Introduction to PLC Programming

Heres a valuable PLC reference that you can use right now. This particularreference is taken from our award-winning textbookProgrammableControllers: Theory and Implementation, 2nd Edition.

In it, youll get an overview of how relay logic can be converted intoPLC logic. Theres also lots of examples, tables, and ladder diagrams tohelp explain the topics.

Best yet, weve included the corresponding chapter from the companionworkbook. Here you can look over the key points as well as see howmuch you learned by answering the review questions. And, yes, theanswers are also included.

This PLC reference is just a sample of what the textbook and workbookhave to offer. If you like it, weve included the product literature pagewith the order number.

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A Special Note ToOur Customers

Introduction to Programmable Controllers

Number Systems and Codes

Logic Concepts

Processors, the Power Supply, and Programming Devices

The Memory System and I/O Interaction

The Discrete Input/Output System

The Analog Input/Output System

Special Function I/O and Serial Communication Interfacing

Programming Languages

The IEC-1131 Standard and Programming Language

System Programming and Implementation

PLC System Documentation

Data Measurements and Transducers

Process Responses and Transfer Functions

Process Controllers and Loop Tuning

Artificial Intelligence and PLC Systems

Fuzzy Logic

Local Area Networks

I/O Bus Networks

PLC Start-Up and Maintenance

System Selection Guidelines

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21 Chapters of PLC Know-HowTABLE OF CONTENTS

4 Follow our 11 major steps in selecting a PLC for an application and avoid using the wrong controller

4 Install sinking and sourcing inputs and outputs properlyone wrong wire and it wont work

4 Implement safety circuits correctly in PLC applications to protect people and equipment

4 Prevent noise, heat, and voltage variations from ruining your PLC system

4 Implement a step-by-step static and dynamic start-up checkout to guarantee smooth PLC system operation

4 Design preventive safety and maintenance into your total control system

SELECTION, INSTALLATION & SAFETY

TROUBLESHOOTING & MAINTENANCE4 Learn no-nonsense troubleshooting procedures to reduce

downtime4 Troubleshoot analog I/O and avoid undesirable count jumps4 Learn 6 preventive maintenance procedures to keep your PLC

system running fault free4 Learn a step-by-step procedure for finding hidden ground loops 4 Learn how to deal with leaky inputs 4 Identify vibration problems and use them for preventive

engineering control4 Control excessive line voltage and avoid intermittent shutdowns

PROGRAMMING4 Learn the number systems and codes used in PLC addressing4 Eliminate the confusion of ladder logic programming4 Master all types of timers and counters used in real-life

applications4 Avoid ladder scan evaluation problems4 Implement a safe circuit with hardware and software interlocking

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The biggest book on PLCs. Written by industry experts, this book covers important, up-to-date, real-world programmable controller topics and applications. This new edition is completely revised and updated to give you the latest developments and insights from the field. At 5 pounds and 1,035 pages, it puts all the PLC information you need at your fingertips. And, since this is a generic PLC reference, it will help you with all of the different makes and models of PLCs in your facility.

But, this book is about more than just PLCsit also thoroughly explains process control, instrumentation, and plant networks. Whether youre already an expert on PLCs or just starting out, our problem-solving approach is guaranteed to help you succeed.

Valuable Maintenance Tips

PLC Reference Book

You covered a huge amount of detail very well. It was very easy to understand.

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Sample pages from the workbook

Imagine having the answers to over 800 PLC problems at your fingertips. Thats what you get with Programmable Controllers: Workbook and Study Guide. At 334 pages, its the perfect companion to Programmable Controllers: Theory and Implementation, 2nd Edition.

This workbook provides not only valuable summaries of each of the text-books twenty-one chapters, but also over 800 review questions. And each of the review questions includes a detailed answer and explanation. Use it on the job to brush up on the essentials and to solve any PLC problem.

Whether youre an expert or just learning about PLCs, youll find plenty to put your skills to the test.

Catalog #ABT-ITV206WKBK $28

Programmable Controllers: Workbook/Study Guide

You Will Learn:

Proper address assignment and interfacing

Basic PLC ladder program implementation

Data measurement

Internal coil assignments

Proper digital and analog interfacing procedures

Advanced function block programming

Network protocols

Analog input and output data handling

Correct PLC installation

A sample problem from Chapter 11 of the workbook:

System Programming and Implementation

Circle the locations where timer traps will be used in the PLCimplementation of this reduced-voltage start motor circuit.

StopStart OL

L1 L2

TR1

TR1

S1

S1

TR1

TR1

TR1

S2

S2

S1 M1

M1

1

2

3

4

5

6

StopStart OL

L1 L2

TR1

TR1

S1

S1

TR1

TR1

TR1

S2

S2

S1 M1

M1

1

2

3

4

5

6

Q.Q.Q.

A.A.A.

Sometimes you think you know it all, but after reading the questions, I often times had to refer back to the theory book.

Ernest Presto, Electrical Engineer, Polyclad Laminates, Inc.

Perfect textbook companion:

800 answers to common PLC problems at your fingertips

Makes a great review tool

Practice PLC addressing and programming

Great on-the-job quick-reference guide

Separate answer section makes quizzing easy

Valuable chapter summaries

Sample Problem

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Introduction to PLCProgramming andImplementationfrom relay logic toPLC logic

Key TermsControl strategythesequence of steps that mustoccur during a process orPLC program to produce thedesired output control.Control taskthe desiredresults of a control program.

Flowchartinga method ofpictorially representing theoperation of a process in asequential manner.

Program codingtheprocess of translating a logicor relay diagram into PLCladder program form.

He that invents a machine augmentsthe power of man and the well-beingof mankind.

Henry Ward Beecher

2Industrial Text & Video Company www.industrialtext.com 1-800-752-8398

Introduction to PLC Programming and Implementationfrom relay logic to PLC logic

1999 by Industrial Text and Video CompanyPublished by Industrial Text and Video CompanyAll rights reserved.

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

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.

3Introduction to PLC Programming and Implementationfrom relay logic to PLC logic


Contents1 CONTROL TASK DEFINITION ..............................................................42 CONTROL STRATEGY .......................................................................43 IMPLEMENTATION GUIDELINES ............................................................54 PROGRAM ORGANIZATION AND IMPLEMENTATION....................................6

CREATING FLOWCHARTS AND OUTPUT SEQUENCES ................................7CONFIGURING THE PLC SYSTEM .....................................................10REAL AND INTERNAL I/O ASSIGNMENT ..............................................10REGISTER ADDRESS ASSIGNMENT ....................................................15ELEMENTS TO LEAVE HARDWIRED ....................................................15SPECIAL INPUT DEVICE PROGRAMMING .............................................17PROGRAM CODING/TRANSLATION.....................................................24

5 DISCRETE I/O CONTROL PROGRAMMING ...........................................25CONTROL PROGRAMMING AND PLC DESCRIPTIONS.............................26SIMPLE RELAY REPLACEMENT..........................................................27SIMPLE START/STOP MOTOR CIRCUIT ...............................................29FORWARD/REVERSE MOTOR INTERLOCKING ........................................33REDUCED-VOLTAGE-START MOTOR CONTROL.....................................37AC MOTOR DRIVE INTERFACE ........................................................40CONTINUOUS BOTTLE-FILLING CONTROL ...........................................44LARGE RELAY SYSTEM MODERNIZATION ............................................47

STUDY GUIDE ..................................................................................54REVIEW QUESTIONS ..........................................................................55ANSWERS........................................................................................64



The implementation of a control program requires complex organizationaland analytical skills, which change depending on the application. Becausethey are so varied, we cannot explain how to solve every specific control task.Nevertheless, we can provide you with techniques and guidelines for com-pleting this problem-solving process. In this handbook, we will introduce astrategy for implementing a control program, which includes program orga-nization, system configuration, and I/O programming. These strategies alsoapply to PLCs with the IEC 1131-3 programming standard. Additionally, wewill present both simple and complex PLC programming examples. After youfinish, you will be ready to learn how to document the PLC systemthe laststep in implementing the control program.

2 CONTROL STRATEGY

After the control task has been defined, the planning of its solution can begin.This procedure commonly involves determining a control strategy, thesequence of steps that must occur within the program to produce the desiredoutput control. This part of the program development is known as thedevelopment of an algorithm. The term algorithm may be new or strange tosome readers, but it need not be. Each of us follows algorithms to accomplish

HIGHLIGHTS

1 CONTROL TASK DEFINITION

A user should begin the problem-solving process by defining the controltask, that is, determining what needs to be done. This information providesthe foundation for the control program. To help minimize errors, the controltask should be defined by those who are familiar with the operation of themachine or process. Proper definition of the task is directly related to thesuccess of the control program.

Control task definition occurs at many levels. All of the departmentsinvolved must work together to determine what inputs are required, so thateveryone understands the purpose and scope of the project. For example, ifa project involves the automation of a manufacturing plant in whichmaterials will be retrieved from the warehouse and sent to the automaticpackaging area, personnel from both the warehouse and packaging areasmust collaborate with the engineering group during the system definition.Management should also be involved if the project requires data reporting.If the control task is currently done manually or through relay logic, theuser should review the steps of the manual procedure to determine whatimprovements, if any, can be made. Although relay logic can be directlyimplemented in a PLC, the procedure should be redesigned, when possible,to meet current project needs and to capitalize on the capabilities of program-mable controllers.



certain tasks in our daily lives. The procedure that a person follows to gofrom home to either school or work is an algorithmthe person exits thehouse, gets into the car, starts the engine, and so on. In the last of a finitenumber of steps, he or she reaches the destination.

The PLC strategy implementation for a control task closely follows thedevelopment of an algorithm. The user must implement the control from agiven set of basic instructions and produce the solution in a finite number ofsteps. If developing an algorithm to solve the problem becomes difficult, heor she may need to return to the control task definition to redefine theproblem. For example, we cannot explain how to get from where we are toBullfrog County, Nevada unless we know both where we are and whereBullfrog County is. As part of the problem definition, we need to know if aparticular method of transportation is required. If there is a time constraint, weneed to know that too. We cannot develop a control strategy until we have allof this problem definition information.

The fundamental rule for defining the program strategy is think first,program later. Consider alternative approaches to solving the problem andallow time to polish the solution algorithm before trying to program thecontrol function. Adopting this philosophy will shorten programming time,reduce debugging time, accelerate start-up, and focus attention where it isneededon design when designing and on programming when programming.

Strategy formulation challenges the system designer, regardless of whetherit is a new application or the modernization of an existing process. In eithercase, the designer must review the sequence of events and optimize controlthrough the addition or deletion of steps. This requires a knowledge of thePLC-controlled field devices, as well as input and output considerations.

3 IMPLEMENTATION GUIDELINES

A programmable controller is a powerful machine, but it can only do what itis told to do. It receives all of its directions from the control program, the setof instructions or solution algorithms created by the programmer. Therefore,the success of a PLC control program depends on how organized the user is.There are many ways to approach a problem; but if the application isapproached in a systematic manner, the probability of mistakes is less.

The techniques used to implement the control program vary according to theprogrammer. Nevertheless, the programmer should follow certain guide-lines. Table1 lists programming guidelines for new applications and modern-izations. New applications are new systems, while modernizations are up-graded existing control systems that have functioned previously without aPLC (i.e., through electromechanical control or individual, analog, loopcontrollers).



As mentioned previously, understanding the process or machine operationis the first step in a systematic approach to solving the control problem. Fornew applications, the strategy should follow the problem definition. Review-ing strategies for new applications, as well as revising the actual method ofcontrol for a modernization project, will help detect errors that were intro-duced during the planning stages.

The programming stage reveals the difference between new and moderniza-tion projects. In a modernization project, the user already understands theoperation of the machine or process, along with the control task. An existingrelay ladder diagram, like the one shown in Figure1, usually defines thesequence of events in the control program. This ladder diagram can be almostdirectly translated into PLC ladder diagrams.

New applications usually begin with specifications given to the person whowill design and install the control system. The designer translates thesespecifications into a written description that explains the possible controlstrategies. The written explanation should be simple to avoid confusion. Thedesigner then uses this explanation to develop the control program.

Table 1. Programming guidelines.

4 PROGRAM ORGANIZATION AND IMPLEMENTATION

Organization is a key word when programming and implementing a controlsolution. The larger the project, the more organization is needed, especiallywhen a group of people is involved.

In addition to organization, a successful control solution also depends on theability to implement it. The programmer must understand the PLC controltask and controlled devices, choose the correct equipment for the job

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(hardware and software), and understand the PLC system. Once thesepreliminary details are understood, the programmer can begin sketching thecontrol program solution. The work performed during this time forms animportant part of the system or project documentation. Documenting a systemonce it is installed and working is difficult, especially if you do notremember how you got it to work in the first place. Therefore, documentingthe system throughout its development will pay off in the end.

CREATING FLOWCHARTS AND OUTPUT SEQUENCES

Flowcharting is a technique often used when planning a program after awritten description has been developed. A flowchart is a pictorial represen-tation that records, analyzes, and communicates information, as well asdescribes the operational process in a sequential manner. Figure 2 illustratesa simple flowchart. Each step in the chart performs an operation, whether itis an input/output, decision, or data process.

In a flowchart, broad concepts and minor details, along with their relationshipto each other, are readily apparent. Sequences and relationships that are hardto extract from general descriptions also become obvious when expressed

Figure 1. Electromechanical relay circuit diagram.

L1 L2

CR1LS7PB14

CR1

CR2

CR3

PL3

PL4

SOL3 UPCR1SOL

PS7

CR3

SOL4 FWDLS9LS8

LS8

CR2PS7

SOL5 DWNReset CR2Start



through a flowchart. Even the flowchart symbols themselves have specificmeanings, which aid in the interpretation of the solution algorithm. Figure 3illustrates the most common flowchart symbols and their meanings.

The main flowchart itself should not be long and complex; instead, it shouldpoint out the major functions to be performed (e.g., compute engineeringunits from analog input counts). Several smaller flowcharts can be used tofurther describe the functions specified in the main flowchart.

Once the flowchart is completed, the user can employ either logic gates orcontact symbology to implement the logic sequences. Logic gates implementa logical output sequence given specific real and/or internal input conditions,

Figure 2. Simple flowchart. Figure 3. Flowchart symbols.

Process A group of one or more instructions that per- form a processing functionInput/Output Any function involving an input /output deviceDecision A point in the program where a branch to alter- nate paths is possiblePreparation A group of one or more instructions that sets the stage for subsequent processingPredefined Process A group of operations not detailed in the flowchart (often a library subroutine)Terminal Beginning, end, or point of interruption in a programConnector Entry from, or exit to, another part of the flowchartFlowline Direction of processing or data flowAnnotation Descriptive comments or explanatory notes provided for clarification

START

Set PresetValues

Is PBPressed?

Read AnalogInput

Store InTemp. Reg.

Is Temp.> 100C

Turn HeaterCoil ON

END

Go ToSubroutine

Yes

NO

No



Figure 4. (a) PLC contact symbology and (b) logic gate representation of a logicsequence.

Figure 5. A combination of logic gates and contact symbology.

while PLC contact symbology directly implements the logic necessary toprogram an output rung. Figure 4 illustrates both of these programmingmethods. Users should employ whichever method they feel most comfortablewith or, perhaps, a combination of both (see Figure 5). Logic gate diagrams,however, may be more appropriate in controllers that use Boolean instructionsets.

Inputs and outputs marked with an X on a logic gate diagram, as in Figure 4b,represent real I/O in the system. If no mark is present, an I/O point is aninternal. The labels used for actual input signals can be either the actualdevice names (e.g., LS1, PB10, AUTO, etc.) or symbolic letters and numbersthat are associated with each of the field elements. During this stage, the usershould prepare a short description of the logic sequence.

(a)

(b)

Reset B(Reset SOL2)

Counter 2330 gallons of B

B Finished(Start of pump

back B)

M

Counter 2330 gallons of B

Reset B(Reset SOL2)

B Finished(Start of pump back B)

B Finished

Count A GallonMeter

SOL1

Clear C1

A Finished

Up

Reset

C1

PV = 500 Gal.

500 Gal. of A



CONFIGURING THE PLC SYSTEM

Table 2. I/O address assignment table for real inputs and outputs.

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tuptuO 0 1 0 2LOS0 1 1 3LP

PLC configuration should be considered during flowcharting and logicsequencing. The PLCs configuration defines which I/O modules will beused with which types of I/O signals, as well as where the modules will belocated in the local or remote rack enclosures. The modules locationsdetermine the I/O addresses that will be used in the control program.

During system configuration, the user should consider the following:possible future expansions; special I/O modules, such as fast-response orwire fault inputs; and the placement of interfaces within a rack (all AC I/Otogether, all DC and low-level analog I/O together, etc.). Consideration ofthese details, along with system configuration documentation, will resultin a better system design.

REAL AND INTERNAL I/O ASSIGNMENT

The assignment of inputs and outputs is one of the most important proceduresthat occurs during the programming organization and implementationstages. The I/O assignment table documents and organizes what has beendone thus far. It indicates which PLC inputs are connected to which inputdevices and which PLC outputs drive which output devices. The assignmentof internals, including timers, counters, and MCRs, also takes place here.These assignments are the actual contact and coil representations that areused in the ladder diagram program. In applications where electromechanicalrelay diagrams are available (e.g., modernization of a machine or process),identification of real I/O can be done by circling the devices and thenassigning them I/O addresses (see Example 1).Table 2 shows an I/O address assignment table for real inputs and outputs,while Table 3 shows an I/O address assignment table for internals. Theseassignments can be extracted from the logic gate diagrams or ladder symbols

11



EXAMPLE 1

For the circuit shown in Figure 7, (a) identify the real inputs and outputsby circling each, (b) assign the I/O addresses, (c) assign the internaladdresses (if required), and (d) draw the I/O connection diagram.

Table 3. I/O address assignment table for internal outputs.

Figure 6. Partial connection diagram for the I/O address assignment in Table 2.

that were used to describe the logic sequences. They can also come from thecircled elements on an electromechanical diagram. The numbers used forthe I/O addresses depend on the PLC model used. These addresses can berepresented in octal, decimal, or hexadecimal. The description section of thetable specifies the field devices that correspond to each address.

The table of address assignments should closely follow the input/outputconnection diagram (see Figure 6). Although industry standards for I/Orepresentations vary among users, inputs and outputs are typically repre-sented by squares and diamonds, respectively. The I/O connection diagramforms part of the documentation package.

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LS2

LS1000 004

001 005

Inputs Outputs

ProgramCoding

RPL1

SOL1

During the I/O assignment, the user should group associated inputs andoutputs. This grouping will allow the monitoring and manipulation of agroup of I/O simultaneously. For instance, if 16 motors will be startedsequentially, they should be grouped together, so that monitoring the I/Oregisters associated with the 16 grouped I/O points will reveal the motorsstarting sequence. Due to the modularity of an I/O system, all the inputs andall the outputs should be assigned at the same time. This practice will preventthe assignment of an input address to an output module and vice versa.



Assume that the PLC used has a modularity of 8 points per module.Each rack has 8 module slots, and the master rack is number 0. Inputsand outputs can have any address as long as the correct module isused. The PLC determines whether an input or output module isconnected in a slot. The number system is octal, and internals start ataddress 10008.

Figure 7. Electromechanical relay circuit.

SOLUTION

(a) Figure 8 shows the circled real input and output connections. Notethat temperature switch TS3 is circled twice even though it is only onedevice. In the address assignment, only one of them is referenced, andonly one of them is wired to an input module.

(b) Table 4 illustrates the assignment of inputs and outputs. It assignsall inputs and all outputs, leaving spare I/O locations for future use.

L1 L2

CR1StartPB1 Stop

PB2

CR1

PL1

CR1 CR2TempTS3

CR1TempTS3

PL2

CR3

PL3

CR2

CR3CR2

SOL2Open

SOL1Open

LevelFS4

LevelFS5

H3Heating

or

H

Ready

13



Figure 8. Identification of real I/O (circled).

Table 4. I/O address assignment.

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L1 L2

CR1StartPB1 Stop

PB2

CR1

PL1

CR1 CR2TempTS3

CR1TempTS3

PL2

CR2

PL3

CR2

CR3CR2

SOL2Open

SOL1Open

LevelFS4

LevelFS5

H3Heating

or

H

Ready



(c) Table 5 presents the output assignments, including a descriptionof each internal. Note that control relay CR2 is not assigned as aninternal since it is the same as the output rung corresponding to PL1.When the control program is implemented, every contact associatedwith CR2 will be replaced by contacts with address 020 (the addressof PL1).

Table 5. Internal output assignment.

Figure 9. I/O connection diagram.

(d) Figure 9 illustrates the I/O connection diagram for the circuit inFigure 7. This diagram is based on the I/O assignment from part (b).Note that only one of the temperature switches, the normally open TS3switch, is a connected input. The logic programming of each switchshould be based on a normally open condition.

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L1 L1 L2L2

StartPB1

StopPB2

TempTS3

000

001 021

Inputs Outputs

ProgramCoding

002

003 023

004

005

006

007

Input Output

Level FS4

Level FS5

020

022

PL1 Ready

PL2

SOL1 Open

SOL2 Open

PL3

H3 Heating

024

026

027

025

15



REGISTER ADDRESS ASSIGNMENT

The assignment of addresses to the registers used in the control program isanother important aspect of PLC organization. The easiest way to assignregisters is to list all of the available PLC registers. Then, as they are used,describe each registers contents, description, and function in a registerassignment table. Table 6 shows a register assignment table for the first 15registers in a PLC system, ranging from address 20008 to address 20168.

Table 6. Register assignment table.

ELEMENTS TO LEAVE HARDWIRED

During the assignment of inputs and outputs, the user should decide whichdevices will not be wired to the controller. These elements will remain partof the electromechanical control logic. These elements usually includedevices that are not frequently switched off after start, such as compressorsand hydraulic pumps. Components like emergency stops and master startpush buttons should also remain hardwired, principally for safety purposes.This way, if the controller is faulty and an emergency occurs, the user can shutdown the system without PLC intervention.

Figure 10 provides an example of system components that are typically lefthardwired. Note that the normally open PLC Fault Contact 1 (orwatchdog timer contact) is wired in series with other emergency conditions.This contact stays closed when the controller is operating correctly, butopens when a fault occurs. The system designer can also use this contact if anemergency occurs to disable the PLC systems operation.

PLC fault contacts are safety contacts that are available to the user whenimplementing or enhancing a safety circuit. When a PLC is operatingcorrectly, the normally open fault contact closes and the normally closed one

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Figure 10. Hardwired components in a PLC system.

opens when the PLC is first turned on. As shown in Figure 10, these contactsare connected in series with the hardwired circuit, so that if the PLC failsduring standard operation, the normally open contacts will open. This willshut down the hardwired circuit at the point where the PLC becomes thecontrolling element. This circuit also uses a safety control relay (SCR) tocontrol power to the rest of the control components. The normally closed faultcontacts are used to indicate an alarm condition.

In the diagram shown in Figure 10, an emergency situation (including a PLCmalfunction) will remove power (L1) to the I/O modules. The turning OFF ofthe safety control relay (SCR) will open the SCR contact, stopping the flowof power to the system. Furthermore, the normally closed PLC fault contact(PLC Fault Contact 2) in the hardwired section will alert personnel of a systemfailure due to a PLC malfunction. The designer should implement this type ofalarm in the main PLC rack, as well as in each remote I/O rack location, since

M2Start

Stop

M2 M3

PLC FaultContact 1

PLC FaultContact 2

M3

SCR

PL1

PLC Fail Alarm

PLC

OLs

OLs

OLs

F1

DisconnectSwich Fuses

1M

M3

OLs

2M

M2

OLs

3M

M1

CoolantPump Motor

HydraulicPump Motor

SpindleMotor

L1 L2

SCR

To I/O System

17



remote systems also have fault contacts incorporated into the remote control-lers. This allows subsystem failures to be signaled promptly, so that theproblem can be fixed without endangering personnel.


SPECIAL INPUT DEVICE PROGRAMMING

Some PLC circuits and input connections require special programming. Oneexample is the programming of normally closed input devices. Rememberthat the programming of a device is closely related to how that device shouldbehave in the control program.

Normally Closed Devices. An input device that is wired as a normallyopen input can be programmed to act as either a normally open or a normallyclosed device. The same rule applies for normally closed inputs. Generally,if a device is wired as a normally closed input and it must act as a normallyclosed input, its reference address is programmed as normally open. As thefollowing example illustrates, however, a normally closed device in ahardwired circuit is programmed as normally closed when it is replaced in thePLC control program. Since it is not referenced as an input, the program doesnot evaluate the device as a real input.

EXAMPLE 2

For the circuit in Figure 11, draw the PLC ladder program and createan I/O address assignment table. For inputs, use addresses 108through 478. Start outputs at address 508 and internals at address 1008.

SOLUTION

Figure 12 shows the equivalent PLC ladder diagram for the circuit inFigure 11. Table 7 shows the I/O address assignment table for thisexample. The normally closed contact (CR10) is programmed asnormally closed because internal coil 100 references it and requiresit to operate as a normally closed contact.

L1 L2LS14

CR10

PS1 CR10

LS15

CR10 SOL7



Figure 12. PLC ladder diagram of the circuit in Figure 11.

Master Control Relays. Another circuit the programmer should be awareof is a master control relay (MCR). In electromechanical circuit diagrams,an MCR coil controls several rungs in a circuit by switching ON or OFFthe power to those rungs. In a hardwired circuit, there is no definite end to anMCR except when the circuit is followed all the way through. For example,in Figure13, the MCR output in line 1 controls the power to the hardwired

Table 7. I/O address assignment table.

Figure 13. Electromechanical relay circuit with a master control relay.

L1 L1 L2L2LS14

50

PS1*

LS15

LS1410

CR10100

CR10100

LS1512

PS111

CR10100

SOL750 SOL7

*Wired NCProgrammed NO

10

11

12

L1 L2MCRLS1PS1

PL1CR1

1

CR100TS20LS10051

2

4

3

HardwiredCircuits

50 HardwiredCircuits

Last hardwiredcircuit

MCR controls power to circuits below until the end of the hardwired circuit

Power to other circuits not controlled by MCR

MCR

sserddAO/I eciveD epyT01 41SL tupnI11 1SP tupnI21 51SL tupnI05 7LOS tuptuO001 01RC lanretnI

19



elements from line 3, where the MCR contact is located, to the last elementin line 51. If the master control relay is ON, power will flow to these rungs(lines 4 through 51). If the master control relay is OFF, power will not flowand these devices will not implement the control action. This configurationis equivalent to a hardwired subprogram or subroutineif the MCR is ON,the rungs are executed; if it is OFF, the rungs are not executed. At line 2in the circuit, power branches to other circuits that are not affected by the MCRsaction. These circuits are the regular hardwired program.

During the translation from a hardwired ladder circuit to PLC symbology,the programmer must place an END MCR instruction after the last rung theMCR should control. Figure14 illustrates the placement of the MCR instruc-tion for the circuit in Figure 13. To provide proper fencing for the programsMCR control section, internal output coil 1000, labeled CR1 (line 1 of PLCprogram), was inserted so that PL1 would not be inside the fenced MCRarea. This is the way the hardwired circuit operates. The END1 instruction

Figure 14. PLC ladder diagram with MCR fence.

L1 L1 L2L2

PS1

LS1

010

011

PS110

LS111

CR1Int 1000

LS100102

TS20103

Int2000

END1

2000PL1040

CR11000 MCR1

040 PL1

TranslatedLogic

TranslatedLogic

LS100

TS20 103

102

Rest of program from line 2 in

hardwired circuit

Fenced byMCR1



ends the MCR fence. The instructions corresponding to the hardwiredcircuits that branch from line 2 in the electromechanical diagram of Figure 13are located after the END1 instruction. Figure15 illustrates a partial ladderrung of a more elaborate circuit with this type of MCR condition. Thecorresponding PLC program should have an END MCR after the rungcontaining the PL3 output.

Figure 15. Electromechanical relay circuit with an MCR.

M1

CR1

CR2

CR1

CR1UpLS1

Run

CR2

CR1

CR3

CR3

TDR1SOL1

SOL3

SOL4

SOL2

CR4

PL2

PL3

OLs

Set Up/Run

MCR

CR3

Enable

Up

MCR

TDR1CR3

LS2

LS3

PL4

CR4

CR4

CR5

CR4

CR3

CR4FeedLS4

CR1 LS5 TDR1

CR25 seconds

MasterControlRelay

Master ON

Up

Sol Up

Sol Dn

Dn ON

Set Up

Set Up ON

Feed Sol

Fast Sol

7

8

9

10

11

12

13

14

15

16

17

18

19

1

2

3

4

5

6

21



Figure 16. MCR-controlled program elements.

EXAMPLE 3

Highlight the sections of the circuit in Figure 15 that will be under thecontrol of a PLC MCR. What additional measures must be taken toinclude or bypass other hardwired circuits within the MCR fence?

SOLUTION

Figure 16 highlights the circuits that must be fenced under the MCRinstruction. Note that solenoid SOL1 and part of its driving logic are notincluded in the MCR fencing because SOL1, CR3, and TDR1 can alsobe turned ON by logic prior to the MCR fence (see Figure 17). For theMCR fence to be properly programmed, the PLC program must

M1

CR1

CR2

CR1

CR1UpLS1

Run

CR2

CR1

CR3

CR3

TDR1SOL1

SOL3

SOL4

SOL2

CR4

PL2

PL3

OLs

Set Up/Run

MCR

CR3

Enable

Up

MCR

TDR1CR3

LS2

LS3

PL4

CR4

CR4

CR5

CR4

CR3

CR4FeedLS4

CR1 LS5 TDR1

CR25 seconds

MasterControlRelay

Master ON

Up

Sol Up

Sol Dn

Dn ON

Set Up

Set Up ON

Feed Sol

Fast Sol

7

8

9

10

11

12

13

14

15

16

17

18

19

1

2

3

4

5

6



include two internal control relays that take SOL1 out of the fence.Figure 18 illustrates the fenced circuit with the additional internals(CR1000 and CR1001). Note that the instructions in this diagram havethe same names as in the hardwired circuit. The solenoid SOL1 will beoutside of the MCR fence because it can be turned ON by either theoutside logic (highlighted section in Figure 17) or the logic inside theMCR fence (highlighted section in Figure 18).

Figure 17. SOL1 activated by logic outside of the MCR fence.

Figure 18. MCR fence.

Up CR3

LS3

TDR1 CR3 SOL2

SOL2 PL3

END1

Up LS2 LS1 CR1001

MCR1

Set Up/Run Up CR4 CR1000

LogicDriving MCR

CR1000

CR1001

CR3 TDR1 SOL1

Fenced byMCR

CR1

CR3

CR3

TDR1 SOL1

SOL2

CR4

PL3

CR3

MCR

TDR1CR3

LS2

LS3

Up

Sol Up

Sol Dn

Dn ON

7

8

9

10

11

Set Up/Run Up

add CR1001

add CR1000

23



Bidirectional Power Flow. The circuit in Figure 19 illustrates anothercondition that can cause programming problems: the possibility of bidirec-tional power flow through the normally closed CR4 contact in line 8. Tosolve the bidirectional flow problem, the programmer must know whether ornot CR4 influences the two output rungs to which it is connected. These rungsare the CR3 control relay output and the solenoid SOL1 output (rungs 7 and9, respectively). Figure 19 illustrates the two paths that can occur in thehardwired circuit. PLCs only allow forward paths; therefore, if a reverse pathis necessary for this circuits logic, the CR4 contact must be included in thelogic driving the CR3 output (see Figure 9b).

Figure 19. (a) Forward and (b) reverse power flow in a hardwired circuit.

Instantaneous Timer Contacts. The electromechanical circuit shown inFigure 15 specifies an instantaneous timer contact (the normally openTDR1 contact in line 10). This type of contact, however, is usually unavail-able in PLCs. To implement an instantaneous timer contact (i.e., a contact

CR1

CR3

CR3

TDR1

SOL2

PL3

CR3

MCR

TDR1CR3

LS2

LS3

Up

Sol Up

Sol Dn

Dn ON

7

8

9

10

11

(a) Forward path

CR1

CR3

CR3

TDR1SOL1

SOL2

CR4

PL3

CR3

MCR

TDR1CR3

LS2

LS3

Up

Sol Up

Sol Dn

Dn ON

7

8

9

10

11

(b) Reverse path

CR4

SOL1



that closes or opens once the timer is enabled), the programmer must use aninternal output to trap the timer, then use the internals contact as aninstantaneous contact to drive the timers logic.

In the electromechanical circuit in Figure 20a, if PB1 and LS1 both close, thetimer will start timing and the instantaneous contact (TMR1-1) will close,thus sealing PB1. If PB1 is released (OFF), the timer will continue to timebecause the circuit is sealed. Figure 20b illustrates the technique for trappinga timer. In this PLC program, an internal output traps the instantaneouscontact from the circuits electromechanical timer. Thus, the contacts fromthis internal drive the timer. If a trap does not exist, the timer will start timingwhen PB1 and LS1 both close, but will stop timing as soon as PB1 isreleased.

Figure 20. (a) An instantaneous timer contact in a hardwired circuit and (b) a trappedtimer in a PLC circuit.

Complicated Logic Rungs. When a logic rung is very confusing, the bestprogramming procedure is to isolate it from the other rungs. Then, reconstructall of the possible logic paths from right to left, starting at the output andending at the beginning of the rung. If a section of a rung, like the onediscussed in Example 3, directly connects or interacts with another rung, itmay be easier to create an internal output at the point where the two rungscross. Then, use the internal output to drive the rest of the logic. For the circuitshown in Figure 15, this cross point is in line 9 at the normally closed contactCR4 between normally open LS1 and normally closed CR3.

PROGRAM CODING/TRANSLATION

Program coding is the process of translating a logic or relay diagram intoPLC ladder program form. This ladder program, which is stored in theapplication memory, is the actual logic that will implement the control of themachine or process. Ease of program coding is directly related to how orderly

L1 L2PB1

TMR1-1

LS1 TMR1

TMR1-2 SOL7

PB1

Internal

Internal

LS1 Internal

TMR1

TMR1 SOL1

(a) (b)

TrapCircuitInstantaneous

Timer Contact

25



the previous stages (control task definition, I/O assignment, etc.) have beendone. Figure 21 shows a sample program code generated from logic gates andelectromechanical relay diagrams (internal coil 1000 replaces the controlrelay). Note that the coding is a PLC representation of the logic, whether it isa new application or a modernization. The next sections examine this codingprocess closer and present several programming examples.

Figure 21. Translation from (a) logic gates and (b) an electromechanical relay diagraminto (c) PLC program coding.

Start PB

SELInternal

Internal

PSLS

Motor

Start PB

CR1

CR1

CR1

SEL

PS

MotorLSM

(a)

(c)

(b)

L1 L1 L2L2Start PB

110

SEL

LS

PB100

CR11000

CR11000

LS102

PS103

SEL101

CR11000

M110

M

100

101

PS103

102

I/O Assignment Program Coding I/O Assignment

5 DISCRETE I/O CONTROL PROGRAMMING

In this section, we will present several programming examples that illustratethe modernization of relay systems. We will also present examples relatingto new PLC control implementations. These examples will deal primarilywith discrete controls. The next section will explain more about analog I/Ointeraction and programming.



CONTROL PROGRAMMING AND PLC DESCRIPTIONS

Figure 22. Example PLC configuration.

The PLC can accept four-channel analog input modules, which can be placedin any slot location. When analog I/O modules are used, discrete I/O cannotbe used in the same slot. The PLC can also accept multiplexed register I/O.These multiplexed modules require two slot positions and provide the enable(select) lines for the I/O devices.Addresses 000 through 777 octal represent input and output device connec-tions mapped to the I/O table. The first digit of the address represents the racknumber, the second digit represents the slot, and the third digit specifies theterminal connection in the slot. The PLC detects whether the slot holds aninput or an output.

Modernization applications involve the transfer of a machine or processscontrol from conventional relay logic to a programmable controller. Con-ventional hardwired relay panels, which house the control logic, usuallypresent maintenance problems, such as contact chatter, contact welding, andother electromechanical problems. Switching to a PLC can improve theperformance of the machine, as well as optimize its control. The machinesnew programmable controller program is actually based on the instructionsand control requirements of the original hardwired system.

Throughout this section, we will use the example of a midsized PLC capableof handling up to 512 I/O points (000 to 777 octal) to explain how toimplement and configure a PLC program. The I/O structure of the controllerhas 4 I/O points per module. The PLC has eight racks (0 through 7), each onewith eight slots, or groups, where modules can be inserted. Figure 22illustrates this configuration.

CPU

0 1 2 3 4 5 6 7

I/O Module Group or Slot

I/O Point Rack 0

27



Point addresses 10008 to 27778 may be used for internal outputs, and registerstorage starts at register 30008 and ends at register 47778. Two types of timerand counter formats can be usedladder format and block formatbut alltimers require an internal output to specify the ON-delay output. Ladderformat timers place a T in front of the internal output address, while blockformat timers specify the internal output address in the blocks output coil.

Throughout the examples presented in this section and the next, we will useaddresses 0008 through 0278 for discrete inputs and addresses 0308 through0478 for discrete outputs. Analog I/O will be placed in the last slot of themaster rack (0) whenever possible. During the development of these ex-amples, you will discover that sometimes the assignment of internals andregisters is performed parallel to the programming stages.

SIMPLE RELAY REPLACEMENT

This relay replacement example involves the PLC implementation of theelectromechanical circuit shown in Figure 23. The hardware timer TMR1requires instantaneous contacts in the first rung, which are used to latch the


L1 L2

TMR1PB1 PS1

CR1

TS1FS1

CR1 LS1 SOL2

SOL1

CR1

CR2

TMR2CR1

CR3

CR3 SOL3TMR2

TMR1

PS2

3 sec

2 sec



rung. If the instantaneous TMR1 contacts are implemented using a PLC time-delay contact, then PB1 must be pushed for the timers required time presetto latch the rung. This instantaneous contact will be implemented by trappingthe timer with an internal output.

Tables 8 and 9 show the I/O address and internal output assignments for theelectromechanical circuits real I/O. Table 10 presents the register assign-ment table. Note that internals do not replace control relays CR1 and CR2since the output addresses 030 and 031 corresponding to solenoids SOL1 andSOL2 are available. Therefore, addresses 030 and 031 can replace the CR1and CR2 contacts, respectively, everywhere they occur in the program. Thenormally open contact LS1 connects limit switch LS1 to the PLC inputinterface; and the normally open LS1 reference, programmed with an exam-ine-OFF instruction, implements the normally closed LS1 in the program.Figure 24 illustrates the PLC program coding solution.


Table 9. Internal address assignment.

Table 10. Register assignment.

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0 0 1 1SP0 0 2 1SF0 0 3 1ST

tupnI 0 0 4 1SL0 0 5 2SP0 0 6 0 0 7

tuptuO 0 3 0 1LOS0 3 1 2LOS0 3 2 3LOS0 3 3

eciveD lanretnI noitpircseD1RMT 0001 1RMTpartotdesU

1RC )030(1LOSsaemaS2RC )130(2LOSsaemaS

1RMT 1001 1RMTremiT2RMT 2001 2RMTremiT

3RC 3001 3RCecalpeR

retsigeR noitpircseD0004 ces3roftnuocremitteserP1004 1001remittnuocdetalumuccA2004 ces2roftnuocremitteserP3004 2001remittnuocdetalumuccA

29



SIMPLE START/STOP MOTOR CIRCUIT

Figure 24. PLC implementation of the circuit in Figure 23.

Figure 25 shows the wiring diagram for a three-phase motor and its corre-sponding three-wire control circuit, where the auxiliary contacts of the starterseal the start push button. To convert this circuit into a PLC program, firstdetermine which control devices will be part of the PLC I/O system; these arethe circled items in Figure 26. In this circuit, the start and stop push buttons(inputs) and the starter coil (output) will be part of the PLC system. The startercoils auxiliary contacts will not be part of the system because an internal willbe used to seal the coil, resulting in less wiring and fewer connections.

L1 L1 L2L2PB1

PS1

PB1000

TMR Trap 1000

PS1001

TMR Trap1000

030

000

001

PS2005

TS1FS1002

TS1003

SOL1030

TMR11001

SOL1030

CR31003

003

FS1TMR Trap 1000

TMR11001

TMR21002

002

TMR21002

PS2005

CR31003

SOL3032

TMR11001

SOL1030

LS1004

LS1TMR11001

SOL1030

LS1004

SOL2031

004

TMR

PR 400030

AR 4001TB = 0.1

TMR

PR 400220

AR 4003TB = 0.1

SOL1

031

SOL2

032

SOL3



Table 11 shows the I/O address assignment, which uses the same addressingscheme as the circuit diagram (i.e., inputs: addresses 000 and 001, output:address 030).To program the PLC, the devices must be programmed in the same logicsequence as they are in the hardwired circuit (see Figure 27). Therefore, thestop push button will be programmed as an examine-ON instruction

Figure 25. (a) Wiring diagram and (b) relay control circuit for a three-phase motor.

Figure 26. Real inputs and outputs to the PLC.

(a)

(b)

L1 L2Start

StopM

OL

M

2 3

Motor

3

2

T1 T2 T3

L1 L2 L3

OL

M

Power

Start

StopM

(a)

Push ButtonStation

(three-wire control)

L1 L2Start

StopM

OL

M

2 3

31





(a normally open PLC contact) in series with the start push button, which isalso programmed as an examine-ON instruction. This circuit will drive output030, which controls the starter. If the start push button is pressed, output 030will turn ON, sealing the start push button and turning the motor ON throughthe starter. If the stop push button is pressed, the motor will turn OFF. Notethat the stop push button is wired as normally closed to the input module. Also,the starter coils overloads are wired in series with the coil.

In a PLC wiring diagram, the PLC is connected to power lines L1 and L2(see Figure 28). The field inputs are connected to L1 on one side and to themodule on the other. The common, or return, connection from the inputmodule goes to L2. The output module receives its power for switching theload from L1. Output terminal 030 is connected in series with the starter coiland its overloads, which go to L2. The output module also directly connectsto L2 for proper operation. Note that, in the motor control circuits wiringdiagram (see Figure 29), the PLC output module is wired directly to thestarter coil.

Although the three-phase motor has a three-wire control circuit, its corre-sponding PLC control circuit has only two wires. This two-wire configurationis similar to a three-wire configuration because it provides low-voltagerelease; however, it does not provide low-voltage protection. Referring to

sserddAO/IeludoM

epyT kcaR puorG lanimreT noitpircseD

tupnI 0 0 0 )CN(BPpotS0 0 1 BPtratS0 0 2 0 0 3

tuptuO 0 3 0 1MrotoM0 3 1 0 3 2 0 3 3

L2L1 L1L2

001

000

Start

Stop M OL

M030

Start001

Stop000

M030

030



Figure 28. PLC wiring diagram of a three-phase motor.

Figure 29. Motor control circuits wiring diagram.

Motor

3

2

T1 T2 T3

L1 L2 L3

M

OL

PLCOutput

030

FromL1

To L2

PLC

M

L1

000

001

L2

Stop

Start

Inputs

L1

L2

L3

MOL

F

Outputs 030

Common

Common

Power

33



Figure 29, the starters seal-in contacts (labeled as 3| |2) are not used andare shown as unconnected. If the motor is running and the overloads open,the motor will stop, but the circuit will still be ON. Once the overloads cooloff and the overload contacts close, the motor will start again immediately.Depending on the application, this situation may not be desirable. Forexample, someone may be troubleshooting the motor stoppage and the motormay suddenly restart. Making the auxiliary contact an input and using itsaddress to seal the start push button can avoid this situation by making thetwo-wire circuit act as a three-wire circuit (see Figure 30). In this configura-tion, if the overloads open while the motor is running, the coil will turn offand their auxiliary contacts will break the circuit in the PLC.

Figure 30. Two-wire circuit configured as a three-wire circuit.

FORWARD/REVERSE MOTOR INTERLOCKING

Figure 31. Hardwired forward/reverse motor circuit.

L2L1 L1L2

001

000

Start

M

Stop M OL

002

M002

Start001

Stop000

M030

030

Figure 31 illustrates a hardwired forward/reverse motor circuit with electricaland push button interlockings. Figure 32 shows the simplified wiringdiagram for this motor. The PLC implementation of this circuit should

L1 L2

Stop Rev R M1

For PL1M1

All OLsFor

F

F M2

Rev PL2M2

R



include the use of the overload contacts to monitor the occurrence of anoverload condition. The auxiliary starter contacts (M1 and M2) are notrequired in the PLC program because the sealing circuits can be programmedusing the internal contacts from the motor outputs. Low-voltage protectioncan be implemented using the overload contact input so that, if an overloadoccurs, the motor circuit will turn off. However, after the overload conditionpasses, the operator must push the forward or reverse push button again torestart the motor.

Figure 32. Forward/reverse motor wiring diagram.

For simplicity, the PLC implementation of the circuit in Figure 31 includesall of the elements in the hardwired diagram, even though the additionalstarter contacts (normally closed R and F in the hardwired circuit) are notrequired, since the push button interlocking accomplishes the same task. Inthe hardwired circuit, this redundant interlock is performed as a backupinterlocking procedure.

Figure 33 shows the field devices that will be connected to the PLC. The stoppush button has address 000, while the normally open sides of the forward andreverse push buttons have addresses 001 and 002, respectively. The overloadcontacts are connected to the input module at address 003. The output

L1 L2 L3

M

F R1 2 3 1 2 3

T1 T3

T2

F

3

2R

3

2

OL

35



devicesthe forward and reverse starters and their respective interlockingauxiliary contactshave addresses 030 and 032. The forward and reversepilot light indicators have address 031 and 033, respectively. Additionally,the overload light indicators have addresses 034 and 035, indicating that theoverload condition occurred during either forward or reverse motor opera-tion. The addresses for the auxiliary contact interlocking using the R and Fcontacts are the output addresses of the forward and reverse starters (030 and032). The ladder circuit that latches the overload condition (forward orreverse) must be programmed before the circuits that drive the forward andreverse starters as we will explain shortly. Otherwise, the PLC program willnever recognize the overload signal because the starter will be turned off inthe circuit during the same scan when the overload occurs. If the latchingcircuit is after the motor starter circuit, the latch will never occur because thestarter contacts will be open and continuity will not exist.

Table 12 shows the real I/O address assignment for this circuit. Figure 34shows the PLC implementation, which follows the same logic as thehardwired circuit and adds additional overload contact interlockings. Notethat the motor circuit also uses the overload input, which will shut down themotor. The normally closed overload contacts are programmed as normallyopen in the logic driving the motor starter outputs. The forward and reversemotor commands will operate normally if no overload condition existsbecause the overload contacts will provide continuity. However, if anoverload occurs, the contacts in the PLC program will open and the motorcircuit will turn OFF. The overload indicator pilot lights (OL Fault Fwd andOL Fault Rev) use latch/unlatch instructions to latch whether the overloadoccurred in the forward or reverse operation. Again, the latching occursbefore the forward and reverse motor starter circuits, which will turn off due


L1 L2

Stop Rev R M1

For PL1M1

All OLsFor

F

F M2

Rev PL2M2

R





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0 0 1 )ONderiw(BPdrawroF0 0 2 )ONderiw(BPesreveR0 0 3 stcatnocdaolrevO

tupnI 0 0 4 BPteseR/LOegdelwonkcA

tuptuO 0 3 0 )DWF(1MretratsrotoM0 3 1 1LPdrawroF0 3 2 )VER(2MretratsrotoM0 3 3 2LPesreveR

tuptuO 0 3 4 DWFnoitidnocdaolrevO0 3 5 VERnoitidnocdaolrevO0 3 6 0 3 7

L2L1L2L1

030

002

000

Reverse

Stop M

031Fwd PL1

003OL

M1030

Rev002

M1030

Stop000

Fwd001

M2032

OL003

032M2032

Rev002

M1030

Stop000

Fwd001

M2032

OL003

M1030

Rev002

PL1031

Stop000

Fwd001

M1030

OL Fwd 034

OL003

001

Forward

034

RM2

033M2032

Rev002

Stop000

Fwd001

PL033

004

ACK OL Reset Rev PL2

LOL Fwd 034

UM2032

OL Rev 035

OL003

035

LOL Rev 035

U

OL Fault Fwd

OL Fault Rev

ACK OL 004

ACK OL 004

37



to the overload. An additional normally open acknowledge overload resetpush button, which is connected to the input module, allows the operator toreset the overload indicators. Thus, the overload indicators will remainlatched, even if the physical overloads cool off and return to their normallyclosed states, until the operator acknowledges the condition and resets it.

Figure 35 illustrates the motor wiring diagram of the forward/reverse motorcircuit and the output connections from the PLC. Note that the auxiliarycontacts M1 and M2 are not connected. In this wiring diagram, both theforward and reverse coils have their returns connected to L2 and not to theoverload contacts. The overload contacts are connected to L1 on one side andto the PLCs input module on the other (input 003). In the event of an overload,both motor starter output coils will be dropped from the circuit because thePLCs output to both starters will be OFF.

Figure 35. Forward/reverse motor wiring diagram.

REDUCED-VOLTAGE-START MOTOR CONTROL

Figure 36 illustrates the control circuit and wiring diagram of a 65% tapped,autotransformer, reduced-voltage-start motor control circuit. This reduced-voltage start minimizes the inrush current at the start of the motor (locked-rotor current) to 42% of that at full speed. In this example, the timer must beset to 5.3 seconds. Also, the instantaneous contacts from the timer in lines 2and 3 must be trapped.

L1 L2 L3

L1

To PLCInput 003

L1

L2L1

F

M

FWD REV

R

OL

3

2M1

3

2M2



Figure 36. (a) Hardwired relay circuit and (b) wiring diagram of a reduced-voltage-startmotor.

Figure 37 illustrates the hardwired circuit with the real inputs and outputscircled. The devices that are not circled are implemented inside the PLCthrough the programming of internal instructions. Tables 13, 14, and 15 showthe I/O assignment, internal assignment, and register assignment, respec-tively. Figure 38 illustrates the PLC implementation of the reduced-voltage-start circuit. The first line of the PLC program traps the timer with internaloutput 1000. Contacts from this internal replace the instantaneous timercontacts specified in the hardwired control circuit. This PLC circuit imple-mentation does not provide low-voltage protection, since the interlockingdoes not use the physical inputs of M1, S1, and S2. If low-voltage protectionis required, then the starters auxiliary contacts or the overload contacts canbe programmed as described in the previous examples. If the auxiliarycontacts or the overloads are used as inputs, they must be programmed as

StopStart OL

L1 L2

TR1

TR1

S1

S1

TR1

TR1

TR1

S2

S2

S1 M1

M1

T1

T2

T3

M

S1S2

M1

S1S2

M1

S1S2

M1

L1

L2

L3

65%

1

2

3

4

5

6

(a)

(b)

39




Table 14. Internal address assignment.



sserddAO/IeludoM


tupnI 0 0 0 )CN(BPpotS0 0 1 )ON(BPtratS

tuptuO 0 3 0 1MretratSrotoM0 3 1 1S0 3 2 2S

StopStart OL

L1 L2

TR1

TR1

S1

S1

TR1

TR1

TR1

S2

S2

S1 M1

M1

1

2

3

4

5

6

retsigeR noitpircseD0004 3.5rofces1.0esabemit,35eulavretsigerteserP

)1001situptuoremit(ces1004 1001tuptuoremitrofretsigerdetalumuccA

eciveD lanretnI noitpircseD 0001 tiucricremitparT

remiT 1001 remiT



normally open (closed when the overloads are closed and the motor isrunning) and placed in series with contact 1000 in line 3 of the PLC program.If the overloads open, the circuit will lose continuity and M1 will turn OFF.


AC MOTOR DRIVE INTERFACE

A common PLC application is the speed control of AC motors with variablespeed (VS) drives. The diagram in Figure 39 shows an operator station usedto manually control a VS drive. The programmable controller implementa-tion of this station will provide automatic motor speed control through ananalog interface by varying the analog output voltage (0 to 10 VDC) to thedrive.

The operator station consists of a speed potentiometer (speed regulator), aforward/reverse direction selector, a run/jog switch, and start and stop pushbuttons. The PLC program will contain all of these inputs except thepotentiometer, which will be replaced by an analog output. The required inputfield devices (i.e., start push button, stop push button, jog/run, and forward/reverse) will be added to the application and connected to input modules,rather than using the operator stations components. The PLC program willcontain the logic to start, stop, and interlock the forward/reverse commands.

L2L1 L1L2

030

001

000

Start

Stop

M1 OL

031 S1

032 S2

Trap1000

Trap1000

Start001

Stop000

S2032

Trap1000

S1031

M1030

TMR1001

Trap1000

TMR1001

S1031

M1030

Trap1000

PR: 4000 = 53AR: 4001TB = 0.1

TMR

S1031

S2032

M1030

Trap1000

TMR1001

41



Figure 39. Operator station for a variable speed drive.

Table 16 shows the I/O address assignment table for this example, whileFigure 40 illustrates the connection diagram from the PLC to the VS drivesterminal block (TB-1). The connection uses a contact output interface toswitch the forward/reverse signal, since the common must be switched. Toactivate the drive, terminal TB-1-6 must receive 115 VAC to turn ON theinternal relay CR1. The drive terminal block TB-1-8 supplies power to thePLCs L1 connection to turn the drive ON. The output of the module (CR1)is connected to terminal TB-1-6. The drives 115 VAC signal is used tocontrol the motor speed so that the signal is in the same circuit as the drive,avoiding the possibility of having different commons (L2) in the drive (thestart/stop common is not the same as the controllers common). In thisconfiguration, the motors overload contacts are wired to terminals TB-1-9and TB-1-10, which are the drives power (L1) connection and the outputinterfaces L1 connection. If an overload occurs, the drive will turn OFF

Star

tSt

op

1

2

3

4

5

6

7

8

9

10

Run

Jog

Reverse

Forward

Reverse

Spare

Spare

Forward

Spee

dPo

tent

iom

eter

TB-1

To Speed Regulator

Common (Controller)

Field Drive

OL

OL

OL

CR1

CR1

115VAC

ChassisGround

Adjust

+12 V



Figure 40. Connection diagram from the PLC to the VS drives terminal block.


sserddAO/IeludoM


tupnI 0 0 0 tratS0 0 1 potS0 0 2 rotcelesesrever/drawroF0 0 3 rotcelesgoj/nuR

tuptuO 0 3 0 )evirdmorf1L(elbaneevirDCAV511 0 3 1

0 3 20 3 3

tuptuO 0 3 4 drawroFtcatnoC 0 3 5 esreveR

0 3 60 3 7

golanA 0 7 0 CDV010ecnereferdeepsgolanAtuptuO 0 7 1

0 7 20 7 3

1

2

3

4

5

6

7

8

9

10

Reverse

Spare

Spare

Forward

TB-1

To Speed Regulator

Common (Controller)

Drive

CR1

CR1

115VAC

ChassisGround

Adjust

+12 V

Analog Output +

Analog Output

Contact Output

L1 Out

115 VACOutput

OLsOutput

30

43



because the drives CR1 contact will not receive power from the outputmodule. This configuration, however, does not provide low-voltage protec-tion, since the drive and motor will start immediately after the overloads cooloff and reclose. To have low-voltage protection, the auxiliary contact fromthe drive, CR1 in terminal TB-1-7, must be used as an input in the PLC, sothat it seals the start/stop circuit.

Figure 41 shows the PLC ladder program that will replace the manual operatorstation. The forward and reverse inputs are interlocked, so only one of themcan be ON at any given time (i.e., they are mutually exclusive). If the jogsetting is selected, the motor will run at the speed set by the analog outputwhen the start push button is depressed. The analog output connection simplyallows the output to be enabled when the drive starts. Register 4000 holds thevalue in counts for the analog output to the drive. Internal 1000, which is usedin the block transfer, indicates the completion of the instruction.

Sometimes, a VS drive requires the ability to run under automatic or manualcontrol (AUTO/MAN). Several additional hardwired connections must bemade to implement this dual control. The simplest and least expensive wayto do this is with a selector switch (e.g., a four-pole, single-throw, single-break selector switch). With this switch, the user can select either theautomatic or manual option. Figure 42 illustrates this connection. Note that

Figure 41. PLC implementation of the VS drive.

L1 L1 L2L2Start PB1 000

StartPB1000

StopPB2001

DriveEn030

XFER OUTPR 4000Slot 7Rack 0Length 1

Fwd* Rev SEL1002

FwdSEL1002

Fwd034

Drive En030

Done1000

Run* Jog SEL2003

RevSEL1002

Rev035

Stop PB2 001Run/Jog

003Drive En

030

030 TB-1-6

TB-1-8

034

035

TB-1-4

TB-1-5

TB-1-3

TB1-2TB1-3

070

A+

*Selector switch is logic 1 (closed) in Fwd/Run position and logic 0 (open) in Rev/Jog position



the start, stop, run/jog, potentiometer, and forward/reverse field devicesshown are from the operator station. These devices are connected to the PLCinterface under the same names that are used in the control program (refer toFigure 41). If the AUTO/MAN switch is set to automatic, the PLC willcontrol the drive; if the switch is set to manual, the manual station willcontrol the drive.

CONTINUOUS BOTTLE-FILLING CONTROL

Figure 42. VS drive with AUTO/MAN capability.

In this example (see Figure 43), we will implement a control program thatdetects the position of a bottle via a limit switch, waits 0.5 seconds, and thenfills the bottle until a photosensor detects a filled condition. After the bottleis filled, the control program will wait 0.7 seconds before moving to the nextbottle. The program will include start and stop circuits for the outfeed motorand the start of the process. Table 17 shows the I/O address assignment, whileTables 18 and 19 present the internal and register assignments, respectively.These assignments include the start and stop process signals.

Start

Stop

1

2

3

4

5

6

7

8

9

10

Run

Jog

ForwardReverse

Reverse

Spare

Spare

Forward

TB-1

To Speed Regulator

Common (Controller)

Field Drive

OL

OL

OL

CR1

CR1

115VAC

ChassisGround

Adjust

+12 V

Auto

Auto

Manual

ManualSpeed

Manual

Manual

Manual

Analog Output+Analog Output

Auto

Auto

ContactOutput

L1 Out

115 VACOutput

45



Table 18. Internal output assignment.


Figure 43. Bottle-filling system.


eciveD lanretnI noitpircseDremiT 1001 tcetednoitisopretfayaledces5.0rofremiTremiT 2001 tcetedlevelretfayaledces7.0rofremiT

3001 1Mrotomdeef,tuodemit,dellifelttoB

Limit Switch

LS

FixedRollers

Outfeed Motor Drive(Always ON During Process)

M2

Feed MotorDriveM1

Photoeye DetectorPerpendicular To Bottle

Filled Bottles

Solenoid Operated ControlFluid

sserddAO/IeludoM


tupnI 0 0 0 1BPssecorptratS0 0 1 )CN(2BPssecorppotS0 0 2 )tcetednoitisop(hctiwstimiL0 0 3 )tcetedlevel(eyeotohP

tuptuO 0 3 0 1MrotomdeeF0 3 1 )NOmetsys(2MrotomdeeftuO0 3 2 lortnocdioneloS0 3 3

retsigeR noitpircseD0004 )1001(ces1.0esabemit,5eulavteserP1004 1001rofeulavdetalumuccA2004 )2001(ces1.0esabemit,7eulavteserP3004 2001rofeulavdetalumuccA



Figure 44 illustrates the PLC ladder implementation of the bottle-fillingapplication. Once the start push button is pushed, the outfeed motor (output031) will turn ON until the stop push button is pushed. The feed motor M1will be energized once the system starts (M2 ON); it will stop when the limitswitch detects a correct bottle position. When the bottle is in position and0.5 seconds have elapsed, the solenoid (032) will open the filling valve andremain ON until the photoeye (PE) detects a proper level. The bottle willremain in position for 0.7 seconds, then the energized internal 1003 will startthe feed motor. The feed motor will remain ON until the limit switch detectsanother bottle.

Figure 44. PLC implementation of the bottle-filling application.

L1 L1 L2L2Start PB1

Stop PB2

PB1000

M2031

Int31003

BottleFilled1003

PB2001

032

000

001

PE

003

LSLS002

M2031

M1030

M2031

002

SOL1

031

LS002

M2031

TMR1001

M2

030 M1

TMR

PR 400050

AR 4001TB = 0.1

TMR1001

PE003

M2031

SOL032

TMR1002

LS002

M2031

BottleFilled1003

PE003

M2031

BottleFilled1003

TMR1002

TMR

PR 400070

AR 4001TB = 0.1

47



LARGE RELAY SYSTEM MODERNIZATION

This example presents the modernization of a machine control system thatwill be changed from hardwired relay logic to PLC programmed logic. Thefield devices to be used will remain the same, with the exception of those thatthe controller can implement (e.g., timers, control relays, interlocks, etc.).The benefits of modernizing the control of this machine are:

a more reliable control system

less energy consumption

less space required for the control panel

a flexible system that can accommodate future expansion

Figure 45 illustrates the relay ladder diagram that presently controls the logicsequence for this particular machine. For the sake of simplicity, the diagramshows only part of the total relay ladder logic.

An initial review of the relay ladder diagram indicates that certain portionsof the logic should be left hardwiredlines 1, 2, and 3. This will keep allemergency stop conditions independent of the controller. The hydraulicpump motor (M1), which is energized only when the master start push buttonis pushed (PB1), should also be left hardwired. Figure 46 illustrates thesehardwired elements. Note that the safety control relay (SCR) will providepower to the rest of the system if M1 is operating properly and no emergencypush button is depressed. Furthermore, the PLC fault contact can be placedin series with the emergency push buttons and also connected to a PLC failurealarm. During proper operation, the PLC will energize the fault coil, thusclosing PLC Fault Contact 1 and opening PLC Fault Contact 2.

Continuing the example, we can now start assigning the real inputs andoutputs to the I/O assignment document. We will assign internal outputaddresses to all control relays, as well as timers and interlocks from controlrelays. Tables 20 and 21 present the assignment and description of the inputsand outputs, as well as the internals. Note that inputs with multiple contacts,such as LS4 and SS3, have only one connection to the controller.

Figure 47 shows the PLC program coding (hardwired relay translation) forthis example. This ladder program illustrates several special coding tech-niques that must be used to implement the PLC logic. Among these tech-niques are the software MCR function, instantaneous contacts from timers,OFF-delay timers, and the separation of rungs with multiple outputs.

An MCR internal output, specified through the program software, performsa function similar to a hardwired MCR. Referring to the relay logic diagramin Figure 45, if the MCR is energized, its contacts will close, allowing powerto flow to the rest of the system. In the PLC software, the internal MCR



PB1MasterStart

PB2MasterStop

PB5 SetupPB6 Reset

PB3Emergency

Stop 1

PB4Emergency

Stop 2

M1 SCR1-1

SCR1-2

MCR

OL1 OL1

4FU

OL1OL1

1M 1M 1M

M1

OL2OL2

2M 2M 2M

M2

1FU

2FU

3FU

L1

L2

L3

H1

X1 X2 X3

H2 H3 H4

L1 L2

M1

SCR1

CR1

TDR1

CR

Documents

Introduction to PLC Programming