Programmable ControllersTheory and ImplementationSecond EditionL.A. BryanE.A. BryanTHEORY AND IMPLEMENTATIONPROGRAMMABLECONTROLLERSAn Industrial Text Company PublicationAtlanta Georgia USASecond EditionL. 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 America03020100999897 1098765432||||| ||||| ||||||| ||||| |||||||||| ||||| |||||||||| ||||| |||||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-TEXTLibrary 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-X1. Programmable controllers.I. Bryan, E.A. II. Title.TJ223.P76B7951997629.8'9dc21 96-49350CIPDue 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 Company1-800-752-8398www.industrialtext.comiiiContentsIndustrial Text & Video Company1-800-752-8398www.industrialtext.comCONTENTSPreface ..................................................................................................... ixAbout the Authors .................................................................................... xHow to Use this Book ............................................................................. xiSECTION 1 INTRODUCTORY CONCEPTSChapter 1 Introduction to Programmable Controllers1-1 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 ............................................................................... 26Chapter 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 .......................................................................... 50Chapter 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 ......................................... 68SECTION 2 COMPONENTS AND SYSTEMSChapter 4 Processors, the Power Supply, and Programming Devices4-1 Introduction ............................................................................................ 824-2 Processors ............................................................................................... 844-3 Processor Scan........................................................................................ 864-4 Error Checking and Diagnostics ............................................................ 924-5 The System Power Supply ..................................................................... 984-6 Programming Devices .......................................................................... 104Chapter 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 ........................................... 119ivContentsIndustrial Text & Video Company1-800-752-8398www.industrialtext.com5-5 Configuring the PLC MemoryI/O Addressing ................................. 1275-6 Summary of Memory, Scanning, and I/O Interaction .......................... 1325-7 Memory Considerations ....................................................................... 133Chapter 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 ............................................................. 1786-10 Summary of Discrete I/O ..................................................................... 182Chapter 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.............................................................. 2077-10 Analog Output Connections ................................................................. 2137-11 Analog Output Bypass/Control Stations .............................................. 214Chapter 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 ........................................................................... 260SECTION 3 PLC PROGRAMMINGChapter 9 Programming Languages9-1 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 ................................................................................ 308vContentsIndustrial Text & Video Company1-800-752-8398www.industrialtext.com9-8 Counter Instructions ............................................................................. 3129-9 Program/Flow Control Instructions ...................................................... 3179-10 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 ............................................................................. 369Chapter 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 .............................................................................................. 439Chapter 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 ............................................................. 521Chapter 12 PLC System Documentation12-1 Introduction to Documentation ............................................................ 53612-2 Steps for Documentation ...................................................................... 53712-3 PLC Documentation Systems............................................................... 54712-4 Conclusion............................................................................................ 549SECTION 4 PLC PROCESS APPLICATIONSChapter 13 Data Measurements and Transducers13-1 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 .............................................................................................. 608Chapter 14 Process Responses and Transfer Functions14-1 Process Control Basics ......................................................................... 61014-2 Control System Parameters .................................................................. 614viContentsIndustrial Text & Video Company1-800-752-8398www.industrialtext.com14-3 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 .............................................................................................. 665Chapter 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) .................................. 72915-10 Proportional-Integral-Derivative Controllers (PID Mode) .................. 73615-11 Advanced Control Systems .................................................................. 74415-12 Controller Loop Tuning ....................................................................... 74715-13 Summary .............................................................................................. 766SECTION 5 ADVANCED PLC TOPICS AND NETWORKSChapter 16 Artificial Intelligence and PLC Systems16-1 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......................................................... 788Chapter 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 ........................................................... 835Chapter 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 ................................................ 862viiContentsIndustrial Text & Video Company1-800-752-8398www.industrialtext.com18-7 Network Protocols ................................................................................ 86618-8 Network Testing and Troubleshooting................................................. 87418-9 Network Comparison and Selection Criteria ....................................... 875Chapter 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 ............................................................ 916SECTION 6 INSTALLATION AND START-UPChapter 20 PLC Start-Up and Maintenance20-1 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 ........................................................ 954Chapter 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 .............................................................................................. 982APPENDICESAppendix 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 ................. 999Glossary.............................................................................................. 1001Index ................................................................................................... 1025This page intentionally left blank.ixPrefaceIndustrial Text & Video Company1-800-752-8398www.industrialtext.comPREFACESince 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 industrythatdoesnotuseprogrammablecontrollersinoneformoranother.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 thebookwithfeaturesliketwo-tonegraphics,keytermslists,well-definedheadingsandsections,callouticons,andarevised,expandedglossarymakes the information presented even easier to understand.This new edition has been a labor-intensive learning experience for all thoseinvolved.Aswithanytasksolarge,wecouldneverhavedoneitalone.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,HoffmanEngineeringCompany,HoneywellMicroSwitchDivision,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 PhilippoEditorxAbout the AuthorsIndustrial Text & Video Company1-800-752-8398www.industrialtext.comABOUT THE AUTHORSLUI SBRYANLuis 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,andcomputerengineering.Duringhisgraduatestudies,Luiswasin-volvedinseveralprojectswithnationalandinternationalgovernmentalagencies.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,includingtheNationalAeronauticsandSpaceAdministration(NASA).Luis is an active member of several professional organizations, including theInstituteofElectricalandElectronicsEngineers(IEEE)andtheIEEEsinstrument and computer societies. He is a senior member of the InstrumentSociety of America, as well as a member of Phi Kappa Phi honor society andEtaKappaNuelectricalengineeringhonorsociety.Luishascoauthoredseveral other books about programmable controllers.ERI CBRYANEric Bryan graduated from the University of Tennessee with a Bachelor ofScience in Electrical Engineering degree, concentrating in digital design andcomputerarchitecture.HereceivedaMasterofScienceinEngineeringdegree from the Georgia Institute of Technology, where he participated in aspecial computer-integrated manufacturing (CIM) program. Erics special-tiesareindustrialautomationmethods,flexiblemanufacturingsystems(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.xiHow to Use this BookIndustrial Text & Video Company1-800-752-8398www.industrialtext.comHOW TO USE THI S 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.BEFOREYOUBEGI NREADI NG 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 HighlightsKey Terms Look at the appendices to see what reference materials have been provided.ASYOUSTUDYEACHCHAPTER 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 beforelookingatthesolution.Thisway,you'lldeterminewhichtopicsyouunderstand and which topics you should study further.WHENYOUFI NI SHEACHCHAPTER 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.I NTRODUCTORYCONCEPTSSECTI ONONE Introduction to Programmable Controllers Number Systems and Codes Logic ConceptsIndustrial Text and Video Company1-800-752-8398www.industrialtext.comThis page intentionally left blank.Industrial Text and Video Company1-800-752-8398www.industrialtext.comI NTRODUCTI ON TOPROGRAMMABLE CONTROLLERSCHAPTERONEI find the great thing in this world is not somuch where we stand as in what direction weare moving.Oliver Wendell HolmesIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConcepts4 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersFigure 1-1. PLC conceptual application diagram.CHAPTERHI GHLI GHTSProgrammablecontrollershavemanydefinitions.However,PLCscanbethought 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 tofoodpackagingusesprogrammablecontrollerstoexpandandenhanceproduction. 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-mablecontrollerstoothertypesofcontrolstohighlightthebenefitsanddrawbacksofeach,aswellaspinpointsituationswherePLCsworkbest.Whenyoufinishthischapter,youwillunderstandthefundamentalsofprogrammablecontrollersandbereadytoexplorethenumbersystemsassociated with them.1-1DEFI NI TI ONProgrammable logic controllers, also called programmable controllers orPLCs,aresolid-statemembersofthecomputerfamily,usingintegratedcircuits instead of electromechanical devices to implement control functions.Theyarecapableofstoringinstructions,suchassequencing,timing,counting,arithmetic,datamanipulation,andcommunication,tocontrolindustrialmachinesandprocesses.Figure1-1illustratesaconceptualdiagram of a PLC application.ProgrammableControllerFieldInputsFieldOutputsMeasure ControlProcessorMachine5CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsThe Hydramatic Division of the General Motors Corporation specified thedesign criteria for the first programmable controller in 1968. Their primarygoalwastoeliminatethehighcostsassociatedwithinflexible,relay-controlledsystems.Thespecificationsrequiredasolid-statesystemwithcomputer 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 andprovideexpandabilityforthefuture.Someoftheinitialspecificationsincluded 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.TheproductimplementationtosatisfyHydramaticsspecificationswasunderway 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.ThefirstPLCsofferedrelayfunctionality,thusreplacingtheoriginalhardwired relay logic, which used electrically operated devices to mechani-callyswitchelectricalcircuits.Theymettherequirementsofmodularity,expandability, programmability, and ease of use in an industrial environment.These controllers were easily installed, used less space, and were reusable.Thecontrollerprogramming,althoughalittletedious,hadarecognizableplant standard: the ladder diagram format.1-2AHI STORI CALBACKGROUNDTHEFI RSTPROGRAMMABLECONTROLLERSECTION1IntroductoryConcepts6 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersInashortperiod,programmablecontrollerusestartedtospreadtootherindustries. 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.THECONCEPTUALDESI GNOFTHEPLCThe first programmable controllers were more or less just relay replacers.Their primary function was to perform the sequential operations that werepreviouslyimplementedwithrelays.TheseoperationsincludedON/OFFcontrol of machines and processes that required repetitive operations, such astransferlinesandgrindingandboringmachines.However,theseprogrammable controllers were a vast improvement over relays. They wereeasilyinstalled,usedconsiderablylessspaceandenergy,haddiagnosticindicators that aided troubleshooting, and unlike relays, were reusable if aproject was scrapped.Programmablecontrollerscanbeconsiderednewcomerswhentheyarecomparedtotheirelderpredecessorsintraditionalcontrolequipmenttechnology, 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, haveimprovedthroughouttheyears,theirspecificationsstillholdtothedesigners original intentionsthey are simple to use and maintain.TODAYS PROGRAMMABLE CONTROLLERSManytechnologicaladvancesintheprogrammablecontrollerindustrycontinuetoday.Theseadvancesnotonlyaffectprogrammablecontrollerdesign, but also the philosophical approach to control system architecture.Changes include both hardware (physical components) and software (con-trolprogram)upgrades.ThefollowinglistdescribessomerecentPLChardware enhancements: Fasterscantimesarebeingachievedusingnew,advancedmicro-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-densityinput/output(I/O)systems(seeFigure1-3)providespace-efficient interfaces at low cost. Intelligent, microprocessor-based I/O interfaces have expanded dis-tributedprocessing.TypicalinterfacesincludePID(proportional-7CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsintegral-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. Specialinterfaceshaveallowedcertaindevicestobeconnecteddirectly to the controller. Typical interfaces include thermocouples,strain gauges, and fast-response inputs. Peripheralequipmenthasimprovedoperatorinterfacetechniques,and system documentation is now a standard part of the system.Figure 1-3. PLC systemwithhigh-densityI/O(64-point modules).Figure 1-2. Small PLC with built-inI/Oanddetachable,handheldprogramming unit.Allofthesehardwareenhancementshaveledtothedevelopmentofprogrammable controller families like the one shown in Figure 1-4. Thesefamiliesconsistofaproductlinethatrangesfromverysmallmicrocontrollers,withasfewas10I/Opoints,toverylargeandCourtesy of Mitsubishi Electronics, Mount Prospect, ILCourtesy of Mitsubishi Electronics, Mount Prospect, ILSECTION1IntroductoryConcepts8 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable Controllerssophisticated PLCs, with as many as 8,000 I/O points and 128,000 words ofmemory.Thesefamilymembers,usingcommonI/Osystemsandprogramming 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: PLCshaveincorporatedobject-orientedprogrammingtoolsandmultiple 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. Diagnosticsandfaultdetectionhavebeenexpandedfromsimplesystemdiagnostics,whichdiagnosecontrollermalfunctions,toincludemachinediagnostics,whichdiagnosefailuresormalfunctions 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.Courtesy of Allen-Bradley, Highland, Heights, OH9CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConcepts Data handling and manipulation instructions have been improved andsimplifiedtoaccommodatecomplexcontrolanddataacquisitionapplicationsthatinvolvestorage,tracking,andretrievaloflargeamounts of data.Programmablecontrollersarenowmaturecontrolsystemsofferingmanymorecapabilitiesthanwereeveranticipated.Theyarecapableofcommunicatingwithothercontrolsystems,providingproductionreports,scheduling production, and diagnosing their own failures and those of themachineorprocess.Theseenhancementshavemadeprogrammablecontrollersimportantcontributorsinmeetingtodaysdemandsforhigherquality 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.PROGRAMMABLECONTROLLERSANDTHEFUTUREThe 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/machineinterfaces(suchasvoicemodules).Theyalsoincludethedevelopmentofinterfacesthatallowcommunicationwithequipment,hardware,andsoftwarethatsupportsartificialintelligence,suchasfuzzylogic I/O systems.Softwareadvancesprovidebetterconnectionsbetweendifferenttypesofequipment, 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 certainlycontinuetocastprogrammablecontrollersasanimportantplayerinthefactory. 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.SECTION1IntroductoryConcepts10 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersFigure 1-5. Programmable controller block diagram.Figure 1-6. Block diagram of major CPU components.ProcessorPowerSupplyMemoryThe 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 supplyOUTPUTSINPUTSCentralProcessingUnit1-3PRI NCI PLES OFOPERATI ONA programmable controller, as illustrated in Figure 1-5, consists of two basicsections: the central processing unit the input/output interface system11CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsFigure 1-7. Illustration of a scan.The operation of a programmable controller is relatively simple. The input/output(I/O)systemisphysicallyconnectedtothefielddevicesthatareencountered 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).Duringitsoperation,theCPUcompletesthreeprocesses:(1)itreads,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,andupdatingtheoutputsisknownasscanning.Figure1-7illustratesagraphic representation of a scan.Theinput/outputsystemformstheinterfacebywhichfielddevicesareconnected to the controller (see Figure 1-8). The main purpose of the interfaceistoconditionthevarioussignalsreceivedfromorsenttoexternalfielddevices. 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 providesallthevoltagesrequiredfortheproperoperationofthevariouscentralprocessing unit sections.(1)(2)(3)SCANREADEXECUTEWRITESECTION1IntroductoryConcepts12 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersFigure 1-9. (a) Personal computer used as a programming device and (b) a mini-programmer unit.Chapters4and5willpresentamoredetaileddiscussionofthecentralprocessing 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.0123456701234567012345670123456701234567012345670 1 2 30123456701234567ProcessorandPower SupplyI/O Interfaces0123456701234567INPUTMODULEOUTPUTMODULE(a)Courtesy of Omron Electronics, Schaumburg, ILCourtesy of Mitsubishi Electronics, Mount Prospect, IL(b)13CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConcepts1-4PLCSVERSUSOTHERTYPESOFCONTROLSPLCSVERSUSRELAYCONTROLForyears,thequestionmanyengineers,plantmanagers,andoriginalequipmentmanufacturers(OEMs)askedwas,ShouldIbeusingaprogrammable controller? At one time, much of a systems engineers timewasspenttryingtodeterminethecost-effectivenessofaPLCoverrelaycontrol. 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 withoutelectroniccontrolequipment.Withrapidtechnologydevelopmentsandincreasing 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.WhendecidingwhethertouseaPLC-basedsystemorahardwiredrelaysystem, 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 inwhichtherequirementslistedaboveareparticularlyimportantfortheeconomic viability of the machine or process operation. A case which speaksforitself,thesystemshowninFigure1-10,showswhyprogrammablecontrollers 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.SECTION1IntroductoryConcepts14 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersIf 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 underelectromechanicalcontroltoincreaseconsiderably.Also,althoughrelaycontrol may cost less initially, this advantage is lost if production downtimedue to failures is high.PLCSVERSUSCOMPUTERCONTROLSFigure 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 placedinanareawithsubstantialamountsofelectricalnoise,electromagneticinterference, mechanical vibration, and noncondensing humidity.Courtesy of Omron Electronics, Schaumburg, IL15CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsA second distinction of PLCs is that their hardware and software are designedfor easy use by plant electricians and technicians. The hardware interfaces forconnectingfielddevicesareactuallypartofthePLCitselfandareeasilyconnected.Themodularandself-diagnosinginterfacecircuitsareabletopinpoint 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 becomemoreintelligent.ComplexPLCsystemsnowprovidemultiprocessorandmultitasking 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.PLCSVERSUSPERSONALCOMPUTERSWith 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.Whilenew,rugged,industrialpersonalcomputerscansometimessustainmidrange 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.Courtesy of Giddings & Lewis, Fond du Lac, WISECTION1IntroductoryConcepts16 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersThe personal computer is, however, being used as the programming device ofchoice for PLCs in the market, where PLC manufacturers and third-partyPLCsupportdeveloperscomeupwithprogramminganddocumentationsystemsfortheirPLCproductlines.PersonalcomputersarealsobeingemployedtogatherprocessdatafromPLCsandtodisplayinformationabouttheprocessormachine(i.e.,theyarebeingusedasgraphicuserinterfaces,orGUIs).Becauseoftheirnumber-crunchingcapabilities,personalcomputersarealsowellsuitedtocomplementprogrammablecontrollers 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.Somecontrolsoftwaremanufacturers,however,utilizePCsasCPUhardware to implement a PLC-like environment. The language they use isbasedontheInternationalElectrotechnicalCommission(IEC)1131-3standard,whichisagraphicrepresentationlanguage(sequentialfunctioncharts) 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.PLCPersonalComputerMainComputerSystem17CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsSinceitsinception,thePLChasbeensuccessfullyappliedinvirtuallyevery segment of industry, including steel mills, paper plants, food-process-ing plants, chemical plants, and power plants. PLCs perform a great varietyofcontroltasks,fromrepetitiveON/OFFcontrolofsimplemachinestosophisticated 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.TYPI CALAREASOFPLCAPPLI CATI ONSC L A C I M E H P / L A C I M E H C O R T E M G N I R U T C A F U N A M / G N I N I H C As s e c o r p h c t a B s e n i h c a m y l b m e s s Ag n i l d n a h t c u d o r p d e h s i n i F g n i r o Bg n i l d n a h s l a i r e t a M s e n a r Cg n i x i M d n a m e d y g r e n Eg n i l l i r d e r o h s - f f O g n i d n i r Gl o r t n o c e n i l e p i P g n i d l o m w o l b / n o i t c e j n It n e m t a e r t e t s a w / r e t a W s r o y e v n o c l a i r e t a Mg n i t s a c l a t e MG S S A L F / M L I g n i l l i Mg n i h g i e w t e l l u C g n i t n i a Pg n i h s i n i F g n i t a l Pg n i m r o F s d n a t s t s e Tl o r t n o c r h e L e h t a l r e c a r Tg n i g a k c a P g n i d l e Wg n i s s e c o r PM S L A T EF D O O B / E G A R E V E l o r t n o c e c a n r u f t s a l Bs r o y e v n o c g n i t a l u m u c c A g n i t s a c s u o u n i t n o Cg n i d n e l B s l l i m g n i l l o Rg n i w e r B t i p g n i k a o Sg n i l d n a h r e n i a t n o Cg n i l l i t s i D M G N I N Ig n i l l i F s r o y e v n o c l a i r e t a m k l u Bg n i m r o f d a o L g n i d a o l n u / g n i d a o Lg n i d a o l n u / g n i d a o l g n i m r o f l a t e M g n i s s e c o r p e r Og n i z i t e l l a P t n e m e g a n a m e t s a w / r e t a Wg n i l d n a h t c u d o r Ps r o y e v n o c g n i t r o S P R E W Ol a v e i r t e r / e g a r o t s e s u o h e r a W l o r t n o c r e n r u Bg n i h g i e W g n i l d n a h l a o Cg n i s s e c o r p h t g n e l - o t - t u CL R E B M U P / P L U P / R E P A l o r t n o c e u l Fs r e t s e g i d h c t a B g n i d d e h s d a o Lg n i l d n a h p i h C g n i t r o Sg n i t a o C g n i s s e c o r p / g n i d n i Wg n i p m a t s / g n i p p a r W g n i k r o w d o o WSECTION1IntroductoryConcepts18 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersTable 1-2. Examples of PLC applications.Becausetheapplicationsofprogrammablecontrollersareextensive,itisimpossible to list them all in this book. However, Table 1-2 provides a smallsample of how PLCs are being used in industry.A E V I T O M O T U. g n i r o t i n o M e n i g n E n o i t s u b m o C l a n r e t n I m o r f d e d r o c e r a t a d s e r i u q c a C L P Ae d u l c n i n e k a t s t n e m e r u s a e M . e n i g n e n o i t s u b m o c l a n r e t n i e h t t a d e t a c o l s r o s n e sl i o , e r u t a r e p m e t t s u a h x e , e u q r o t , s M P R , e r u t a r e p m e t l i o , e r u t a r e p m e t r e t a w. g n i m i t d n a , e r u s s e r p d l o f i n a m , e r u s s e r p. g n i t s e T n o i t c u d o r P r o t e r u b r a C e v i t o m o t u a f o s i s y l a n a e n i l - n o e d i v o r p s C L Pt s e t e h t e c u d e r y l t n a c i f i n g i s s m e t s y s e h T . e n i l y l b m e s s a n o i t c u d o r p a n i s r o t e r u b r a c, m u u c a v , e r u s s e r P . s r o t e r u b r a c y t i l a u q r e t t e b d n a d l e i y r e t a e r g g n i d i v o r p e l i h w , e m i t. d e t s e t s e l b a i r a v e h t f o e m o s e r a w o l f r i a d n a l e u f d n a. s e n i h c a M n o i t c u d o r P e v i t o m o t u A g n i r o t i n o M , s t r a p l a t o t s r o t i n o m m e t s y s e h T. y c n e i c i f f e e n i h c a m d n a , e m i t e l c y c e n i h c a m , d e c u d o r p s t r a p , s t r a p d e t c e j e r. t f i h s h c a e r e t f a r o e m i t y n a r o t a r e p o e h t o t e l b a l i a v a s i a t a d l a c i t s i t a t S. g n i t s e T d n a y l b m e s s A e v l a V g n i r e e t S r e w o P a s l o r t n o c m e t s y s C L P e h Tg n i n r u t t h g i r d n a t f e l e z i m i x a m o t d n a s e v l a v e h t f o e c n a l a b r e p o r p e r u s n e o t e n i h c a m. s o i t a rC L A C I M E H D N A P L A C I M E H C O R T E. g n i s s e c o r P e n e l y h t E d n a a i n o m m A d n a r o t i n o m s r e l l o r t n o c e l b a m m a r g o r Pe h T . g n i r u t c a f u n a m e n e l y h t e d n a a i n o m m a g n i r u d d e s u s r o s s e r p m o c e g r a l l o r t n o cr o s s e r p m o c , s t e k c o p e c n a r a e l c f o n o i t a r e p o , s e r u t a r e p m e t g n i r a e b s r o t i n o m C L Pd n a , e r u s s e r p , s e r u t a r e p m e t e g r a h c s i d , n o i t a r b i v , n o i t p m u s n o c r e w o p , d e e p s. w o l f n o i t c u s. s e y D y e h T . y r t s u d n i e l i t x e t e h t n i d e s u g n i s s e c o r p e y d e h t l o r t n o c d n a r o t i n o m s C L P. s e u l a v d e n i m r e t e d e r p o t s r o l o c d n e l b d n a h c t a m. g n i h c t a B l a c i m e h C s l a i r e t a m e r o m r o o w t f o o i t a r g n i h c t a b e h t s l o r t n o c C L P e h Th c a e f o e g r a h c s i d f o e t a r e h t s e n i m r e t e d m e t s y s e h T . s s e c o r p s u o u n i t n o c a n id n a d e g g o l e b n a c s e p i c e r h c t a b l a r e v e S . s d r o c e r y r o t n e v n i s p e e k d n a l a i r e t a m. r o t a r e p o e h t m o r f d n a m m o c n o r o y l l a c i t a m o t u a d e v i e r t e r. l o r t n o C n a F l a c i m e h c a n i s e s a g c i x o t f o s l e v e l n o d e s a b s n a f l o r t n o c s C L Pl e v e l t e s e r p a n e h w s e s a g s e v o m e r y l e v i t c e f f e m e t s y s s i h T . t n e m n o r i v n e n o i t c u d o r pd n a , g n i l c y c , p o t s / t r a t s n a f e h t s l o r t n o c C L P e h T . d e h c a e r s i n o i t a n i m a t n o c f o. d e z i m i n i m s i n o i t p m u s n o c y g r e n e e l i h w d e n i a t n i a m e r a s l e v e l y t e f a s t a h t o s , s d e e p s. n o i t u b i r t s i D d n a n o i s s i m s n a r T s a G d n a r o t i n o m s r e l l o r t n o c e l b a m m a r g o r Ps i a t a D . s m e t s y s n o i t u b i r t s i d d n a n o i s s i m s n a r t s a g f o s w o l f d n a s e r u s s e r p e t a l u g e r. m e t s y s C L P e h t o t d e t t i m s n a r t d n a d l e i f e h t n i d e r u s a e m d n a d e r e h t a g. l o r t n o C n o i t a t S p m u P e n i l e p i P r o f s p m u p r e t s o o b d n a e n i l n i a m l o r t n o c s C L Ph g i h / w o l k n a t d n a , e g r a h c s i d , n o i t c u s , w o l f e r u s a e m y e h T . n o i t u b i r t s i d l i o e d u r ca t a D d n a l o r t n o C y r o s i v r e p u S ( A D A C S h t i w n o i t a c i n u m m o c e l b i s s o P . s t i m i l. e n i l e p i p e h t f o n o i s i v r e p u s l a t o t e d i v o r p n a c s m e t s y s ) n o i t s i u q c A. s d l e i F l i O o t t n e n i t r e p a t a d f o g n i s s e c o r p d n a g n i r e h t a g e t i s - n o e d i v o r p s C L Pd n a s l o r t n o c C L P e h T . s g i r g n i l l i r d f o y t i s n e d d n a h t p e d s a h c u s s c i t s i r e t c a r a h c. s n o i t c n u f l a m e l b i s s o p y n a f o r o t a r e p o e h t s t r e l a d n a n o i t a r e p o g i r l a t o t e h t s r o t i n o m19CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsTable 1-2 continued.G S S A L P G N I S S E C O R. l o r t n o C r h e L g n i l a e n n A s s e r t s l a n r e t n i e h t e v o m e r o t d e s u r h e l e h t l o r t n o c s C L Pg n i l a e n n a e h t g n i w o l l o f y b n o i t a r e p o e h t s l o r t n o c m e t s y s e h T . s t c u d o r p s s a l g m o r fg n i l o o c d i p a r d n a , g n i n i a r t s , g n i l a e n n a , g n i t a e h e r e h t g n i r u d e v r u c e r u t a r e p m e tn i e d a m e r a s t n e m e v o r p m I . s e n o z g n i l o o c d n a g n i t a e h t n e r e f f i d h g u o r h t s e s s e c o r p. n o i t a z i l i t u y g r e n e d n a , t s o c r o b a l n i n o i t c u d e r , p a r c s o t s s a l g d o o g f o o i t a r e h t. g n i h c t a B s s a l G s s a l g d e r o t s o t g n i d r o c c a m e t s y s g n i h g i e w h c t a b e h t l o r t n o c s C L Pd n a o t d e e f n i r o f s r e d e e f c i t e n g a m o r t c e l e e h t s l o r t n o c o s l a m e t s y s e h T . s a l u m r o f. t n e m p i u q e r e h t o d n a , s e t a g f f o - t u h s l a u n a m , s r e p p o h h g i e w e h t m o r f d e e f t u o. g n i h g i e W t e l l u C t e l l u c y r o t a r b i v e h t g n i l l o r t n o c y b m e t s y s t e l l u c e h t t c e r i d s C L Pd n a n o i t a r e p o f o s e c n e u q e s l l A . r o y e v n o c e l t t u h s d n a , e l a c s t l e b - t h g i e w , r e d e e f. e s u e r u t u f r o f C L P e h t y b t p e k e r a d e h g i e w s e i t i t n a u q f o y r o t n e v n i. t r o p s n a r T h c t a B t l e b e l b i s r e v e r g n i d u l c n i , m e t s y s t r o p s n a r t h c t a b e h t l o r t n o c s C L Pe l t t u h s , s r e p p o h g n i d l o h , e s u o h t e l l u c e h t o t s r o y e v n o c r e f s n a r t , s r o y e v n o ce g r a h c s i d e h t r e t f a n o i t c a s e k a t r e l l o r t n o c e h T . s r o t a r a p e s c i t e n g a m d n a , s r o y e v n o cs i t i e r e h w , e l t t u h s e c a n r u f e h t o t h c t a b d e x i m e h t s r e f s n a r t d n a r e x i m e h t m o r f. r e p p o h d e e f e c a n r u f e h t f o h t g n e l l l u f e h t o t d e g r a h c s i dM G N I R U T C A F U N A M / G N I N I H C A. s e n i h c a M n o i t c u d o r P n o i t c u d o r p c i t a m o t u a s r o t i n o m d n a s l o r t n o c C L P e h Td n a n o i t c u d o r p t n u o c - e c e i p s r o t i n o m o s l a t I . s e t a r y c n e i c i f f e h g i h t a s e n i h c a ma s t c e t e d C L P e h t f i y l e t a i d e m m i n e k a t e b n a c n o i t c a e v i t c e r r o C . s u t a t s e n i h c a m. e r u l i a f. s e n i h c a M e n i L r e f s n a r T g n i n i h c a m e n i l r e f s n a r t l l a l o r t n o c d n a r o t i n o m s C L Ps e v i e c e r m e t s y s e h T . n o i t a t s h c a e n e e w t e b g n i k c o l r e t n i e h t d n a s n o i t a r e p o n o i t a t sd e t n u o m - e n i l e h t n o s n o i t i d n o c g n i t a r e p o e h t k c e h c o t r o t a r e p o e h t m o r f s t u p n ie n i h c a m r e t a e r g s e d i v o r p t n e m e g n a r r a s i h T . s n o i t c n u f l a m y n a s t r o p e r d n a s l o r t n o c. s l e v e l p a r c s r e w o l d n a , s t c u d o r p y t i l a u q r e h g i h , y c n e i c i f f e. e n i h c a M e r i W - e r i w a f o s e l c y c F F O / N O d n a e m i t e h t s r o t i n o m r e l l o r t n o c e h Tf o n o i t a z i n o r h c n y s d n a l o r t n o c g n i p m a r s e d i v o r p m e t s y s e h T . e n i h c a m g n i w a r de h t n i a t b o o t d n a m e d n o d e t r o p e r d n a d e d r o c e r e r a s e l c y c l l A . s e v i r d r o t o m c i r t c e l e. C L P e h t y b d e t a l u c l a c s a y c n e i c i f f e s ' e n i h c a m. g n i g n a h C l o o T l a r e v e s h t i w e n i h c a m g n i t t u c l a t e m s u o n o r h c y s a s l o r t n o c C L P e h Td e s a b , d e c a l p e r e b d l u o h s l o o t h c a e n e h w f o k c a r t s p e e k m e t s y s e h T . s p u o r g l o o ts t n e m e c a l p e r d n a t n u o c e h t s y a l p s i d o s l a t I . s e r u t c a f u n a m t i s t r a p f o r e b m u n e h t n o. s p u o r g l o o t e h t l l a f o. g n i y a r p S t n i a P e h T . g n i r u t c a f u n a m o t u a n i s e c n e u q e s g n i t n i a p e h t l o r t n o c s C L Pt r a p e h t s k c a r t d n a n o i t a m r o f n i r o l o c d n a e l y t s s r e t n e r e t u p m o c t s o h a r o r o t a r e p oe h t s e d o c e d r e l l o r t n o c e h T . h t o o b y a r p s e h t s e h c a e r t i l i t n u r o y e v n o c e h t h g u o r h tn u g y a r p s e h T . t r a p e h t t n i a p o t s n u g y a r p s e h t s l o r t n o c n e h t d n a n o i t a m r o f n i t r a p. t u p h g u o r h t t r a p e s a e r c n i d n a t n i a p e v r e s n o c o t d e z i m i t p o s i t n e m e v o mM S L A I R E T A H G N I L D N A. e n i L g n i t a l P c i t a m o t u A , t s i o h d e t a m o t u a e h t r o f n r e t t a p t e s a s l o r t n o c C L P e h T. s n o i t u l o s g n i t a l p s u o i r a v e h t h g u o r h t n w o d d n a , p u , t h g i r , t f e l e s r e v a r t n a c h c i h w. s e m i t l l a t a s i t s i o h e h t e r e h w s w o n k m e t s y s e h TSECTION1IntroductoryConcepts20 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersTable 1-2 continued.. s m e t s y S l a v e i r t e R d n a e g a r o t S n i m e h t y r r a c d n a s t r a p d a o l o t d e s u s i C L P Ae n a l a e k i l n o i t a m r o f n i s k c a r t r e l l o r t n o c e h T . m e t s y s l a v e i r t e r d n a e g a r o t s e h t n i s e t o tr a l u c i t r a p a n i s t r a p f o y t i t n a u q e h t d n a , s e n a l c i f i c e p s o t d e n g i s s a s t r a p e h t , s r e b m u nr o d e d a o l s t r a p f o s u t a t s e h t n i s e g n a h c d i p a r s w o l l a t n e m e g n a r r a C L P s i h T . e n a ld n a s t u o t n i r p y r o t n e v n i s e d i v o r p o s l a r e l l o r t n o c e h T . m e t s y s e h t m o r f d e d a o l n u. s n o i t c n u f l a m y n a f o r o t a r e p o e h t s m r o f n i. s m e t s y S r o y e v n o C , s m r a l a , s n o i t a r e p o l a i t n e u q e s e h t f o l l a s l o r t n o c m e t s y s e h Tt I . r o y e v n o c e n i l n i a m a n o s t r a p e t a l u c r i c d n a d a o l o t y r a s s e c e n c i g o l y t e f a s d n ae z i m i t p o o t g n i t r o s e n a l e l u d e h c s n a c d n a s e n a l t c e r r o c r i e h t o t s t c u d o r p s t r o s o s l ad e n i a t b o e b n a c s t c e j e r o t s t r a p d o o g f o o i t a r e h t g n i l i a t e d s d r o c e R . y t u d r e z i t e l l a p. t f i h s h c a e f o d n e e h t t a. g n i s u o h e r a W d e t a m o t u A f o t n e m e v o m e h t s e z i m i t p o d n a s l o r t n o c C L P e h T, d e t a m o t u a n a n i s t s e u q e r s l a i r e t a m f o d n u o r a n r u t h g i h s e d i v o r p d n a s e n a r c g n i k c a t se s a c d n a s r o y e v n o c e l s i a s l o r t n o c o s l a C L P e h T . e s u o h e r a w l a c i t r e v , e b u c - h g i hs e r u g i f l o r t n o c y r o t n e v n I . s t n e m e r i u q e r r e w o p n a m e c u d e r y l t n a c i f i n g i s o t s r e z i t e l l a p. t s e u q e r n o d e d i v o r p e b n a c d n a d e n i a t n i a m e r aM S L A T E. g n i k a M l e e t S n i l a t e m e c u d o r p o t s e c a n r u f s e t a r e p o d n a s l o r t n o c C L P e h Tn e g y x o s e t a l u c l a c o s l a r e l l o r t n o c e h T . s n o i t a c i f i c e p s t e s e r p h t i w e c n a d r o c c a. s t n e m e r i u q e r r e w o p d n a , s n o i t i d d a y o l l a , s t n e m e r i u q e r. s y o l l A f o g n i d a o l n U d n a g n i d a o L g n i d a o l d n a g n i h g i e w e t a r u c c a h g u o r h Td n a , e r o n o r i , l a o c f o y t i t n a u q e h t s r o t i n o m d n a s l o r t n o c m e t s y s e h t , s e c n e u q e sa o t l e e t s e h t f o e c n e u q e s g n i d a o l n u e h t l o r t n o c o s l a n a c t I . d e t l e m e b o t e n o t s e m i l. r a c o d e p r o t. g n i t s a C s u o u n i t n o C - s u o u n i t n o c e h t o t e l d a l t r o p s n a r t l e e t s n e t l o m e h t t c e r i d s C L P. n o i t a c i f i d i l o s r o f d l o m d e l o o c - r e t a w a o t n i d e r u o p s i l e e t s e h t e r e h w , e n i h c a m g n i t s a c. g n i l l o R d l o C d e h s i n i f o t n i s t c u d o r p d e h s i n i f i m e s f o n o i s r e v n o c e h t l o r t n o c s C L Pt c e r r o c n i a t b o o t d e e p s r o t o m s l o r t n o c m e t s y s e h T . s l l i m g n i l l o r - d l o c h g u o r h t s d o o g. l a i r e t a m d e l l o r e h t f o g n i g u a g e t a u q e d a e d i v o r p d n a n o i s n e t. g n i k a M m u n i m u l A e r a s e i t i r u p m i h c i h w n i , s s e c o r p g n i n i f e r e h t r o t i n o m s r e l l o r t n o Ce r o e h t s e x i m d n a s d n i r g m e t s y s e h T . s l a c i m e h c d n a t a e h y b e t i x u a b m o r f d e v o m e re r a y e h t e r e h w , s r e n i a t n o c e r u s s e r p o t n i m e h t s p m u p n e h t d n a s l a c i m e h c h t i w. s l a c i m e h c e r o m h t i w d e n i b m o c d n a , d e r e t l i f , d e t a e hP R E W O. m e t s y S r e w o P t n a l P n o i t u b i r t s i d r e p o r p e h t s e t a l u g e r r e l l o r t n o c e l b a m m a r g o r p e h Te s u o h r e w o p s r o t i n o m C L P e h t , n o i t i d d a n I . m a e t s r o , s a g , y t i c i r t c e l e e l b a l i a v a f oe h T . s t r o p e r n o i t u b i r t s i d s e t a r e n e g d n a , y g r e n e f o n o i t u b i r t s i d s e l u d e h c s , s e i t i l i c a fd a o l c i t a m o t u a e h t s a l l e w s a , t n a l p e h t f o n o i t a r e p o g n i r u d s d a o l e h t s l o r t n o c C L P. s e g a t u o r e w o p g n i r u d g n i r o t s e r r o g n i d d e h s. t n e m e g a n a M y g r e n E e h t , s e r u t a r e p m e t e d i s t u o d n a e d i s n i f o g n i d a e r e h t h g u o r h Tm e t s y s C L P e h T . t n a l p g n i r u t c a f u n a m a n i s t i n u g n i l o o c d n a g n i t a e h s l o r t n o c C L Pf o k c a r t g n i p e e k d n a s e l c y c d e n i m r e t e d e r p g n i r u d m e h t g n i l c y c , s d a o l e h t s l o r t n o cs e d i v o r p m e t s y s e h T . e m i t e l c y c e h t g n i r u d f f o r o n o e b d l u o h s h c a e g n o l w o h. s t i n u g n i l o o c d n a g n i t a e h e h t y b d e s u y g r e n e f o t n u o m a e h t n o s t r o p e r d e l u d e h c s21CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsTable 1-2 continued.. g n i s s e c o r P n o i t a z i d i u l F l a o C s i y g r e n e h c u m w o h s r o t i n o m r e l l o r t n o c e h Tg n i x i m d n a g n i h s u r c l a o c e h t s e t a l u g e r d n a l a o c f o t n u o m a n e v i g a m o r f d e t a r e n e gs e r u t a r e p m e t , s e t a r g n i n r u b s l o r t n o c d n a s r o t i n o m C L P e h T . e n o t s e m i l d e h s u r c h t i w. s e v l a v t e j f o l o r t n o c g o l a n a d n a , s e v l a v f o g n i c n e u q e s , d e t a r e n e g. l o r t n o C y c n e i c i f f E r o s s e r p m o C l a c i p y t a t a s r o s s e r p m o c l a r e v e s l o r t n o c s C L Pn w o d t u h s / p u t r a t s , s k c o l r e t n i y t e f a s s e l d n a h m e t s y s e h T . n o i t a t s r o s s e r p m o ct a g n i n n u r s r o s s e r p m o c p e e k s C L P e h T . g n i l c y c r o s s e r p m o c d n a , s e c n e u q e s. s r o s s e r p m o c e h t f o s e v r u c r a e n i l n o n e h t g n i s u y c n e i c i f f e m u m i x a mP P L U D N A P R E P A. g n i d n e l B h c t a B p l u P t n e i d e r g n i , n o i t a r e p o e c n e u q e s s l o r t n o c C L P e h Ts w o l l a m e t s y s e h T . s s e c o r p g n i d n e l b e h t r o f e g a r o t s e p i c e r d n a , t n e m e r u s a e ms e d i v o r p d n a , y r a s s e c e n f i , y t i t n a u q h c a e f o s e i r t n e h c t a b y f i d o m o t s r o t a r e p o. d e s u s t n e i d e r g n i f o g n i t n u o c c a r o f d n a l o r t n o c y r o t n e v n i r o f s t u o t n i r p y p o c d r a h. g n i s s e c o r P g n i k a M - r e p a P r o f n o i t a r a p e r P h c t a B f o l o r t n o c e d u l c n i s n o i t a c i l p p Ah c a e r o f s e p i c e R . g n i r u t c a f u n a m r e p a p r o f m e t s y s n o i t a r a p e r p k c o t s e t e l p m o c e h tk c a b d e e f l o r t n o c n a c s C L P . s e i r t n e r o t a r e p o a i v d e t s u j d a d n a d e t c e l e s e r a k n a t h c t a be h t t A . s l a n g i s t n e m e r u s a e m l e v e l k n a t n o d e s a b n o i t i d d a l a c i m e h c r o f c i g o ls l a i r e t a m n o s t r o p e r t n e m e g a n a m s e d i v o r p m e t s y s C L P e h t , t f i h s h c a e f o n o i t e l p m o c. e s u. r e t s e g i D l l i M r e p a P d o o w m o r f p l u p r e p a p g n i k a m f o s s e c o r p e h t l o r t n o c s C L Pd n a y t i s n e d n o d e s a b s p i h c f o t n u o m a e h t s l o r t n o c d n a s e t a l u c l a c m e t s y s e h T . s p i h cd n a d e t a l u c l a c s i s r o u q i l g n i k o o c d e r i u q e r f o t n e c r e p e h t , n e h T . e m u l o v r e t s e g i dg n i k o o c e h t s d l o h d n a s p m a r C L P e h T . e c n e u q e s e h t o t d e d d a e r a s t n u o m a e s e h t. d e t e l p m o c s i g n i k o o c e h t l i t n u e r u t a r e p m e t. n o i t c u d o r P l l i M r e p a P d n a t h g i e w s i s a b e g a r e v a e h t s e t a l u g e r r e l l o r t n o c e h T, s e v l a v w o l f m a e t s e h t s e t a l u p i n a m m e t s y s e h T . e d a r g r e p a p r o f e l b a i r a v e r u t s i o m. w o l f l a t o t s l o r t n o c d n a s r o t i n o m d n a , t h g i e w e t a l u g e r o t s e v l a v k c o t s e h t s t s u j d aR R E B B U D N A P C I T S A L. g n i r o t i n o M s s e r P g n i r u C - e r i T r o f g n i r o t i n o m s s e r p l a u d i v i d n i s m r o f r e p C L P e h Te h t s t r e l a m e t s y s e h T . e l c y c s s e r p h c a e g n i r u d e r u t a r e p m e t d n a , e r u s s e r p , e m i td e r o t s s i s u t a t s e n i h c a m g n i n r e c n o c n o i t a m r o f n I . s n o i t c n u f l a m s s e r p y n a f o r o t a r e p oy r a m m u s a e d u l c n i t f i h s h c a e r o f s t u o t n i r p n o i t a r e n e g t r o p e R . e s u r e t a l r o f s e l b a t n i. s n o i t c n u f l a m o t e u d e m i t n w o d s s e r p d n a s e r u c d o o g f o. g n i r u t c a f u n a M e r i T s m e t s y s e r u c / s s e r p e r i t r o f d e s u e r a s r e l l o r t n o c e l b a m m a r g o r Pe h t r o f t i f e r i t a o t n i e r i t w a r a s m r o f s n a r t t a h t s t n e v e f o g n i c n e u q e s e h t l o r t n o c o tn i a t b o o t r e b b u r e h t g n i r u c d n a n r e t t a p d a e r t e h t g n i d l o m s e d u l c n i l o r t n o c s i h T . d a o re c a p s e h t s e c u d e r y l l a i t n a t s b u s n o i t a c i l p p a C L P s i h T . s c i t s i r e t c a r a h c t n a t s i s e r - d a o r. t c u d o r p e h t f o y t i l a u q e h t d n a m e t s y s e h t f o y t i l i b a i l e r s e s a e r c n i d n a d e r i u q e r. n o i t c u d o r P r e b b u R d n a , s n o i t c n u f c i g o l r e x i m , l o r t n o c e l a c s e t a r u c c a e d i v o r p s C L Pn o i t c u d o r p e h t n i d e s u t n e m g i p d n a , l i o , k c a l b n o b r a c f o n o i t a r e p o a l u m r o f e l p i t l u mn o i t c u d o r p g n i r u d s l o o t e n i h c a m f o n o i t a z i l i t u s e z i m i x a m m e t s y s e h T . r e b b u r f od e r i u q e r l e n n o s r e p d n a e m i t s e c u d e r d n a , s e i r o t n e v n i s s e c o r p - n i s k c a r t , s e l u d e h c s. s t r o p e r d n e - t f i h s e h t d n a y t i v i t c a n o i t c u d o r p e h t e s i v r e p u s o t. g n i d l o M n o i t c e j n I c i t s a l P e r u t a r e p m e t s a h c u s , s e l b a i r a v s l o r t n o c m e t s y s C L P Am e t s y s e h T . s s e c o r p g n i d l o m n o i t c e j n i e h t e z i m i t p o o t d e s u e r a h c i h w , e r u s s e r p d n ao t d e m m a r g o r p e b n a c s l e v e l y t i c o l e v l a r e v e s e r e h w , n o i t c e j n i p o o l - d e s o l c s e d i v o r p. e m i t e l c y c n e t r o h s d n a , s t c e f e d e c a f r u s e c u d e r , g n i l l i f t n e t s i s n o c n i a t n i a mSECTION1IntroductoryConcepts22 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersFigure 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 completethesegmentation.Figure1-14showsseveralPLCsthatfallintothiscategory classification.The A, B, and C overlapping areas in Figure 1-13 reflect enhancements, byaddingoptions,ofthestandardfeaturesofthePLCswithinaparticularsegment.Theseoptionsallowaproducttobecloselymatchedtotheapplicationwithouthavingtopurchasethenextlargerunit.Chapter20ABC12345Complexity and Cost32 8192 4096 1024 2048 512 128 64I/O Count1-5PLCPRODUCTAPPLI CATI ONRANGESFigure 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 PLCs2. small PLCs3. medium PLCs4. large PLCs5. very large PLCs23CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptscovers, in detail, the differences between PLCs in overlapping areas. Thesedifferences include I/O count, memory size, programming language, soft-warefunctions,andotherfactors.AnunderstandingofthePLCproductranges 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)Courtesy of Mitsubishi Electronics, Mount Prospect, ILCourtesy of Giddings & Lewis, Fond du Lac, WI(c)Courtesy of Omron Electronics, Schaumburg, IL(e)Courtesy of PLC Direct, Cumming, GA(b)Courtesy of Allen-Bradley, Highland Heights, OH(f)Courtesy of Allen-Bradley, Highland Heights, OH(d)SECTION1IntroductoryConcepts24 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable Controllers1-6LADDERDI AGRAMSANDTHEPLCThe 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.Programmablecontrollerscanimplementalloftheoldladderdiagramconditionsandmuchmore.Theirpurposeistoperformthesecontroloperations 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 areconnectedtoinputandoutputinterfaces,whiletheladderprogramisimplemented 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-gizesthecorrespondingcircuitelementaccordingtotheprogram,andcontrols a real output device via the output interfaces.Figure 1-15. Simple electrical ladder diagram.L1 L2PLLS1PB1LS225CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsEXAMPLE 1-1In 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 willcausethepilotlightconnectedtoanoutputmoduletoturnON.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.SOLUTI ONFigure1-17showsseveralpossibleconfigurationsforthecircuitinFigure 1-15. The highlighted blue lines indicate that power is presentatthatconnectionpoint,whichisalsothewayaprogrammingormonitoringdevicerepresentspowerinaPLCcircuit.Thelasttwoconfigurations in Figure 1-17 are the only ones that will turn PL ON.Asyouwillseelater,eachinstructionisrepresentedinsidethePLCbyareference address, an alphanumeric value by which each device is known inthe PLC program. For example, the push button PB1 is represented inside thePLCbythenamePB1(indicatedontopoftheinstructionsymbol)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 L2 L2PL PB1LS1LS2PB1 LS1 PLLS2represents input modulerepresents output moduleSECTION1IntroductoryConcepts26 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersFigure 1-17. Possible configurations of inputs and corresponding outputs.1-7ADVANTAGES OFPLCSIn general, PLC architecture is modular and flexible, allowing hardware andsoftware elements to expand as the application requirements change. In theeventthatanapplicationoutgrowsthelimitationsoftheprogrammablecontroller, 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.PB1LS1PLLS2PB1PB1 LS1 PL PLPLPLPLPLLS1LS2LS2No EventTakes PlacePB1 is OpenLS1 is OpenLS2 is OpenPL is OFFPB1LS1PLLS2PB1PB1 LS1 PLLS1LS2LS2PB1 is ClosedLS1 is OpenLS2 is OpenPL is OFFPB1LS1PLLS2PB1PB1 LS1 PLLS1LS2LS2PB1 is ClosedLS1 is OpenLS2 is ClosedPL is OFFPB1LS1PLLS2PB1PB1 LS1 PLLS1LS2LS2PB1 is ClosedLS1 is ClosedLS2 is OpenPL is ONPB1LS1PLLS2PB1PB1 LS1 PLLS1LS2LS2PB1 is OpenLS1 is ClosedLS2 is ClosedPL is ONHardwired Description PLC27CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsTable 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 therearenophysicalconnectionsbetweenthefieldinputdevicesandoutputdevices (see Figure 1-18), as in hardwired systems. The only connection isthrough the control program, which can be easily altered.Table1-3listssomeofthemanyfeaturesandbenefitsobtainedwithaprogrammable controller.s e r u t a e F t n e r e h n I s t i f e n e Bs t n e n o p m o c e t a t s - d i l o S y t i l i b a i l e r h g i H y r o m e m e l b a m m a r g o r P s e g n a h c s e i f i l p m i S l o r t n o c e l b i x e l F e z i s l l a m S s t n e m e r i u q e r e c a p s l a m i n i M d e s a b - r o s s e c o r p o r c i M y t i l i b a p a c n o i t a c i n u m m o C e c n a m r o f r e p f o l e v e l r e h g i H s t c u d o r p y t i l a u q r e h g i H y t i l i b a p a c l a n o i t c n u f i t l u M s r e t n u o c / s r e m i t e r a w t f o S e r a w d r a h e t a n i m i l E s t e s e r p d e g n a h c y l i s a E s y a l e r l o r t n o c e r a w t f o S t s o c g n i r i w / e r a w d r a h e c u d e R s t n e m e r i u q e r e c a p s e c u d e R e r u t c e t i h c r a r a l u d o M y t i l i b i x e l f n o i t a l l a t s n I d e l l a t s n i y l i s a E t s o c e r a w d r a h s e c u d e R y t i l i b a d n a p x E s e c a f r e t n i O / I f o y t e i r a V s e c i v e d f o y t e i r a v a s l o r t n o C l o r t n o c d e z i m o t s u c s e t a n i m i l E s n o i t a t s O / I e t o m e R s n u r t i u d n o c / e r i w g n o l e t a n i m i l E s r o t a c i d n i c i t s o n g a i D e m i t g n i t o o h s e l b u o r t e c u d e R n o i t a r e p o r e p o r p l a n g i S e c a f r e t n i O / I r a l u d o M l e n a p l o r t n o c f o e c n a r a e p p a t a e N d e n i a t n i a m y l i s a E d e r i w y l i s a E s t c e n n o c s i d O / I k c i u Q g n i r i w g n i b r u t s i d t u o h t i w e c i v r e S s e l b a i r a v m e t s y Sa t a d y r o m e m n i d e r o t se c n a n e t n i a m / t n e m e g a n a m l u f e s U m r o f t r o p e r n i t u p t u o e b n a C SECTION1IntroductoryConcepts28 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersA 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.Asetofthumbwheelswitchesandapushbuttoncanbeeasilyconfigured to input new preset values to any number of software timers. Thetimesavingsbenefitofalteringsoftwaretimers,asopposedtoalteringseveral hardware timers, is obvious.Thehardwarefeaturesofprogrammablecontrollersprovidesimilarflexibility and cost savings. An intelligent CPU is capable of communicatingwithotherintelligentdevices.Thiscapabilityallowsthecontrollertobeintegratedintolocalorplantwidecontrolschemes.WithsuchacontrolFigure 1-18. Programmable controller I/O connection diagram showing no physicalconnections between the inputs and outputs.OUTPUTSCommon For InputsL1 L2IN0 1 2 3 4 5 6 7OUT0 1 2 3 4 5POWERRUNOKPROG-ECPU-EL101234567L2L1L2GroundPLCPower L1AC PowerFor OutputsPLCCommon L229CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsconfiguration,aPLCcansendusefulEnglishmessagesregardingthecontrolledsystem 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 LS2SOLLS1 LS2SOLLS1 LS3SOLLS1 LS3SOLLS1LS2SOLLS1SOLLS2LS2 LS2(a) SERIES(b) PARALLEL(c) Adding One LS In SeriesHARDWIRED PLC(a) Series(b) Parallel(c) Adding one LS in seriesEASEOFI NSTALLATI ONSeveral 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.Actualchangeovercanbemadequicklybysimplyconnectingtheinput/output devices to the prewired terminal strips.SECTION1IntroductoryConcepts30 Industrial Text and Video Company1-800-752-8398www.industrialtext.comCHAPTER1Introduction toProgrammable ControllersInlargeinstallations,remoteinput/outputstationsareplacedatoptimumlocations (see Figure 1-21). A coaxial cable or a twisted pair of wires connectsthe remote station to the CPU. This configuration results in a considerablereductioninmaterialandlaborcostsascomparedtoahardwiredsystem,which would involve running multiple wires and installing large conduits.The remote subsystem approach also means that various sections of a totalsystemcanbecompletelyprewiredbyanOEMorPLCvendorpriortoreaching 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.PLCMainPlantLocation01234567012345670123456701234567Sub-systemRemote Location01234567012345670123456701234567Sub-systemRemote Location01234567012345670123456701234567Sub-systemRemote LocationWiring to manyI/O field devicesfrom I/O modulesCoaxial cable ortwisted pair ofwires used forsubsystemcommunicationCourtesy of Omron Electronics, Schaumburg, IL31CHAPTER1Introduction toProgrammable ControllersIndustrial Text and Video Company1-800-752-8398www.industrialtext.comSECTION1IntroductoryConceptsEASEOFMAI NTENANCEANDTROUBLESHOOTI NGFigure 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 easeofmaintenanceinmind.Withvirtuallyallcomponentsbeingsolid-state,maintenance is reduced to the replacement of modular, p