning, and instruction programmingis vital to the proper implementation of a control system.

Figure 1-16 illustrates the PLC transformation of the simple diagram shownin Figure 1-15 to a PLC format. Note that the real I/O field devices areconnected to input and output interfaces, while the ladder program isimplemented in a manner, similar to hardwiring, inside the programmablecontroller (i.e., softwired inside the PLCs CPU instead of hardwired in apanel). As previously mentioned, the CPU reads the status of inputs, ener-gizes the corresponding circuit element according to the program, andcontrols a real output device via the output interfaces.

Figure 1-15. Simple electrical ladder diagram.

L1 L2

PLLS1

PB1

LS2

25

CHAPTER1



SECTION1


EXAMPLE 1-1

In the hardwired circuit shown in Figure 1-15, the pilot light PL will turnON if the limit switch LS1 closes and if either push button PB1 or limitswitch LS2 closes. In the PLC circuit, the same series of events willcause the pilot lightconnected to an output moduleto turn ON.Note that in the PLC circuit in Figure 1-16, the internal representationof contacts provides the equivalent power logic as a hardwired circuitwhen the referenced input field device closes or is pushed. Sketchhardwired and PLC implementation diagrams for the circuit in Figure1-15 illustrating the configurations of inputs that will turn PL ON.

SOLUTION

Figure 1-17 shows several possible configurations for the circuit inFigure 1-15. The highlighted blue lines indicate that power is presentat that connection point, which is also the way a programming ormonitoring device represents power in a PLC circuit. The last twoconfigurations in Figure 1-17 are the only ones that will turn PL ON.

As you will see later, each instruction is represented inside the PLC by areference address, an alphanumeric value by which each device is known inthe PLC program. For example, the push button PB1 is represented inside thePLC by the name PB1 (indicated on top of the instruction symbol) andlikewise for the other devices shown in Figure 1-16. These instructions arerepresented here, for simplicity, with the same device and instruction names.Chapters 3 and 5 further discuss basic addressing techniques, while Chapter6 covers input/output wiring connections. Example 1-1 illustrates the similar-ity in operation between hardwired and PLC circuits.

Figure 1-16. PLC implementation of Figure 1-15.

L1 L1 L2L2

PLPB1

LS1 LS2

PB1 LS1 PL

LS2

represents input module represents output module

SECTION1



CHAPTER1


Figure 1-17. Possible configurations of inputs and corresponding outputs.

1-7 ADVANTAGES OF PLCS

In general, PLC architecture is modular and flexible, allowing hardware andsoftware elements to expand as the application requirements change. In theevent that an application outgrows the limitations of the programmablecontroller, the unit can be easily replaced with a unit having greater memoryand I/O capacity, and the old hardware can be reused for a smaller application.A PLC system provides many benefits to control solutions, from reliabilityand repeatability to programmability. The benefits achieved with program-mable controllers will grow with the individual using themthe more youlearn about PLCs, the more you will be able to solve other control problems.

PB1 LS1 PL

LS2

PB1 PB1 LS1 PL PL

PL

PL

PL

PL

LS1

LS2

LS2

No EventTakes Place

PB1 is OpenLS1 is OpenLS2 is OpenPL is OFF

PB1 LS1 PL

LS2

PB1 PB1 LS1 PL

LS1

LS2

LS2

PB1 is ClosedLS1 is OpenLS2 is OpenPL is OFF

PB1 LS1 PL

LS2

PB1 PB1 LS1 PL

LS1

LS2

LS2

PB1 is ClosedLS1 is Open

LS2 is ClosedPL is OFF

PB1 LS1 PL

LS2

PB1 PB1 LS1 PL

LS1

LS2

LS2

PB1 is ClosedLS1 is ClosedLS2 is Open

PL is ON

PB1 LS1 PL

LS2

PB1 PB1 LS1 PL

LS1

LS2

LS2

PB1 is OpenLS1 is ClosedLS2 is Closed

PL is ON

Hardwired Description PLC

27

CHAPTER1



SECTION1


Table 1-3. Typical programmable controller features and benefits.

Without question, the programmable feature provides the single greatestbenefit for the use and installation of programmable controllers. Eliminatinghardwired control in favor of programmable control is the first step towardsachieving a flexible control system. Once installed, the control plan can bemanually or automatically altered to meet day-to-day control requirementswithout changing the field wiring. This easy alteration is possible since thereare no physical connections between the field input devices and outputdevices (see Figure 1-18), as in hardwired systems. The only connection isthrough the control program, which can be easily altered.

Table 1-3 lists some of the many features and benefits obtained with aprogrammable controller.

serutaeFtnerehnI stifeneB

stnenopmocetats-diloS ytilibailerhgiH

yromemelbammargorP segnahcseifilpmiSlortnocelbixelF

ezisllamS stnemeriuqerecapslaminiM

desab-rossecorporciM ytilibapacnoitacinummoCecnamrofrepfolevelrehgiH

stcudorpytilauqrehgiHytilibapaclanoitcnufitluM

sretnuoc/sremiterawtfoS erawdrahetanimilEsteserpdegnahcylisaE

syalerlortnocerawtfoS tsocgniriw/erawdrahecudeRstnemeriuqerecapsecudeR

erutcetihcraraludoM ytilibixelfnoitallatsnIdellatsniylisaE

tsocerawdrahsecudeRytilibadnapxE

secafretniO/IfoyteiraV secivedfoyteiravaslortnoClortnocdezimotsucsetanimilE

snoitatsO/IetomeR snurtiudnoc/eriwgnoletanimilE

srotacidnicitsongaiD emitgnitoohselbuortecudeRnoitareporeporplangiS

ecafretniO/IraludoM lenaplortnocfoecnaraeppataeNdeniatniamylisaE

deriwylisaE

stcennocsidO/IkciuQ gniriwgnibrutsidtuohtiwecivreS

selbairavmetsySatadyromemniderots

ecnanetniam/tnemeganamlufesUmroftropernituptuoebnaC

SECTION1



CHAPTER1


A typical example of the benefits of softwiring is a solenoid that is controlledby two limit switches connected in series (see Figure 1-19a). Changing thesolenoid operation by placing the two limit switches in parallel (see Figure 1-19b) or by adding a third switch to the existing circuit (see Figure 1-19c)would take less than one minute in a PLC. In most cases, this simple programchange can be made without shutting down the system. This same change toa hardwired system could take as much as thirty to sixty minutes of downtime,and even a half hour of downtime can mean a costly loss of production. Asimilar situation exists if there is a need to change a timer preset value or someother constant. A software timer in a PLC can be changed in as little as fiveseconds. A set of thumbwheel switches and a push button can be easilyconfigured to input new preset values to any number of software timers. Thetime savings benefit of altering software timers, as opposed to alteringseveral hardware timers, is obvious.

The hardware features of programmable controllers provide similarflexibility and cost savings. An intelligent CPU is capable of communicatingwith other intelligent devices. This capability allows the controller to beintegrated into local or plantwide control schemes. With such a control

Figure 1-18. Programmable controller I/O connection diagram showing no physicalconnections between the inputs and outputs.

OUTPUTS

Common For Inputs

L1 L2

IN0

1 2

3 4

5 6

7

OUT

0 1

2 3

4 5

POWERRUNOK

PROG-ECPU-E

L1

0

1

2

3

4

5

6

7

L2

L1

L2

Ground

PLCPower L1

AC PowerFor Outputs

PLCCommon L2

29

CHAPTER1



SECTION1


configuration, a PLC can send useful English messages regarding thecontrolled system to an intelligent display. On the other hand, a PLC canreceive supervisory information, such as production changes or schedulinginformation, from a host computer. A standard I/O system includes a varietyof digital, analog, and special interface modules, which allow sophisticatedcontrol without the use of expensive, customized interface electronics.

Figure 1-19. Example of hardwiring changes as opposed to softwiring changes.

LS1 LS2 SOL LS1 LS2 SOL

LS1 LS3 SOL LS1 LS3 SOL

LS1

LS2

SOL LS1 SOL

LS2

LS2 LS2

(a) SERIES

(b) PARALLEL

(c) Adding One LS In Series

HARDWIRED PLC

(a) Series

(b) Parallel

(c) Adding one LS in series

EASE OF INSTALLATION

Several attributes make PLC installation an easy, cost-effective project. Itsrelatively small size allows a PLC to be conveniently located in less than halfthe space required by an equivalent relay control panel (see Figure 1-20). Ona small-scale changeover from relays, a PLCs small, modular constructionallows it to be mounted in the same enclosure where the relays were located.Actual changeover can be made quickly by simply connecting the input/output devices to the prewired terminal strips.

SECTION1



CHAPTER1


In large installations, remote input/output stations are placed at optimumlocations (see Figure 1-21). A coaxial cable or a twisted pair of wires connectsthe remote station to the CPU. This configuration results in a considerablereduction in material and labor costs as compared to a hardwired system,which would involve running multiple wires and installing large conduits.The remote subsystem approach also means that various sections of a totalsystem can be completely prewired by an OEM or PLC vendor prior toreaching the installation site. This approach considerably reduces the timespent by an electrician during an on-site installation.

Figure 1-21. Remote I/O station installation.

Figure 1-20. Space-efficient design of a PLC.

PLCMainPlantLocation

01234567

01234567

01234567

01234567

Sub-system

Remote Location

01234567

01234567

01234567

01234567

Sub-system

Remote Location

01234567

01234567

01234567

01234567

Sub-system

Remote Location

Wiring to manyI/O field devicesfrom I/O modules

Coaxial cable ortwisted pair ofwires used for

subsystemcommunication

Cou

rtes

y of

Om

ron

Ele

ctro

nics

, Sch

aum

bur

g, I

L

31

CHAPTER1



SECTION1


EASE OF MAINTENANCE AND TROUBLESHOOTING

Figure 1-23. Failures in a PLC-based system.

Figure 1-22. (a) A PLC processor and (b) an intelligent module containing severalstatus indicators.

Fielddevicefailures85%

I/O10%

CPU5%

From the beginning, programmable controllers have been designed with easeof maintenance in mind. With virtually all components being solid-state,maintenance is reduced to the replacement of modular, plug-in components.Fault detection circuits and diagnostic indicators (see Figure 1-22), incorpo-rated in each major component, signal whether the component is workingproperly or malfunctioning. In fact, most failures associated with a PLC-based system stem from failures directly related to the field input/outputdevices, rather than the PLCs CPU or I/O interface system (see Figure 1-23).However, the monitoring capability of a PLC system can easily detect andcorrect these field device failures.

Cou

rtes

y of

Alle

n-B

rad

ley,

Hig

hlan

d H

eig

hts,

OH

(b)

Cou

rtes

y of

Alle

n-B

rad

ley,

Hig

hlan

d H

eig

hts,

OH

(a)

SECTION1



CHAPTER1


Figure 1-24. A programming device being used to monitor inputs and outputs,with highlighted contacts indicating an ON condition.

KEYTERMS

With the aid of the programming device, any programmed logic can beviewed to see if inputs or outputs are ON or OFF (see Figure 1-24).Programmed instructions can also be written to enunciate certain failures.

These and several other attributes of the PLC make it a valuable part of anycontrol system. Once installed, its contribution will be quickly noticed andpayback will be readily realized. The potential benefits of the PLC, like anyintelligent device, will depend on the creativity with which it is applied.

It is obvious from the preceding discussion that the potential benefits ofapplying programmable controllers in an industrial application aresubstantial. The bottom line is that, through the use of programmablecontrollers, users will achieve high performance and reliability, resulting inhigher quality at a reduced cost.

addresscentral processing unit (CPU)executehardwareinput/output systeminterfaceladder diagramprogrammable logic controller (PLC)programming devicereadrelay logicscansoftwaresolid-statewrite

NUMBER SYSTEMSAND CODES

CHAPTERTWO

I have often admired the mystical ways ofPythagoras and the secret magic of numbers.

Sir Thomas Browne


34

SECTION1



Number Systemsand Codes

CHAPTER2

CHAPTERHIGHLIGHTS

In this chapter, we will explain the number systems and digital codes that aremost often used in programmable controller applications. We will firstintroduce the four number systems most frequently used during input/outputaddress assignment and programming: binary, octal, decimal, and hexadeci-mal. Then, we will discuss the binary coded decimal (BCD) and Gray codes,along with the ASCII character set and several PLC register formats. Sincethese codes and systems are the foundation of the logic behind PLCs, a basicknowledge of them will help you understand how PLCs work.

2-1 NUMBER SYSTEMS

A familiarity with number systems is quite useful when working withprogrammable controllers, since a basic function of these devices is torepresent, store, and operate on numbers, even when

plc programmable logic controllers

Documents