906

5.4 PLCs: Programmable Logic Controllers

W. N. CLARE, G. T. KAPLAN, D. R. SADLON, A. C. WIKTOROWICZ (1985)

R. A. GILBERT (1995) C. W. WENDT (2005)

Reviewed by B. G. Lipták (1995)

Types of Input/Output (I/O): Discrete I/O: 120 VAC, 220 VAC, 0 to 5 V DC, 0 to 24 V DC, transistor-transistorlogic (TTL)Analog I/O: 4 to 20 mA, 1 to 5 V DC, 0 to 10 V DC, –5 to +5 V DC, –10 to +10 V DCSpecial I/O: Thermocouple, RTD, stepper motor (pulses), strain gauge, high-speedcounters, PID

Typical Specifications: Scan Time per 1000 Words (1 K) of Logic: 0.1 to 50 ms, depending on manu-facturer and enhanced software featuresWord Size: 4, 8, 16, or 32 bit (typical)Amount of I/O: Nano PLC—under 15 I/O, Small or Micro PLC—15 to 128 I/O;Medium PLC—128 to 512 I/O; Large PLC—512 and greater I/OSize of Memory: Small PLC—256 to 2 K words (K = 1024 bits, bytes, or wordsof digital data); medium PLC—2 K to 12 K words; large PLC—12 K words andlargerType of Memory: CMOS (complementary metal oxide semiconductor) RAM (ran-dom access memory); BBRAM (battery-backed RAM); EEPROM (electricallyerasable PROM); and Flash memoryEnvironmental Conditions: 0 to 60°C (32 to 140°F), relative humidity, to 95%noncondensing, 115 VAC ± 15% and 230 VAC ± 15%

Costs: Nano PLC Hardware: Basic PLC with CPU, 12 I/O points costs $99 to $200Small PLC Hardware: Basic PLC with CPU, 2 K to 8 K RAM memory, 20 to 40I/O points costs $200 to $1000 (extra discrete I/O when available costs $10/point)Medium PLC Hardware: Basic system with 256 I/O points costs $5,000 to $8,000;with 512 I/O points costs $10,000 to $20,000; and with 1024 points costs $14,000to $28,000 (RAM quantity adjusted to handle designated I/O count)Large PLC Hardware: $15,000 to $70,000 (cost driven by network and informa-tion display requirements)Software and Engineering: Software costs range from 50 to 100% of the hardwarecost. System engineering costs and documentation costs each range from 25 to50% of hardware cost. Therefore, the total cost (without installation labor) is abouttwice (if not more) the hardware cost.

Support Equipment Cost: Handheld programmers cost $100 to $500; PC-supported programming soft-ware costs $2500 to $5000

Partial List of PLC Suppliers: ABB (Elsag-Bailey Controls) (www.ABB.com)Allen-Bradley/Rockwell Automation (www.AB.com)Automation Direct (www.Automationdirect.com)Danaher (Eagle Signal Controls) (www.Dancon.com)Eaton (Cutler-Hammer) (www.EatonElectrical.com)Emerson (Westinghouse) (www.EmersonProcess.com)Fuji Electric Corp. (www.FujiElectric.com)G.E. Fanuc Automation (www.GEIndustrial.com)Giddings & Lewis (www.GLControls.com)Idec Corp. (www.Idec.com)

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 907

International Parallel Machines Inc. (www.ipmiplc.com)Mitsubishi Electric (www.meau.com)Modicon/Schneider Electric (www.Modicon.com)Moeller Corp. (www.Moeller.net)Omega Engineering (www.Omega.com)Omron Electronics Inc. (www.Omron.com)Reliance Electric Co./Rockwell Automation (www.Reliance.com)Siemens (www.sea.siemens.com)Toshiba Inc. (www.Toshiba.com)Triconex/Invensys (www.Triconex.com)Uticor Technology Inc. (www.Uticor.com)

(Note: The most popular PLCs are Allen-Bradley, Modicon, Siemens, and GEFanuc.)

Partial List of HMI GE Fanuc (iFix & Cimplicity) (www.gefanucautomation.com)Software Suppliers: Rockwell Software (RSView) (www.software.rockwell.com)

Invensys (Wonderware InTouch) (www.wonderware.com)

INTRODUCTION

A programmable logic controller (PLC) is an industrially hard-ened computer-based unit that performs discrete or continuouscontrol functions in a variety of processing plant and factoryenvironments. Originally intended as relay replacement equip-ment for the automotive industry, the PLC is now used in vir-tually every industry imaginable. Though they were commonlyreferred to as PCs before 1980, PLC became the accepted abbre-viation for programmable logic controllers, as the term “PC”became synonymous with personal computers in recent decades.

PLCs are produced and sold worldwide as stand-aloneequipment by several major control equipment manufacturers.In addition, a variety of more specialized companies producePLCs for original equipment manufacturer (OEM) applications.

Typically, PLC vendors can supply large volumes ofapplication notes for their products. Most major PLC vendorsalso publish detailed articles about applications in technicaljournals and prepare papers for engineering societies andindustrial symposia on control, automation, and so forth. Eachmanufacturer’s software package usually has its own applica-tion programming techniques. Vendors also are a valuablesource of “how-to” information, providing training courses intheir local office or at the factory as well as actual hands-onexperience to help users gain familiarity with the PLC. Mostvendors offer an applications or programming manual thatprovides insight on how to use available programming fea-tures. Of course, familiarity with one brand of PLC generallyhelps the engineer learn to use other brands quickly.

HISTORY

The automotive industry fostered the development of the PLCprimarily because of the massive rewiring required each time amodel change occurred. Solid-state logic is much easier tochange than relay panels, and this advantage was reflected in thecost of installing and operating PLCs instead of traditional relay

systems. Table 5.4a lists some of the milestones in the develop-ment of PLCs. Table 5.4b provides a summary of the differencesamong various technologies that perform logic control.

Bedford Associates, the forerunner of Modicon, designedthe first PLC in 1968 for the Hydramatic Division of GeneralMotors Corporation to eliminate costly scrapping of assembly-line relays during model changeovers and to replace unreliableelectromechanical relays. The objectives of the program were to:

• Extend the advantages of static circuits to 90% of themachines in the plant

• Reduce machine downtime related to controls problems• Provide for future expansion• Include full logic capabilities, except for data reduction

functions1

Richard Morley, a Bedford engineer, is credited withthe original PLC design and the creation of ladder logicprogramming.2 Morley says the diagrams on which ladderlogic is based, however, probably originated in Germany yearsbefore to describe relay circuitry.3 Describing the technologyof the day, Morley explains, “Automatic control of industrialmetalworking and assembly operations in 1969 were mostlyby relays and clock-driven electromechanical devices. Relaysran hot and tended to beat themselves to death with theirpersistent 60-cycle hum and contact arcings. Electromechan-ical timers/sequencers were much like enlarged music boxmechanisms and were maintenance headaches.”4 Describingthe early years of the PLC, Morley recounts, “We had somereal problems in the early days of convincing people that abox of software, albeit cased in cast iron, could do the samething as 50 feet of cabinets, associated relays, and wiring.”

Morley recounted that in 1969, “all computers required aclean, air-conditioned environment, yet were still prone tofrequent malfunctions. … Thus, even though PLCs were andare special, dedicated computers, considerable effort was madeto not identify PLCs as computers due to the poor reliabilityof computers and the fact that they were not things procured

© 2006 by Béla Lipták

908 PLCs and Other Logic Devices

by manufacturing operations.”4 Unlike computers of that era,the programmable controller was designed to be reliable. Mor-ley explains, “No fans were used, and outside air was notallowed to enter the system for fear of contamination andcorrosion. Mentally, we had imagined the programmable con-troller being underneath a truck, in the open, and being drivenaround—driven around in Texas, driven around in Alaska.Under those circumstances, we wanted it to survive. The otherrequirement was that it stood on a pole, helping run a utilityor a microwave station which was not climate controlled, andnot serviced at all. Under those circumstances, would it workfor the years that it was intended to be? Could it be walledin? Could it be bolted in a system that was expected to last20 years?” Upon arriving for a client demonstration, the PLC

was accidentally dropped on the floor. The client was surprisedthat not only did the PLC work perfectly, but that Modiconstaff expected it to work after being dropped.3

Bedford Associates demonstrated the Modicon 084 solid-state sequential logic solver at GM in 1969.5 The fundamentaladvantage that PLCs introduced was that they used program-ming rather than rewiring to configure for a new application.PLC programming could be done much quicker than the tra-ditional rewiring methods. In addition, solid-state devicesoffered greater reliability, required less maintenance, and hada longer life than mechanical relays, all in a smaller footprint.2,6

The 1970s and 1980s were dominated by proprietary sys-tems and software. Odo Struger, often called the father of Allen-Bradley’s PLC, is credited with the PLC acronym.5 The early

TABLE 5.4aHistory of Programmable Logic Controller (PLCs)

1968 PLC designed for General Motors Corporation to eliminate costly scrapping of assembly-line relays during model changeovers.

1969 First commercial PLCs manufactured for automotive industry as electronic equivalent of relays.

1971 First application of PLCs outside the automotive industry.

1972 Allen-Bradley (AB) adds the first computer interface to its PLC, read-write controller, and off-line software documentation package.

1973 Introduction of “smart” PLCs for arithmetic operations, printer control, data move, matrix operations, CRT interface, etc.

1974 First CRT-based program panel for PLC.

1975 Introduction of analog proportional, integral, derivative (PID) control, which made possible the accessing of thermocouples, pressure sensors, etc.

Allen-Bradley coins the PLC acronym.

1976 First use of PLCs in hierarchical configurations as part of an integrated manufacturing system.AB introduces Remote I/O.

1977 Introduction of PLCs based on microprocessor technology.

1978 PLCs gain wide acceptance; sales approach $80 million.USDATA introduced REACT, the industry’s first microprocessor-based, user-configurable, interactive color graphics workstation for use with PLCs.2

1979 Integration of plant operation through a PLC communications system with AB’s DataHighway.

1980 Introduction of intelligent input and output modules to provide high-speed, accurate control in positioning applications.

1981 IBM introduces PC/XT using DOS.Data highways enable users to interconnect many PLCs up to 15,000 feet from each other. More 16-bit PLCs become available. Color graphics CRTs are available from several suppliers.

1982 Larger PLCs with up to 8192 I/O become available.

1983 “Third-party” peripherals, including graphics CRTs, operators’ interfaces, “smart” I/O networks, panel displays, and documentation packages, become available from many sources.

1985 IBM-PC programming terminal introduced.AB PLC-5/15 introduced with built-in RIO and peer-to-peer DH+ directly on processor card.6

Intellution introduces the FIX software, the first DOS-based process control system for IBM PCs.2,5

1986 Various other PC HMI software introduced.

1990 Wonderware introduces InTouch, the first MS Windows-based HMI application, and starts a major transition to Windows-based “open” technology.2,5

1991 Small (Brick) PLCs enter the market place.

1992 Networked block I/O developed.Moore Products manufactures APACS, the first control system using IEC 1131-3 standard allowing DCS function blocks, PLC ladder, sequential

function chart, and structured text to be used and combined within a single controller.2

1993 Ethernet and TCP/IP connectivity appear on PLCs.

1994 AB launches DeviceNet, an open device-level network.

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 909

1980s saw a cross pollination between PLCs and distributedcontrol systems (DCSs). PLCs, for example, had alreadybegun incorporating distributed control functions so they couldbe linked much in the way that DCSs were linked. Buildingon the trend, software companies sprang up in great numbersduring this time.2

During the 90s, standardization and open systems werethe main themes. Ethernet peer-to-peer networking becameavailable from virtually all PLC manufacturers. EEPROMand Flash memories replaced the EPROMs of the 1980s. PCsand CRTs in general became accepted and started to replaceswitches and lights on control system panels. Small PLCscalled “Bricks” were introduced to the marketplace. Redun-dancy for PLCs became a standard product. Many PLC sys-tems were upgraded to address Y2K concerns. The first fewyears of the 21st century have seen a consolidation of PLCmanufacturers. Very small nano or pico PLCs, some as smallas industrial relays, have appeared. Safety PLCs featuringtriple redundancy were introduced. LCD base operator inter-face panels have largely displaced CRTs, especially on theplant floor.

PLC SIZES

Today’s PLC is at best a distant relative of the first- andsecond-generation PLCs built during the 1970s and 1980s.There are now many PLC sizes to select from, ranging fromnano- and micro-size devices with 12–30 I/O, to large super-visory control units with built-in PC and networking capa-bilities.7

The modern medium-sized PLC performs all the relayreplacement functions expected of it but also adds many other

functions—including counting, timing, and complex mathe-matical applications — to its repertoire. Most medium-sizedPLCs can perform proportional, integral, derivative (PID), feed-forward, and other control functions as well (see Chapter 2). Inaddition, medium-sized and large-scale PLCs have data high-way capabilities and can function well in DCS environments.

Nano PLCs

In the 1990s, small PLCs were introduced with limited func-tionality. Today’s small PLCs generally contain all of the soft-ware and networking power that used to be confined to the largerunits.8 A small PLC can be amazingly inexpensive. This small,dedicated controller is enclosed in a single-mounted hardenedcase. It is intended to be a relay replacement unit and providereliable control to a stand-alone section of a process. SmallPLCs offer many of the same functions as larger models butwith limited flexibility of adding I/O points. They are especiallysuited to limited applications with only basic communicationsrequirements. “For instance, a $99 PLC with four PID loopsand two serial ports, matched with a $79 four channel analogplug-in option card, is a cost-effective choice over traditionalrelays and a single-loop controllers.”9

BASIC PLC COMPONENTS

A PLC manufactured by virtually any company has severalcommon functional parts. Figure 5.4c illustrates a genericPLC architecture. The diagram shows a central processor,memory, I/O, power supply, and programming and peripheraldevice subsections. Each is discussed below.

TABLE 5.4bCost Advantages over Relay

Relays Solid-State Controls Microprocessor Minicomputer PLCs

Hardware cost Low Equal Low High High to low, depending onnumber of controls

Versatility Low Low Yes Yes Yes

Usability Yes Yes No No Yes

Troubleshooting maintainability

Yes No No No Yes

Computer-compatible No No Yes Yes Yes

Arithmetic capability No No Yes Yes Yes

Information gathering No No Yes Yes Yes

Industrial environment

Yes No No No Yes

Programming cost (Wiring) High (Wiring) High Very high High Low

Reusable No No Yes Yes Yes

Space required Largest Large Small OK Small

© 2006 by Béla Lipták

910 PLCs and Other Logic Devices

Central Processor Unit (Real Time)

The central processor unit (CPU) performs the tasks neces-sary to fulfill the PLC function. Among these tasks are scan-ning, I/O bus traffic control, program execution, peripheraland external device communications, special function or datahandling execution (enhancements), and self-diagnostics.Central processing units that use microprocessor-based(VLSI) systems are both more powerful and more flexiblethan earlier technologies such as TTL or CMOS. It shouldbe noted that the CPU and memory units are consideredseparate functions.

One common method for rating how a PLC performsthese tasks is its scan time. Scan time is roughly defined as thetime it takes for the PLC to interrogate the input devices, exe-cute the application program, and provide updated signals tothe output devices. Scan times can vary from 0.1 ms per 1 K(1024) words of logic to more than 50 ms per 1 K of logic.Although scan times are often given as performance measures,there are factors that make them misleading. Word size variesfrom 4 bits to 32 bits depending on the PLC model and man-ufacturer. Special features, which vary from preprogrammeddrum times to full floating-point mathematics, have differentprocessing times and may generate longer scan times. Becauseof these variables, other performance factors must be consid-ered when selecting a PLC. The user should take into accountthe application as well as the speed of the controller. Generallyspeaking, process applications need to take advantage of micro-processor power, whereas machine control applications are usu-ally more concerned with program execution speed.

Newer CPUs are equipped with communications portssuch as ASCII (RS-232), remote I/O (RIO), and Ethernetbuilt into the CPU module. Some CPUs now even supporttimed interrupts.

Redundancy The new breed of data highways can providea process with a “hot” backup PLC to take over in the eventof a PLC processor failure. Figure 5.4d depicts a PLC systemwith redundant CPUs and I/O.

Until recently, it was difficult or impossible to configurean ordinary PLC into a redundant hot backup system. Siemenswas one of the first PLC makers to introduce redundant con-figurations for PLCs. For some people, redundancy is veryimportant. Why is redundancy important? Safety in areas con-taining explosives or flammables, unattended operation, andenvironmental liabilities lead the list of concerns addressedin part by redundancy.10

Safety PLCs For several years, Triconex was one of a fewmanufacturers that offered a triplicate PLCs for use in safetysystems. Fault tolerant systems called programmable safetysystems (PSSs) are now offered by a number of PLC manu-facturers. “The big difference between a regular PLC and aPSS is that all the triple redundant features needed in bothhardware and software are built into the latter. That can bedone with regular PLCs but it’s a significant engineeringchallenge.”11 See Section 5.8 for further information on safetyPLC systems.

It’s important to acknowledge that having redundantCPUs doesn’t necessarily provide system-wide redundancy.Nor does it protect against all types of errors. A programmingerror, such as divide by zero, or an infinite loop will stop aredundant CPU just as fast as a nonredundant CPU.

Memory Unit

The memory unit12 of the PLC serves several functions. It isthe library where the application program is stored. It is alsowhere the PLC’s executive program is stored. An executiveprogram functions as the operating system of the PLC, whichserves as the program that interprets, manages, and executesthe user’s application program. Finally, the memory unit is thepart of the PLC where process data from the input modules andcontrol data for the output modules are temporarily stored asdata tables. This includes I/O status bits, counter values, timerpreset and accumulated values, and other stored constants orvariables. Typically, an image of these data tables is used bythe CPU and, when appropriate, sent to the output modules.

The basic PLC memory element is the word. A word isa collection of 4, 8, 16, or 32 bits that is used to transfer dataabout the PLC. As word length increases, more informationcan be stored in a memory location. Even with the ambiguityassociated with word length, PLCs that provide the equivalentof 32 K of 8-bit memory locations can execute applicationprograms that are moderately complicated and interact with50–100 discrete I/O points.

Memory can be volatile or nonvolatile. Volatile memoryis erased if power is removed. Obviously, this is undesirable,and most units with volatile memory provide battery backupto ensure there will be no loss of program in the event of apower outage. This is often referred to as battery-backed

FIG. 5.4cPLC architecture.

Programmingdevice

I/O Systemmodules

Outputdevices

Inputdevices

Power supply CPU Memory

I/O Bus

Solenoidsmotor starters

etc.

Switchespushbuttons

etc.

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 911

RAM. When batteries are used, they must be replaced on aregular basis, typically 1–2 years. Some small PLCs use alarge capacitor instead of a battery to avoid this maintenanceissue.

Nonvolatile memory does not change state on loss ofpower and is used in cases in which extended power outagesor long transportation times to job sites (after program entry)are anticipated. Core memory was widely used in the 60s andearly 70s but is no longer used. In the late 70s a solid-statenonvolatile memory called programmable read-only memory(PROM) became popular. The user programmed the PROMby applying a high current pulse in order to fuse internallinks to set individual bits to 1 or 0. This procedure wasirreversible.13 This required a separate piece of equipmentcalled a PROM burner. Then erasable PROMs became avail-able. Like the PROMs, EPROMs could be programmed by

the user. The advantage was that EPROMs could be erasedoptically, through exposure to ultraviolet light through aquartz window on the chip, and then rewritten electrically. Adrawback to EPROM devices was that they had to be removedfrom the equipment to be reprogrammed.

EEPROM and Flash Memory New memory technologies,electrically erasable (or Alterable) PROMs (EEPROMs,E2PROMs, or EAPROMs) and Flash memories, became avail-able in the 90s and have essentially replaced the earlier types.EEPROM and Flash memories can be electrically repro-grammed by the user, but without removal from the system,and without the use of exterior programming devices such asPROM programmers.14 Flash is also used for the firmwareused by PLCs and intelligent modules. Revision changes to

FIG. 5.4dHot backup PLC networking. (Courtesy of Allen-Bradley.)

Hostcomputer

LA-120computerterminal

10 MBWinchester

diskLDT

terminal

T4 industrialterminal

SparePLC-3

#3

7890 micro 11supervisorycomputer

1797engineering

terminal

1797supervisorterminal

LA-120reportprinter

LA-120alarm

printer

1797operatorterminal

Terminalhighway

Systemcontrolroom Programming

highway

Data acquisition highway

Backup dataacquisition

highway

Area 1raw

mtls.

Area 2premix

Area 3mixing

Area 4cooking

PID

AI

DI

DO

PID

AI

DI

DO

PID

AI

DI

DO

PID

AI

DI

DO

PID

AI

DI

DO

PID

AI

DI

DO

PID

AI

DI

DO

PID

AI

DI

DO

Area 5drying

Plant

Centralcontrolroom

Remote I/O

Plantareas

Area 6processutilities

Area 7packaging

Area 8productstorage

MainPLC-3

# 1

BackupPLC-3

# 1

MainPLC-3

# 2

BackupPLC-3

# 2

© 2006 by Béla Lipták

912 PLCs and Other Logic Devices

the firmware can be “Flash upgraded” in the field, withoutequipment disassembly. EEPROM and Flash memory are thecurrent choices for nonvolatile memory.

I/O Systems

The I/O group includes all types of modules, from dry contactmodules to intelligent I/O to remote I/O capabilities. Inputsare defined as real-world electrical signals that give the con-troller real-time status of process variables such as level, pres-sure, flow, temperature, weight, analysis results, position, orstatus (hand switches, pushbuttons, alarm contacts, and so on).These signals can be analog or digital, low or high frequency,maintained or momentary. Typically, they are presented to thePLC as a varying voltage, current, or resistance value.15,16

Based on the status sensed or values measured, the PLC con-trols various output modules to operate devices such as valves,motors, pumps, and alarms. PLC manufacturers are providingmore and more types of input and output capabilities in theirproducts. There are, however, many third-party peripheralsthat aid the PLC in interfacing to the field devices.

One of the most important functions of the I/O is itsability to filter, condition, and isolate real-world signals(0–120 VAC, 0–24 V DC, 4–20 mA, 0–10 V, and thermocou-ples) and convert these to the low signal levels (typically 0–5V DC max) in the PLC I/O bus. This is accomplished by useof optical isolators. A light-emitting diode (LED) generatesa light that optically turns on a phototransistor. Because theonly thing normally connecting the input to the output islight, there is an excellent isolation between input and outputvoltages. Only the electrical breakdown of the isolator (typ-ically over 1000 V) limits the voltage isolation achieved withthese devices. Typical discrete I/O schematics are shown inFigures 5.4e, 5.4f, and 5.4g.

Most I/O systems are modular in nature; that is, a systemcan be arranged by use of modules that contain multiples ofI/O points. These modules can be composed of 1, 4, 8, 16,or 32 points and plug into the existing bus structure. Thebus structure is really a high-speed multiplexer that carriesinformation back and forth between the I/O modules andthe central processor unit. Higher point densities are possi-ble, but their selection may involve a trade-off in wire sizeused, as well as the ease of wire harness installation to themodule.

In small PLCs, the CPU and I/O are generally containedin a single enclosure and may or may not be expandable.These have been nicknamed “Bricks” because the size of theenclosure is similar to the size of a brick. In most mediumand large PLCs, plug-in I/O modules are used to convert theI/O signal level to one that is compatible with the bus archi-tecture. These modules can be composed of 1, 4, 8, 16, or 32points, depending on the manufacturer’s standard design. Forsmall projects (20–256 I/O), I/O requirements are usually easyto define and group. A systematic approach is required formedium-sized projects (256–1024 I/O), however, in order toavoid confusion of I/O allocation. Obviously, the organization

of I/O for large systems (1024 I/O and above) requires carefulplanning.

PLC manufacturers are typically very good at backwardcompatibility. When new models of CPUs are introduced,

FIG. 5.4eTypical input area; typical AC input unit.4

FIG. 5.4fElectrical optical isolator.4

Electricaloptical

isolationFilter

Electricaloptical

isolationFilter

Electricaloptical

isolationFilter

Electricaloptical

isolationFilter

LS−1

LS−2

Stop

Start

Inputbus

To theprocessor

Address lines

Logiccircuitry

resholddevice

Photoisolator

To I/O Bus

Remotecontact

Circuitstatus

indicator

Commonexternal

AC source

Hot115 V

Filter

LED Light sensitivetransistor

Signaloutput

Signalinput

Light path

Optical

Isolator

On

OffInput

On

Off Output

Filtering

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 913

they are generally able to use the I/O modules made yearsbefore. For example, at least one PLC manufacturer’s currentPLC still works with I/O modules introduced nearly 30 yearsago — an impressive feat considering the advances in tech-nology during that time span. In other cases, when manufac-turers have changed lines, other companies have stepped into support and expand the older PLC equipment.

Some manufacturers assign I/O memory locations based onI/O type and position in the rack. Care must be taken in thesesystems when adding I/O to these systems, as it might cause anautomatic readdressing of existing I/O modules. Because theprogram doesn’t alter itself, it would need to be updated for itto function like it did before the I/O module addition.

Local I/O There are at least three different types of racks:local, expansion, and remote. A local I/O base can hold theCPU module, power supply, and optionally I/O modules.Expansion racks that are close by (usually within the samepanel) are either plugged into the side of the local I/O baseor connected by a short parallel cable. Standard cabling isprovided for connection from the remote I/O base to the PLCprocessor. These cables are generally high-speed serial cablesto simplify installation. The PLC processor generally plugsinto a dedicated slot in the rack. Some manufacturers allowmultiple processors in a single rack.

The I/O base (rack or housing) is used to hold the I/Omodule in place and to provide a termination point for thewiring. The bases may be mounted anywhere in the controlenclosure; however, there are cable length and free spacerequirements that must be met. Some racks are sized to mountdirectly into a standard 19-in. rack enclosure. The majorityof bases mount horizontally to allow proper module cooling.Some manufacturers define how much space must bereserved around the rack to permit proper circulation. A ter-minal strip is built into the mounting base for field connec-tions so that no wiring need be disturbed in order to removeor replace a module. These bases typically hold various quan-tities of I/O — anywhere from 1 to 128 I/O points.

Some PLC vendors provide wire-way as part of the rackstructure to “clean up” the panel design. As each new gen-eration of I/O becomes increasingly dense, the systemsbecome progressively harder to wire. Interposing terminalblocks between the field connections and the rack maybecome standard in the future. Some manufacturers alreadyoffer prewired field terminals that mount in a panel and areequipped with a cable that plugs directly onto the I/O module.

Digital Inputs Pushbuttons, limit switches, or even electro-mechanical relay contacts are familiar examples of digital,contact closure type signals. Input modules are typically tran-sistor-triggered and have built-in time delays to protectagainst contact bounce. The input signal from a field device(limit switch) has to be energized for some amount of timein order for the module to notify the processor of a true “on”condition. The modules are available in a number of voltagesincluding DC voltages (14, 15–30, 24, 120, 240), TTL(5 V DC), and AC voltages (24, 48, 120, 240). They come in8, 16, or 32 points per module.

Discrete I/O modules come equipped with an LED indi-cator light to indicate the status of each I/O point in themodule (on or off) for troubleshooting. The input moduleLED indicates field side status of the pushbutton.

Modules, especially high-density modules, often share acommon neutral between multiple inputs. This means that theinput voltages must be referenced to the same neutral. ACsignals powered from different power sources cannot be mixedwithin the shared group. Using a different module for thedifferent sources or using isolating relays solves the problem.

Digital Outputs Digital or discrete outputs (DOs) can beused to power pilot lights, solenoid valves, or annunciatorwindows (lamp box). The discrete output module uses asolid-state switch (triac) to power a field device, such as amotor starter, a valve, or lights. Outputs are available forvoltage ranges of 5–240 V at currents up to 5 A, with typical120 V outputs operating at 1–2 A maximum. Solid-statedrivers of this type are not intended to drive large loadsdirectly (e.g., a large motor starter). Highly inductive loadsor those with a high surge current may also require an inter-posing dry contact relay in order to power the field deviceor a resistor-capacitor (RC) network to control transients.

FIG. 5.4gTypical output area; typical AC output unit.4

StorageElectrical

opticalisolation

Powerdrives

MS1

StorageElectrical

opticalisolation

Powerdrives

MS2

StorageElectrical

opticalisolation

Powerdrives

SOL1

StorageElectrical

opticalisolation

Powerdrives

SOL2

Addresslinesfrom

memoryOutput bus

From the processor

Remotedevice

Fuse

Blown fuse indicator Circuit status indicator

Commonexternal

Hot115 V

AC source

FilterSignal

Varistor

© 2006 by Béla Lipták

914 PLCs and Other Logic Devices

Both manufacturers and third-party vendors offer drycontact modules. These modules solve the problems nor-mally associated with triacs: low power and uncertainty offailure state. Triac outputs also “leak” some current whenthey are off. When attached to high impedance devices suchas Sonalerts or LED pilot lights, this leakage current cancause continuous or intermittent operation. A voltage-drop-ping resistor can be added across the load to solve this, butthe resistors are often large and run hot. Some designerssimply add inexpensive isolation relays as needed to addressthis problem.

Manufacturers rate their equipment output current at dif-ferent temperatures. All current ratings for solid-state outputswill vary with ambient temperatures. Therefore, one shouldbe sure to check the PLC output rating for each applicationand manufacturer. Built-in fusing on output modules isbecoming standard in the industry and provides good protec-tion for overload conditions. The type of fuse depends on themodule, and the way in which fuses are accessed varies fromone PLC manufacturer to another. Some wiring arms haveindividual fuses for each output.

Discrete I/O modules come equipped with an LED indi-cator light to indicate the status of each output on the module(on or off) for troubleshooting. The output module LEDgenerally indicates logic side status.

Analog Inputs Analog modules are sometimes referred to asA/D (analog-to-digital) modules. They provide optical isolationfor electrical noise protection and are typically arranged in aquad module or an eight-point module. The analog I/O systemis designed to interface with analog field devices such as flow-meters, pressure transmitters, and valve positioners. This sys-tem accepts inputs of 0–5 V DC, 0–10 V DC, –5 to +5 V DC,–10 to +10 V DC, 0–20 mA, or 4–20 mA. Precisions of 1 in4,096 (12-bit), 1 in 32,768 (15-bit), or 1 in 65,536 (16-bit) arecommonly available.

Direct thermocouple and RTD input modules typicallyaccept eight to ten points each. On-board cold junction com-pensation is often included with thermocouple input modules,and many different types of thermocouples (B, C, E, J, K, N,R, S, T ), RTD (100 Ω platinum, 120 Ω nickel, 10 Ω copper),and mV signals (0–50 mV, 0–100 mV) can be accommodated.

Analog Outputs Analog outputs can drive signals to vari-able-speed drives, control valves, and other analog devices.The output analog module can output a signal of 4–20 mAcurrent loop as well as 0–10 V DC, 0–5 V DC, –5 to +5 VDC, and –10 to +10 V DC. Most source their power from theback plane. Some loops require external loop power supplies.

Register I/O Systems One additional type of input signal,the register input, reflects the computerized nature of the PLC.The register input is particularly useful when the processcondition is represented by a collection of digital signals deliv-ered to the PLC at the same time. A binary-coded decimal

(BCD) thumbwheel is a good example of an input device thatis compatible with a register input port. If the thumbwheelrepresents three and one-half digits of process data, then all14 data output wires from the thumbwheel would providetheir digital signal directly to the PLC register input unit,which would, in turn, signal the condition and transfer thedata to the central processor unit.

The register I/O system provides direct interface to multi-bit data field devices, such as thumbwheel switches, positionencoders, and digital readouts to the PLC. These devices aretypically TTL level, which allows interface to other types ofelectronic hardware as well. Intelligent I/O and other special-purpose I/O requirements are becoming increasingly common.

Specialty Modules Specialty modules are designed tosolve a single interface problem. X/Y positioner modules canbe included in this category, as well as servo axis controllers,stepper motor outputs, and even maintenance access mod-ules. These modules are a further extension of the distributedtechnology. Combination I/O modules combine either digitalor analog inputs and outputs on a single module. They areparticularly useful in small systems.

High-Speed Modules Fast-response I/O is currently offeredin both discrete and analog versions. Discrete rapid responsemodules are facilitated by the PLC logic, but the output doesnot rely on ladder logic scan times to get updated. High-speedanalog modules provide quicker analog-to-digital and digital-to-analog (D/A) conversions. This gives PLCs the ability tocontrol faster PID loops and to make analog measurementsof assembly-line parts (weight, for example). High-speedpulse counter modules provide the ability to interface withturbine meters, stepper motors, and optical encoders. High-speed pulses cannot normally be interfaced to PLC inputsbecause of the scan time of the ladder logic. These modulesprovide an interface that does not rely on the scan time, sothe PLC is able to monitor fast pulses that indicate positionor flow. They can often be configured as high-speed upcounters, up/down counters for quadrature use, high-speedinterrupts, pulse catchers for very fast pulses, and adjustablefilter inputs. PID loop coprocessor and temperature controllermodules are also available.

Intelligent I/O Modules Intelligent I/O modules include allmodules that are able to perform processing functions.Because the tasks performed by the PLC are further distrib-uted, greater speed and reliability for the overall system canbe realized. Intelligent I/O modules give the PLC multipleadditional capabilities, which may include memory storageand retrieval, computing tasks, and communications. Memorymodules provide additional room to store data points, alarmmessages, lookup tables, and the like. This approach leavesthe main operating memory free for the control tasks. Com-puting modules give PLCs the ability to perform true computerfunctions using a language like Basic. Basic modules can beused for a variety of things, such as complex math abilities.

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 915

Because they usually come with two or more serial ports, theycan be used for handling specialized ASCII protocols and theycan demultiplex a multiplexer data stream. Of course they canbe programmed as Modbus masters or slaves. Again, the real-time tasks are left in the main memory, but tasks such assetpoint calculation, formation of data, and some operatorinterface tasks may be placed in the computer module. Otherin-rack modules include complete computers, state languagecoprocessors, and real-time interrupt modules.

Clock modules that fit into the I/O bus may be consideredpart of this group. These modules provide real-time andday/date functions upon interrogation from the PLC. Mostare backed up by a battery to ensure time keeping duringpower outages.

I/O Simulators Some I/O simulators used to develop anddebug programs can be categorized in the I/O enhancementgroup. These specific devices are typically hardware modulesthat can be plugged into the PLC. I/O simulators generallylook like input cards except switches are built into the modulefor testing.

PLC Power Supply

The power supply may be integral or separately mounted.It always provides the isolation necessary to protect solid-state components from most high-voltage line spikes. Thepower supply converts power line voltages to those requiredby the solid-state components. All PLC manufacturers pro-vide the option to specify line voltage conditions (typically120 VAC, 240 VAC, or 12/24 V DC). In addition, the powersupply is rated for heat dissipation requirements for plantfloor operation. This dissipation capability allows PLCs tohave high-ambient-temperature specifications and representsan important difference between PLCs and PCs for industrialapplications.

The power supply drives the I/O logic signals, the centralprocessor unit, the memory unit, and possibly some periph-eral devices. As I/O is expanded, some PLCs may requireadditional power supplies in order to maintain proper powerlevels. The power supply generally does not provide powerto field devices, though some manufacturers provide externalterminals to permit this use. The additional power suppliesmay also be separate or part of the I/O structure. The powerrequirements of all the I/O modules and CPU or remote I/Oadapters are added together for each voltage used to makesure that the power supply is large enough.

ADDITIONAL PLC COMPONENTS

Communications Modules

Communications modules can provide the PLC with a rangeof capabilities, from simple ASCII output strings to commu-nications networking. The storage of ASCII messages for a

printer or display can be contained outside the main memoryof the PLC, and the data can be output when required. Full-system communications networking capabilities are providedwith network modules, giving the designer the ability tomultidrop PLCs off a single operator interface device or asupervisory computer. A wide variety of devices communi-cate using ASCII, including weight scales, bar code scanners,some operator interfaces, and modems. Modules included inthis category also include telephone and radio modem mod-ules. Remote I/O and peer-to-peer are specialized communi-cations modules that are discussed below.

Remote I/O

Robust industrial networks make distributed I/O feasible.Networks enable I/O modules to be located close to fielddevices, resulting in labor and material savings for wiring.8,11

Often manufacturers provide a remote I/O distribution panelor module to serve the efficient multiplexing of the moduleson the remote I/O rack back to the CPU. A remote I/O baseis similar to the local I/O base except it holds a remote I/Oadapter (or has it built-in) instead of the CPU. Most medium-sized PLCs can support several remote racks, which in turncontain 4, 8, or 16 I/O modules. It may or may not bepossible to mix various modules in a remote rack. Specificinformation about module compatibility and remote I/Omultiplexing is available from the manufacturer. This infor-mation is required to facilitate PLC selection and sizing forspecific applications.

Whereas in most systems the module has the intelligenceto communicate with the CPU, some systems require the useof serial interface modules. In any case, some provision ismade to accept register input data from the input modulesand to send this data (on or off status of field device) in serialformat to the PLC processor. Serial data are also convertedinto register data to be sent to the output module. This isnormally a proprietary protocol.

Remote I/O adapters need to be configured for a partic-ular drop, either by programming or a switch setting on themodule itself. Remote I/O is very often an RS-485-basedsystem. The end modules in the RIO network are usuallyterminated with a resistor that matches the impedance of thecable used. RIO communications speeds range from 56 K to230 Kbps. Remote I/O is broken down into two distinct types:the integral type, which allows a limited transmission dis-tance (up to 15,000 ft, or 4,500 m); and the transmitter/receiver type, which allows virtually unlimited transmissioncapability. Most PLC manufacturers and third-party periph-erals manufacturers can provide some form of either type.Technology is advancing rapidly in this area, as systemschange from fiber optics to microwave and radio transmis-sion. Many manufacturers are now offering flexible remoteI/O solutions. Often these are modular, plug-together systemsinstead of being rack based. Designed to be located next tothe wireway, they incorporate built-in terminal blocks fordirect wiring.

© 2006 by Béla Lipták

916 PLCs and Other Logic Devices

Open vs. Proprietary I/O Networks Proprietary networkshave a compelling advantage: They are easy to put together.If you buy all your network components from one supplier,even if they’re for an open network like DeviceNet, you avoidpotential configuration problems. Even so, offering both openand proprietary systems is important to most manufacturers.11

Besides proprietary remote I/O networks, a variety of openprotocols exist to enable I/O data interchange. These includeModbus, DeviceNet, Profibus, HART, and Fieldbus, amongothers. Because these are open, a number of different manu-facturers offer products based on them. Ethernet is becominga major factor by providing a backbone that can carry thediverse protocols of nonproprietary I/O communications.17

Peer-to-Peer Communications

Many PLC manufacturers have responded to the increasingindustry demands for equipment that allows communicationsamong multiple process areas.18,19,20,21 The original mastercontrol layout shown in Figure 5.4h is adequate for PLCcontrol of moderate-size applications of perhaps 100–500 I/Opoints, but as the application grows the PLC becomes over-burdened under this arrangement. The hierarchical plan illus-trated in Figure 5.4i provides a supervisory PLC controllingnetwork that reduces this burden. The plan allows for a “mas-ter” PLC that oversees the process and controls a set of “slave”PLCs that control the actual process activities in the plant.

An alternate approach is illustrated in Figure 5.4j. Thisdistributed control system allows dedicated PLCs to controlsections of the process with an interface to management. Asindicated in the figure, this interface could be to a humanoperator. This person would monitor selected data from thecollection of PLCs. Any management decisions generatedfrom this data are passed back to the PLCs by an inputterminal. Under normal conditions it is expected that eachdedicated PLC can monitor and control its section of theprocess.

Most major PLC manufacturers currently offer their ownnetwork communications for their products (e.g., DataHigh-way+, ControlNet, Modbus +, and TI-WAY). Various net-work protocols exist and efforts to generate standards aregaining momentum. Instrument and control organizationssuch as the Instrumentation, Systems, and Automation (ISA)Society constantly exert pressure on the PLC industry toagree on one or at least a few communications standards. Inany event, there are several communications philosophies ofinterest. Some of these are outlined below.

FIG. 5.4hWith master control, one PLC controls a number of relatedmachines and processes. This system is simple but may require longruns of multiple-wire cables, and the entire system is vulnerable tothe failure of the one PLC. (Courtesy of Allen-Bradley.)

Computernumerical

controlPackagingConveyorsAssemblyMachine

tool

Materialhandling Inspection Testing

PLCMaster control

FIG. 5.4iHierarchical control. (Courtesy of Allen-Bradley.)

FIG. 5.4jDistributed control. (Courtesy of Allen-Bradley.)

SupervisoryPC

PLC

PLC

PLC

PLC

PLC

PLC PLC PLC

Rawmaterials

ProcessA

ProcessB

ProcessC

ProcessD

Inspect

Package

Inspect

Manage-ment

controlManage-

mentreports

Hierarchical control

Managementinput

Machine

Machine

Machine

Machine

Machine

Process

Process

Conveyors Packaging

Test andinspection

Managementdata

PLC PLC

PLC PLC PLC PLC PLC PLC

PLC PLC PLC PLC PLC

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 917

The computer interface device group is a rapidly expand-ing section of PLC peripheral devices. These devices allowpeer-to-peer communications (i.e., one PLC connecteddirectly to another), as well as network interaction with var-ious computer systems. In fact, this group of devices willcertainly expand in number as communications standardsbecome commonly accepted and more and more products areprovided to facilitate such network interactions.22

Most PLC manufacturers have already addressed theneed for peer-to-peer communications among PLCs on adistributed network. Without this feature, every time one PLCneeds to know the status of another part of the machine orplant, it must interrupt the activities of the supervisory com-puter in order to get the information (Figure 5.4k) or interfacethrough status I/O points. The ability of one PLC to “talk”to another along the data highway greatly speeds the controlactivities of each machine and allows the supervisory com-puter to “concentrate” on its tasks.

Some PLC manufacturers and third parties are offeringuniversal communications networking based on open proto-cols such as Modbus. This is likely to be the favored methodin the future, because the need for networking differentbrands of PLCs together will increase.

More PLC vendors are adding Ethernet compatibility totheir systems. Ethernet has become the open architecture of

choice for a variety of industries. However, care must be takento ensure that the PLC is never connected to the Internet. Doingso opens the door to all sorts of security issues, especiallyhackers.11 Using a PC in combination with firewalls, encryp-tion, and passwords helps address some of these issues.23

Peripheral Devices

The popularity of PLCs has led to the creation of a strong third-party peripheral manufacturing industry. These companies arealways developing new products that assist the PLC userwith interfacing a specific application to a PLC. Two cate-gories of these products— operator stations and programming/documentation tools—are presented below. The operator sta-tions facilitate operator interface with the PLC-controlled pro-cess to monitor process variables, to alter program parameters,to conduct on-line program alterations, and to conduct trouble-shooting procedures. Programming and documentation toolsinclude products supplied by the manufacturers or made avail-able by third-party vendors.24,25

Local Operator Interface

Operational aids include a variety of resources that rangefrom switches and lights, to color graphics CRTs, to equip-ment or support programs that can give the operator specific

FIG. 5.4kPeer-to-peer communications. (Courtesy of Texas Instruments.)

Bridge

Tiwayadaptor

To otherTI networks

Tiwayadaptor

Tiwayadaptor

Robot CNC Non TIPLC

Operatorinterface

Dataexchange Programmer TI530 TI520

PM550 TI510

5TI

Computer

Datastorage

Tiway II

GatewayNetworkcontroller

GatewayTI 5100

Peer to peer communications

Gateway

To other publicindustrial networks

5TI data bus line Tiway I

5TI 5TI 5TI TI510 TI520 TI500 5TI PM550 Operatorinterface

© 2006 by Béla Lipták

918 PLCs and Other Logic Devices

access to processor parameters. In this situation the operatoris usually allowed to read and modify timer, counter, andloop parameters but not access the program itself. Some aidsfacilitate the interaction between the PLC and printers todeliver process information in a desired format. Some deviceshave the ability to set up an entire panel and plug into thePLC through external RS-232-C ports, saving enormouspanel and wiring costs.

The most basic of operator interfaces is switches andlights. These are directly wired to the input and output cardsand relay information back and forth to the PLC. Analogsignals can also be used with potentiometers, panel meters,and chart recorders. Individual loop controllers can be wiredto accept a remote set point from the PLC. Register I/O isused with BCD thumbwheels and BCD displays. Annunciatorpanels can also be used. Although just about all control panelshave some switches and lights, the cost of using a displayhas fallen to such an extent that it is often not cost-effectiveto use switches and lights as the primary interface. PLCbackup systems that are used if the PLC is being servicedstill use these simple devices.

Annuniciator panels are still used in some industries.They are light boxes that have alarm descriptions illuminatedwhen an output is activated. The PLC generally supplies oneoutput point per alarm. The lamp test, silence, and acknowl-edge functions are built into the annunciator assembly. Textdisplay units are also available in a number of sizes, whichdisplay a preprogrammed message when signaled by thePLC. Other message systems are available that can dial apreprogrammed phone number and deliver a page, a textmessage, or a verbal description of the triggering event. Someswitches and light assemblies became available in standardarrangements that connected directly using remote I/O tech-nology. Some are available that communicate using the Mod-bus protocol.

Operator stations include those provided by manufactur-ers intended to be used with their particular PLC and thoseoffered by third parties for use with either a particular brandor anyone’s PLC. These stations may include devices such astimer/counter access modules (TCAMs), loop access modules(LAMs), data terminals, color graphics consoles, computers,printers, and manual backup stations. Most PLC manufactur-ers provide an operator interface unit (OIU) designed specif-ically for their PLC. These are either part of the standardsystem or offered as an option. They are sometimes mounteddirectly on the PLC or can be panel-mounted and cabled backto the controller. Functions include access to read/write reg-ister data, simple programming, and diagnostics.

Some specialized devices, such as TCAMs, LAMs, andOIUs, provide operator interaction with PLC internal regis-ters and loop tables. This gives the systems designer theability to provide real-time changing of variables, loop tuningand inspection, manual control of analog outputs, and theability to provide batch- or menu-type information at lowcost. Communications with the PLC are multidropped overan RS-422 or a similar differential line. Unauthorized data

entry is prevented with software locks, keylock protection,or both. TCAMs and LAMs have become less popular as thecost of graphic interface panels has dropped, because theirfunctions are easily added to these newer interface panels.Some PLCs can support communications directly with dumbdata terminals. However, dumb terminals have been replacedalmost completely by PCs.

Many PLCs are able to provide communications directlyto printers. A stand-alone PLC system can often provideperformance reports, alarm logging, and the like withoutever involving a computer. This feature is usually somewhatlimited, because PLCs were designed primarily to controlthe process machine. Large amounts of data, sophisticatedprint logs, and multiple alarms are not really within therealm of a stand-alone PLC system. This type of data manip-ulation is too cumbersome and requires too much memoryfor most PLCs. In addition, many PLCs are located in theprocess floor, which is generally not a good environment forprinters.

Small touch screen panels have become very popular inrecent years. They provide basic reporting of status informa-tion and commands in a small format. Some also providebuilt-in alarm management functions. They are meant toreplace the traditional switches, lights, and meters. The maindisadvantage these small screens have is that they are funda-mentally different from the computers in the control room.The screen graphics for the local panels must be developedseparately. They generally are limited to the individual con-trol panel they are associated with.

With the advent of plant floor Ethernet and relativelyinexpensive computers, it has become practical to provide thesame screens at the local control panel that are displayed inthe control room. An industrial computer or a regular desktopcomputer in a sealed computer enclosure is used. The samesoftware that is used in the control room is loaded onto thelocal computer, which uses the local area network (LAN) tocommunicate with the supervisory control and data acquivi-sion (SCADA) server. This has many advantages. Becausethe screens are shown on the same type of hardware, theycan be simply copied from the control room computers. Obvi-ously, this helps reduce development and maintenance costs.Because these communicate with the entire plant, an alarmcan be investigated at any control panel throughout the plant.

Human-Machine Interface

Intelligent human-machine interfaces (HMIs) can be net-worked into a total distributed system to give a redundant orlocal interface to the system (Figure 5.4l). This technique canbe used to provide the control room interface, while thesupervisory computer supports its own interfaces. Anotherpowerful technique is to allow the networking of a few PLCstogether solely for the purpose of providing a single human-machine interface to all parts of the system (Figure 5.4m).This solution is ideal for small PLC users because it is veryeconomical yet allows expansion as the plant grows.

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 919

Color graphics consoles offer process graphics and com-munications facilities simultaneously to many brands of PLCs.Because of this ability, HMIs are sometimes used to tie mul-tiple PLC systems together. These systems range from thosethat can simply be purchased and put on-line with a minimumof engineering effort to those that require some programming.The basic differences are in flexibility. Those that do notrequire programming may not be able to provide the custommenus and graphics that are required. The ease of communi-cations with different types of PLCs also varies according tomanufacturer. Finally, the method of generating the graphicspages differs greatly. Most color graphics consoles offer mul-tiple graphics pages that are animated by reading data tablesin the PLCs. Operators enter the data by means of standardkeyboards, user-configurable industrial keyboards, light pens,touch screens, and the like. Different graphics pages may beselected with preformatted menus or custom menus pro-grammed by the user or the systems house. Developmentstations are often required to give the final user the ability tochange graphics menus or key commands after the initialproject is completed.

Computer systems can be made to perform human-machine interface functions. Indeed, the color graphics con-soles described in the previous paragraph are simply com-puters with standard graphics and communications softwarepackages. Most PLC manufacturers provide board-level addi-tions or modules that give the PLC the ability to conversevia the RS-232 or Ethernet with nearly any computer. Ofcourse, both the communications software and the particularapplications software must be generated to provide an inter-face. Many vendors and systems houses are providing com-munications packages for various PLCs to run on microcom-puters and PCs. These small systems offer low-cost operatorinterfaces to PLCs, providing data handling capabilities andthe ability to be networked into a true distributed architecture.Microcomputers that have the ability to multitask, accesslarge amounts of both RAM and nonvolatile memory, haveproper software support, and are able to be networked willprovide a good investment in terms of operator interfacefunctions as well as total system capability.

Printers

Printers have always been an important part of the PLCsystem both as a development tool and for handling some ofthe operator interface functions. Two types of printers arecommonly found in control rooms: dot matrix printers andlaser printers. The dot matrix printers are used to print alarmsand events as they occur. They are unique because as soonas a line of text is sent to the printer, the operator can readit. This is not possible with most types of printers currentlypopular. The laser printers are used for reports or screenprintout and are either color or black and white.

Programmers and Workstations

The programmer unit provides an interface between the PLCand the user during program development, start-up, and trou-bleshooting. The instructions to be performed during each scanare coded and inserted into memory with the programmer.

Programmers vary from small handheld units the size ofa large calculator to desktop stand-alone intelligent CRT-based units. These units come complete with documentation,reproduction, I/O status, and on-line and off-line program-ming ability.26,27 Most PLCs can use a PC as the programmingtool. A program for the PC allows it to interface with a serialor Ethernet port in the PLC.

Programming units are the liaison between what the PLCunderstands (words) and what the engineer desires to occurduring the control sequence. Some programmers have theability to store programs on floppy disks or on a hard drive.Another desirable feature is automatic documentation of theexisting program. This is accomplished by a printer attachedto the programmer. With off-line programming, the user canwrite a control program on the programming unit, then takethe unit to the PLC in the field and load the memory withthe new program, all without removing the PLC. Selection

FIG. 5.4lLocal or redundant HMI in network.

FIG. 5.4mEntry-level networking grows with user’s needs.

Intelligent HMI

Intelligentmultiplexer

Supervisorycomputer

PLC PLC PLC PLC

Data hiway

Terminals

Operator’s MMI

Intelligentmultiplexer

PLC PLC PLC PLC PLC

Supervisorycomputer

Terminal

Reportprinter

Operator’sMMI

Stage 1 Stage 2

Data hiway

Stage 3

© 2006 by Béla Lipták

920 PLCs and Other Logic Devices

of these features depends on user requirements and budget.On-line programming allows cautious modification of theprogram while the PLC is controlling the process or themachine.

Manufacturers of PLCs provide two basic programmingtools. These are handheld programmers and programmingsoftware that runs on PCs. The programming software maynot be an option if a small PLC is purchased. Dedicated CRTprogrammers, once manufactured for the sole purpose ofprogramming PLCs, have been replaced with PCs runningPLC programming software.

The handheld programmer enables the operator to entera program one contact at a time. These units are widely usedbecause they are rugged, portable, and easy to operate. Theyare very cost-effective and give an engineer the capability toenter a program and to diagnose trouble in logic and fielddevices.

The programming software on a PC provides the engineerwith a visual picture of the program in the PLC. Ladderdiagrams are drawn on the screen, just as they would bedrawn on paper. Design and troubleshooting time is reduced.With menu-driven software, programming training time isdecreased. Information is stored on either floppy disks or thehard drive. Programs can be copied from one disk to anotherand then verified without need for loading the PLC’s memory.With stand-alone programming an engineer can develop aprogram on the PC and then load it into the PLC.

PLC programming software generally provides completedocumentation capability, including ladder diagrams, cross-reference listing, I/O listing, user-defined contacts, and coilnames along with commentary above each network or rung.All of this is can be sent to a printer for hard copy documen-tation. The screen typically shows eight rungs of ladder logicby 11 contacts across. The ladder diagrams can be placedinto the real-time mode, which allows visual contact status.A whole screen of contacts and coils can be updated in asfast as a few seconds. Laptop PCs are often used for porta-bility. With a modem connection, these programs can be usedat remote locations for programming and troubleshooting.

PLC programming software may be restricted for use ononly one PC. Security keys are one way to obtain this isola-tion. Security keys are devices that plug into the back of thecomputer or a special key disk. Without the key the softwarewill not run. Some manufacturers use alternate securityschemes such as providing extra keys at group prices orissuing site license agreements. In any case, it is unlikely thata PLC manufacturer will ever allow the use of programmingsoftware on an unlimited number of PCs.

Some PC-compatible software allows the PLC to beemulated by the PC. This software is sold by the PLC man-ufacturer or a licensee and is often model-specific. If thesoftware also offers on-line programming and troubleshoot-ing characteristics, it might be usable on only a single spe-cific PLC. This restriction is achieved by means of softwareor hardware keys that come with each copy of the softwarepurchased.

On-line engineering of PLC systems can be configuredfrom a remote location like a control room with the combi-nation of the programming/documentation tools and the dis-tributed network. Systematic start-up and debugging of pro-cesses are available with this technique. Figure 5.4n depictson-line engineering as part of a distributed system.

JUSTIFICATION FOR THE USE OF PLCs

For a given control problem, several technologies besidesPLCs can be applied to achieve a solution: relays and stand-alone controllers, DCSs, and software running on PCs. PLCsdiffer from these technologies as described below.

PLCs vs. Relays and Stand-Alone Controllers

The main purpose of any control system is to get the processvariable under control effectively and reliably. When controloptions are available, several factors can be taken into con-sideration in making an implementation decision. Some ofthese factors include the controller’s cost, versatility, flexi-bility, and expandability.

Cost The ideas associated with cost are discussed firstbecause this is usually the first issue users consider whenselecting a control technology. In most cases, the two thingsinitially known about a problem are the results desired andthe budget available for fixing the problem. Unfortunately, atthe beginning of a project the understanding of the problemis often limited. If the user’s focus on the problem is toonarrow, the solution might solve only the perceived symptoms

FIG. 5.4nNetworking with on-line engineering.

DevelopmentMMI

Terminal

Alarmprinter

Operator’sMMI

Engineering

Supervisorycomputer

Reportprinter

PLC PLC PLC PLC PLC

Supervisor’sterminal

Data hiway

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 921

of the problem while the problem itself may reappear in analternate form at some other point in the process.

The budget assigned to solve a problem should be basednot only on the initial investment required for the PLC hard-ware, but also on the costs associated with labor, mainte-nance, and downtime. Purchasing a larger and therefore moreexpensive PLC might in some situations be cost-effectivewhen labor, maintenance, and downtime costs are also con-sidered. Similarly, while PLCs may be more expensive thanindividual solid-state control units, if the indirect costs arealso considered then the PLC becomes a cost-competitivealternative. As the cost of PLCs continues to drop, the costof small PLCs may be equal to or even be less expensivethan the cost of industrial relays.

Although significant, cost reductions alone are not the onlyreason that PLCs are the major replacement candidate fortraditional relay logic. Compared with electromechanical relaysystems, PLCs offer the following additional advantages:

• Ease of programming and reprogramming in the plant• A programming language that is based on relay wiring

symbols familiar to most plant electrical personnel• High reliability and minimal maintenance• Small physical size• Ability to communicate with computer systems in the

plant• Moderate to low initial investment cost• Rugged construction• Modular design

Versatility The multifunction capability of a PLC allowscontrol logic decision-making, enabling versatility rarely pos-sible with other systems. The ability to combine discrete andanalog logic is a powerful tool for the controls engineer. Thisis particularly evident in the control of batch processes. Entirestart-up and shutdown sequences can be performed by thesequencer logic, and analog logic can be brought in duringthe batch run. Control of critical start-up parameters, such astemperature and pressure, can be precisely preprogrammedfor each start-up step. Temperature stepping is easily pro-grammed, as are the feedforward calculations that are used insome polymer reactors. All of these types of PLC applicationsare currently in use today and are well documented. 28,29,30,31

Flexibility As a process goes on-line and is refined, thecontrol equipment should be easily reconfigured to accom-modate such modifications. The multifunction use of the PLChas already been discussed. In addition, digital blendingapplications, boiler control of either carbon monoxide orexcess oxygen, and some other forms of optimizing controlare also within the capabilities of PLCs. Because one com-mon device performs multiple functions in a plant, fewerspare parts are needed, and the programming language istechnician-friendly. In addition, the digital nature and self-diagnostic capabilities are strong additional justification forthe PLC.

Expandability As a process matures, it is inevitable thatenhancements will be added. These usually require moreinputs and outputs. For hand-wired relay systems this usuallynecessitates extensive panel changes, which generally areproblematic. A PLC easily accommodates the additional I/Owithout requiring changes in the existing wiring: The newpoints are merely placed in the system. If a PID loop or twois being added, no panel rework is necessary; only the wiringof the new points and some reprogramming to incorporatethem are required. Conversely, if the initially selected PLCis “tight,” additional I/O bases might be necessary. For thisreason, most manufacturers recommend sizing the system toallow for 10 to 20% expansion.

Another advantage of the PLC is that it allows piecemealimplementation of projects. Systems can be brought on-linequickly and can be gradually converted to the PLC while on-line. The ability of the PLC to be reprogrammed while oper-ating permits automation of processes that are too expensiveto shut down. This technique is valuable to new as well asto retrofit projects (revamps).32,33

PLC vs. DCS

The capabilities of PLCs and DCSs have changed to theextent that today many applications that used to be the exclu-sive province of one or the other can be handled by both.The technology differences between PLCs and DCSs haveevolved to be subtle or nonexistent. Manufacturers that hadbeen making relays for logic and interlock applications devel-oped PLCs, while DCS systems were developed by processcontrol manufacturers with substantial experience in PID-type analog control. In the past it made good sense to useeach type of controller in its area of superior experience. Ifthe bulk of the I/O was digital (discrete), the logical choicewas to use a PLC. If the I/O was mostly analog, a DCS systemwas selected. This logic is no longer universally true, andpersonal preference and end-user familiarity have becomedecisive factors in system selection. PLC and DCS compar-isons have been taking place for as long as the two systemshave existed. What’s odd about this debate is that PLCs defineonly the controllers, whereas DCSs also include the software,operator interface, and network.34

In terms of pros and cons between the two designs, PLCI/O is likely to be more rugged and PLCs are likely to handlediscrete logic faster than DCS systems. PLCs are also likelyto be more desirable because their languages, such as ladderlogic, are usually more familiar to plant personnel, and there-fore there is less resistance to using them. On the other hand,ladder logic-type languages can be undesirable in some sit-uations because they are not well suited to analog processcontrol.

PLCs tend to be sold in a more piecemeal fashion ascompared to the integrated systems approach common toDCSs. While PLCs are less expensive than DCSs, the enduser often becomes more deeply involved with programmingand systems integration issues. Nonetheless, many users find

© 2006 by Béla Lipták

922 PLCs and Other Logic Devices

PLCs to be a cost-effective solution for small to medium-sized process plants.35

Some users have overcome the limitations of PLCs bycoupling them to PCs using custom-coded programming. Thedisadvantage of this approach is that such a nonstandardizedsystem is usually understood fully only by its designer, andwhen that individual leaves the organization, the system canbe ruined. When it comes to communications redundancyand data security, the DCS systems are superior. The DCSsystems are also superior in their programming library, inadvanced or optimizing control, in self-tuning algorithms,and, particularly, in their total plant architecture and infor-mation management capabilities.

PLC vs. Personal Computers

The PC is designed to be flexible and handle thousands ofdifferent types of applications, but PLCs are especiallydesigned for control.35 PCs started showing up on the factoryfloor in the mid-80s in programming and HMI applications.However, now the PC is migrating toward actual control. Inresponse, PLC manufacturers have adapted PC technology,such as Ethernet. Some systems actually provide a “PC on acard” to plug into the PLC back plane.11 PLCs are oftenthought of as computers. To a certain extent this is true;however, there are important differences between PLCs andcomputers.

Real-Time Operation/Orientation The PLC is designed tooperate in a real-time control environment. The first priorityof the CPU is to scan the I/O for status, make sequentialcontrol decisions (as defined by the program), implementthose decisions, and repeat this procedure all within the allot-ted scan time. Most PLCs have internal clocks and “watchdogtimers” built into their operations to ensure that a softwareerror like “divide by zero” or an endless loop does not sendthe central processor into an undefined state. When the watch-dog time is exceeded, the processor shuts down in a prede-termined manner and usually turns off all outputs. In real-time systems, reliability is a big concern. PLC manufacturers’experience shows mean time between failures (MTBF) rang-ing from 20,000 to 400,000 hours. “This is far in excess ofalmost any other type of electronic or control equipment.”3,6

Environmental Considerations PLCs are designed to oper-ate near the equipment they control. This means they functionin hot, humid, dirty, noisy, and dusty industrial environments.Typical PLCs can operate in temperatures as high as 140°F(60°C) and as low as 32º F (0°C), with tolerable relativehumidity ranging from 0 to 95% noncondensing. In addition,they have electrical noise immunities comparable with thoserequired in military specifications.

Programming Languages and Techniques PLC languagesare designed to emulate the popular relay ladder diagramformat. This format is read and understood worldwide by

maintenance technicians as well as engineers. Unlike com-puter programming, PLC programming does not requireextensive special training. Applications know-how is muchmore important. Although certain special techniques areimportant for programming efficiency, they are easilylearned. The major goal is the control program performance.Another difference between computers and PLCs is thesequential operation of the PLC. Program operations are per-formed by the PLC in the order they were programmed(Figure 5.4o). This is an extremely useful feature that allowseasy programming of shift registers, ring counters, drumtimers, and other useful indexing techniques for real-timecontrol applications.

Maintenance and Troubleshooting As a plant floor control-ler, the plant electrician or the instrument technician mustmaintain the PLC. It would be highly impractical to requirecomputer-type maintenance service. To this end, PLC man-ufacturers build in self-diagnostics to allow for easy trouble-shooting and repair of problems. Most PLC components aremodular and simple to isolate; remove-and-replace (systemmodules) diagnostic techniques are usually implemented.

SUMMARY

Table 5.4p together with Figures 5.4q and 5.4r provide illus-trative summaries of the PLC characteristics discussed thusfar. Table 5.4p illustrates an I/O list for a milling application,while Figure 5.4q shows in some detail the PLC and all itskey parts. The CPU is shown in the center of the figure asthe PLC processor. Power for this unit is delivered by the

FIG. 5.4oPLC versus computer. Program structure for a PLC (A) requiressequential execution with a scan, starting with task 1 and proceed-ing through task 4. The program structure for a general-purposecomputer (B) permits task execution in any order.

Start Start

Task1

Task2

Task3

Task4 Task

1Task

2Task

3Task

4

Eventoccur

Sort

No

(A) (B)

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 923

power supply shown to its right. The programming unit con-nected directly to it on the left is the way a ladder logicprogram line, like the one shown just above the CPU, isentered into the controller.

Representations of two I/O modules are shown inFigure 5.4r. The input module is on the left of the drawing andindicates a variety of contacts directly attached to seven of theeight points on the module. The point at address zero is shownas a spare for use as a replacement or future enhancement site.The output module shown has all eight of its output addresses(from address 140 through 147) in use.

Figure 5.4q shows an example of a remote I/O rack.Although the rack shown (rack No. 2) is powered by the mainpower supply, that is not a requirement. A remote I/O rackfunctions the same way as its direct I/O counterpart does.Various modules can be inserted in the rack to match theapplication’s control needs.

PROJECT EXECUTION

Like any major enterprise, the PLC project must take intoaccount the important considerations of schedule and budget.The PLC can facilitate the transition, however, by simulta-neously pursuing several activities, thereby condensing theoverall project schedule. A review of each major activity ispresented in the following paragraphs.

Systems Analysis

The control system should be analyzed as a whole to deter-mine plant control requirements. The PLC plays an integralpart in these analyses, and its capabilities should be thor-oughly understood by the controls engineer. Vital to systemsanalysis are the process and instrument diagram (P&ID),the descriptive operational sequence, and a logic diagram

TABLE 5.4pExample of a Detailed I/O List for a Milling Machine

Input Definition Used in Rung(s)*

XO MOA Automatic 1– 46 40– 47 40–48 43 44 45

X1 Right Shot Pin In L.S. 1 3 – 3 9

X2 Left Shot Pin In L.S. 2 3 – 3 9

X3 Machine Slide Adv L.S. 3 7 – 22

X4 Machine Slide Ret L.S. 4 3 8 – 39

X5 Shot Pins Out L.S. 5 & 6 6 – 52

X6 Pump #1 3M 1

X7 Right Bore Slide Ret L.S. 7 11 46

X8 Left Bore Slide Ret L.S. 8 12 47

X9 Right Bore Slide Adv L.S. 9 3 15 – 19

X10 Left Bore Slide Adv L.S. 10 3 15 – 21

X11 Swing Clamp On P.S. 1 6

X12 Pull Clamp On P.S. 2 5

X13 Swing Clamp Off P.S. 3 16

X14 Cycle Start 2 3 – 17 – 36 – 36 –37

– 45

X15 Cycle Stop 1 2

X16 Auto Lube On/Off 40 48

X17 Lube Complete Pin – 40 – 48

X18 Hi Lube Level 37 – 40

X19 Lo Lube Level – 40 – 48

X20 Manual Light – 1 49 50 51 52

X21 Pull Clamp On/Off 49 – 52

X22 Shot Pins In/Out 50 – 52

X23 Swing Clamp On/Off 51 – 52

* Rung is a grouping of PLC instructions that controls one output or storage bit. This is represented as one section of a logic ladderdiagram.

© 2006 by Béla Lipták

924 PLCs and Other Logic Devices

or electrical schematic. Part of this evaluation will be sys-tem sizing and selection. Once the appropriate PLC isselected and purchase orders are placed, two activitiesshould begin immediately: engineering design and softwaredevelopment.

Systems Integrators One of the first decisions is whetherto perform the work in-house or have a systems integratordo it. The general trend in the industry is that systems aremore likely to be sold to OEMs and system integrators than

to end users.10 If an integrator will be used, it should beselected carefully. Some things to consider include:

• How well does the integrator know the PLC/HMI/report-ing software?

• How well does it know your industry?• How familiar is the integrator with your operations?• How well can you define the operation of your project?• How good is its training?• Will it have time and budget to fine-tune the system?

FIG. 5.4qConfiguration drawing. (Courtesy of Allen-Bradley.)

If LS15 and PB4 are both closed,SOL 3 is energized through thePLC instructions addressed to:

LS15

PB4SOL 3

+ +] [ ] [ ( )12102 12105 02504

PLC program panel PLC processor

PLC (main)power supply

WireNo. 02504

SOL 3

SOL 3

L2 1

1

L1 1

SOL 3025 04

LS 15121 02121 03121 04PB4121 05

(Other I/O devices not shown)

PB 4

LS 15

Wireno. 12102

Wireno. 12105

LS 15

PB 4

Remote I/Odistribution

panel

Rack2

Remote I/Orack no. 2

User-supplied I/O power

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 925

• After warranty, will you be able to make changeswithout the integrator or will you have to pay for it toreturn?36

For budgeting, whether the work is done in-house or byan integrator, software costs range from 50 to 100% of thehardware cost. System engineering costs and documentationcosts each range from 25 to 50% of hardware cost. Therefore,the total cost (without installation labor) is about twice (ifnot more) the hardware cost.

Open Systems

Another consideration is whether to use open systems orproprietary. A major benefit of using open systems is that anumber of different manufacturers can supply equipment forthe system. “The trend today is definitely towards open sys-tems and open languages, but the bulk of the PLC market isstill proprietary—about 75% proprietary and 25% open orstandards-based. Even so, a large number of proprietary PLCsare shipped with Profibus, DeviceNet, or other open network-ing capabilities.”7

Distributed Control “One of the effects of better networksand intelligent devices is that control function no longer hasto reside in a central controller. It can be scattered about, eitherin a network of small PLCs or in intelligent I/O bases.…Some distributed processing is good to have because you candivide an operation into logical pieces, but it becomes diffi-cult to partition programs and logic and so forth into themachines.”11 A potential benefit dividing a single large systeminto separate areas of control is that starting and troubleshoot-ing a smaller piece of the system is more manageable.37

Redundancy Another issue is the amount of redundancythat will be built into the control system. Some areas toconsider when evaluating the need for redundancy includepreventing production impacts, equipment damage, businessinterruption and quality impacts, liability costs, public healthand safety, site safety, and environmental impacts.38 Redun-dant PLCs are available and I/O can be made redundant butit is often better to provide diversity in redundancy planning.Instead of using PLCs, manual controllers can be used. Man-ual control stations are important as backups in case of failure

FIG. 5.4rPoint-to-point wiring diagram.14

0 Spare 140

141

142

143

144

145

146

147

1

2

3

4

5

6

7

A

A

A

W

G

B

Outputs 2 LTauto cycle

1 LTManual cycle

Chuck clamped10 LT

20 SOLUnclamp chuck

21 SOLClamp chuck

Clamp and jackretracted

13 LTJack advanced

11 LTClamp advanced

12 LT

75

76

77

78

79

80

81

82

2

210

10 Inputs

14

15

16

17

18

19

20

2SSmanual

Cycleoff Auto

8 PBCycle start

5 PSClosed when

chuck clamped

ClampChuck

Unclamp

4 PSClosed when

chuck advanced

© 2006 by Béla Lipták

926 PLCs and Other Logic Devices

of the PLC controlling PID loops. Operator stations and HMIoften provide manual control capabilities but still rely on theintegrity of the PLC, so they are not truly manual in thehardwired sense. A manual control station is an importantpart of the distributed control system because it gives truemanual control of the loops locally or in the control room,even when the local control systems are down. It is importantto look at the overall system when evaluating redundancy.Unfortunately, the term “control-system redundancy” hascome to refer to only the CPU, possibly the I/O but not thefield devices. Even redundant PLCs can “fail” if they arepowered from a single power source.

PLC Hardware, System Sizing, and Selection

Despite the variety of available PLC models, system sizingis relatively simple. Hardware and system size can be deter-mined by an analysis of the following system characteristics:

1. I/O quantity and type2. Remote I/O requirements3. Memory quantity and type4. Programming requirements5. Programmers6. Peripheral requirements

Although sizing is generally straightforward, selection ofthe right PLC requires considerable judgment regardingtrade-offs between future requirements and present cost. Thefirst three are examined below. The others are addressedelsewhere in this chapter.

I/O Quantity and Type (Example 1) For this simple system,the first step is development of the I/O list (Table 5.4p). Thisdetailed document will be used extensively and should bedeveloped with great care. (Once I/O numbers are assigned,it becomes very difficult to change all references to thesenumbers.) If switches and pilot lights are used to operate thecontrols, include I/O for them. Include any alarm outputsused for alarm annunciators. An intrusion switch on the paneldoor is useful for larger control panels. Also, consider allow-ing 20% spare I/O of each type used. The I/O list is followedby the configuration drawing.

The configuration drawing (Figure 5.4q) shows thearrangement of the I/O and support hardware. The point-to-point wiring diagram (Figure 5.4r) is used by the panel shopand the installation contractor to make the I/O device inter-connect. Panel, or enclosure, design should now be coordi-nated with the addition of panel instrumentation, such as lightswitches, meters, and recorders. Once these steps are com-pleted, panel fabrication and assembly can begin.

I/O Quantity and Type (Example 2) In this somewhat morecomplicated example, the user should arrange any special I/Otypes as well as the commonly available modules accordingto an I/O matrix by logical area, as shown in Table 5.4s.

In this table, I/O types are listed across the first row and plantareas are listed down the first column. In this way, one canaccurately reconstruct the decision-making process concern-ing I/O quantity and type. It is important to include at least10 to 20% spare rack space in all I/O considerations.

For example, consider a typical process application.Assume a total I/O count of 764, broken down into 436 inputsand 328 outputs. This application falls into the medium PLCcategory. Because the majority of the field devices are locateda good distance from the CPU, a PLC with remote I/O isdesirable. The I/O requirements by locations are as follows:

Process Area: Total of 390 inputs and outputs, of which230 are 24 V (discrete DC inputs), 24 are 4–20 mAanalog inputs, and 136 are 24 V DC discrete outputs.

Tank Farm #1: Total of 98 I/O, of which 62 are120 VAC discrete inputs (DIs) and 36 are 120 VACdiscrete outputs.

Tank Farm #2: Total of 86 I/O, of which 52 are120 VAC discrete inputs and 34 are 120 VAC dis-crete outputs.

Loading Station: Total of 190 I/O, of which 68 are120 VAC discrete inputs and 122 are discrete out-puts (of which 24 are 120 VAC and 98 are 240 VAC).

Let us assume that the PLC system being considered has thefollowing features: (1) No constraints on input and outputmixture; (2) the I/O modules are available in two formats,16 points per module and 8 points per module; and (3) 10%spare I/O is required. For the sake of illustration, we will usethe 16 points per module I/O structure in the process areaand the 8 points per module structure in the tank farms andloading station. The I/O distribution (including spares) perlocation would now be as follows:

Process Area: Total of 390 I/O points, which requirethe following modules:

• 230 + 10% = 253 24 V DC inputs with 16 points/module = 15.8 modules; use 16 modules

• 24 + 10% = 26.4 4–20 mA analog inputs at 16 pointsper module = 1.6; use 2 modules

• 136 + 10% = 149.6 24 V DC discrete outputs at16 points per module = 9.35; use 10 modules

Tank Farm #1: Total of 98 I/O points, which requirethe following modules:

• 62 + 10% = 68.2 are 120 VAC discrete inputs at8 points per module = 8.5; use 9 modules

• 36 + 10% = 39.6 are 120 VAC discrete outputs at8 points per module = 4.9; use 5 modules

Tank Farm #2: Total of 86 I/O points, which requirethe following modules:

• 52 + 10% = 57.2 are 120 VAC discrete inputs at8 points per module = 7.2; use 8 modules

• 34 + 10% = 37.4 are 120 VAC discrete outputs at8 points per module = 4.7; use 5 modules

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 927

Loading Station: Total of 190 I/O points, which requirethe following modules:

• 68 + 10% = 74.8 are 120 VAC discrete inputs at8 points per module = 9.4; use 10 modules

• 24 + 10% = 26.4 are 120 VAC discrete outputs at8 points per module = 3.3; use 4 modules

• 98 + 10% = 107.8 are 240 VAC discrete outputs at8 points per module = 13.5; use 14 modules

If we assume that one remote communications channelcan service up to 128 I/O in groups of sixteen 8-point mod-ules or eight 16-point modules, the system becomes:

• Process Area—390 I/O; 28 16-point modules; fourremote channels

• Tank Farm #1—98 I/O; 14 8-point modules; oneremote channel

• Tank Farm #2—86 I/O; 13 8-point modules; oneremote channel

• Loading Station—190 I/O; 28 8-point modules; tworemote channels

Remote I/O vs. Distributed Control A unique feature of thePLC is the multiplexed nature of the I/O bus. This can be usedto great advantage to reduce overall wiring cost. If I/O racksare centralized in logical clusters, plant wiring requirements

can be greatly reduced. Wiring between racks and the CPUcan be reduced to a few twisted pairs of wires or a single cable.The tremendous cost savings that result can be realized withouta compromise of control accuracy or capability.

A system configuration diagram (such as that shown inFigure 5.4q), when used in conjunction with the I/O matrixin Table 5.4s, aids in keeping track of the overall systemconfiguration.

It is important to remember the major weakness of remoteI/O systems. If the bus is cut or interrupted, the effects of I/Ofailure will be relatively unpredictable. One must consider theeffect of a possible system failure on each step in the sequence.Some users install a redundant version of remote I/O commu-nications to guard against the loss of remote I/O communica-tions. For this reason, duplication of smaller CPUs at eachremote location is often considered preferable to a large centralCPU. This is actually an extension of distributed control withinthe network of the PLC itself. This approach can be very cost-effective, because requirements for the central unit size can bereduced. Serious consideration should be given to distributedversus centralized architecture in remote I/O systems in whichcontrol system integrity is important.

Memory Quantity and Type The type and quantity of PLCmemory used depends on the controller’s size and thecompany that manufactured it. Most small PLCs come with

TABLE 5.4sI/O Matrix

Plant Area Analog InAnalog

Out

Processarea1

24 4–20 mA

0

Tank farm #13

0 0

Tank farm #23

0 0

Loading station3

0 0

Notes: (1) Discrete in 16 pts/card

Discrete out 16 pts/card

Analog in 16 pts/card

Analog out

(2) Add model numbers after award of contract.

(3) DI and DO are 8 points per card

Model NoQty

.2 Model NoQty

.2 Discrete InVoltage

Model NoQty

.2 Discrete OutVoltage

Model NoQty

.

2230

24 VDC 16136

24 VDC 10

62120 VAC 9

36120 VAC 5

52120 VAC 8

34120 VAC 5

68120 VAC 10

24120 VAC 4

98240 VAC 14

© 2006 by Béla Lipták

928 PLCs and Other Logic Devices

a fixed quantity of RAM. Although this is usually 2 K to4 K of memory, the actual number of memory locations isnot as important as the average size application programthe PLC can be expected to handle. (In this case, size refersto the number of I/O points that are to be controlled andthe average number of logic, timer, counter, and mathe-matics operations that are to be performed.) Some manu-facturers may provide an extra-expense option of EEPROMor Flash memory with their small PLCs.

Midsize and large PLCs provide users an option for almostany type of memory desired. This includes various types ofnonvolatile memory. Quantity limits imposed by the PLC willexceed most application demands. When this is not the case,it usually suggests that a more efficient control scheme is inorder or that the application really does not belong on a PLCin the first place.

Total memory, as stated in the manufacturer’s literature,does not necessarily mean the entire content is available tothe user. Some manufacturers reserve large blocks for thePLC executive. A system with 4 K of 16-bit words of usermemory may comfortably accommodate a program, whereasanother system with 8 K of 8-bit words may have too littlememory for the same program.

Special programming language features are an importantaspect of memory sizing, especially in process control. ThePID algorithm is a perfect example: One manufacturerrequires 33 words of user-available memory, whereas anothermay need in excess of 1000 words. Obviously, the memorysizing for a loop control program would vary in these twosystems. Another example is the use of special functions, suchas shift registers. An alternative way of developing a shiftregister in ladder logic is to use a special function shift registeror handling data to require less user memory. Word (or reg-ister) moves are also powerful in terms of memory efficiency.Programming languages, which can be binary- or octal-basedor alphanumeric Boolean, affect memory use. The closer thelanguage is to a machine code (binary-based), the more usermemory is required to perform the more complex functions.The closer the language is to alphanumeric Boolean, the lessmemory will be required for complex functions.

The best way to determine program memory prerequi-sites is to write a representative sample program reflectingsome actual project requirements and to request informationabout user memory size from the various manufacturers. Ifthe manufacturer’s suggestions are followed, the user can bereasonably assured that the memory will not be undersized.

The final area of caution about memory size concernsthe consideration of data storage. Data tables, scratch pads,and historical data retrieval requirements can inflate the sizeof the PLC memory. It should be remembered that the pri-mary task of a PLC is control of the process. If data require-ments are large, connection to auxiliary devices, such asmini- and microcomputers, should be given serious consid-eration. Many of these devices are currently available andare of an industrial grade; furthermore, the price of thesesystems is coming down rapidly. It is not good engineering

practice to degrade control capabilities by burdening the PLCwith excessive data acquisition functions. As a plant goes on-line, operational requirements for data generally increaseastronomically. These will be easily accommodated by amini- or microcomputer but not by the PLC memory.

PLC Installation and Panel Design

Installation of PLC systems is not a difficult or mysteriousprocedure, but the following general rules will save time andtrouble for the systems designer or installer. The basic princi-ples of PLC installation are the same as those for the instal-lation of relay or other control systems. Safety rules and prac-tices governing proper use of electrical control equipment ingeneral should be observed. These include correct groundingtechniques, placement of disconnect devices, proper selectionof wire gauge, fusing, and logical layout of the device. PLCscan often be retrofitted into existing hardwired relay enclo-sures because they are designed to withstand the typical plantenvironment. PLC vendors provide installation manuals uponrequest.

After the PLC equipment is selected, the support equip-ment will need to be identified. Some panel shops that buildPLC panels can assist or perform this additional design. Atthe panel level this includes terminal blocks, wireway, wiretype and size both for discrete and analog, loop power supply(usually 24 V DC), circuit breakers and fuses, isolation relays,intrinsic safety barriers (if I/O wiring enters flammable orexplosive environments as defined by national electric code),main isolation transformer, pilot lights, pushbuttons andswitches, internal panel work lights, and receptacle. Alsoincluded are any door-mounted equipment like chart recorders,digital panel meters, and operator interface panels. A UPS maybe required. Separate isolated grounding is sometimes usedfor loop shields. Motor starters and small variable frequencydrives may also be in the same enclosure.

After all the equipment in the panel is determined, a heatcalculation should be done for the ambient extremesexpected, particularly if the cabinet is outdoors, to verify theinternal temperature is well below the maximum temperatureof all components. Generally the PLC is not the limitingfactor here, but other electronics and power supplies maylimit the maximum allowable temperature. If the interiortemperature is too high, a heat exchanger or air conditionermay be required. If the panel is installed in a cold area, aheater may be needed to prevent condensation, which canruin electronic equipment.

Safety Considerations Perhaps the most important safetyfeature, which is often neglected in PLC system design, isemergency stop and master control relays. This feature mustbe included whenever a hardwired device is used in order toensure operator protection against the unwanted application ofpower. Emergency stop functions should be completely hard-wired (Figure 5.4t). In no way should any software functions

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 929

be relied upon to shut off the process or the machine. Discon-nect switches and master control relays should be hardwiredto cut off power to the output supply of the PLC. This isnecessary because most PLC manufacturers use triacs for theiroutput switching devices, and triacs are just as likely to failon as off. This feature is often required by local or nationalcodes. In the United States, NFPA 79, “Electrical Standardfor Industrial Machinery,” covers this subject.39,40,41,42

Implementation Planning ahead is every bit as important indesigning a complete PLC system as in laying out a relaylogic panel. Care in counting I/O points in the beginning—and leaving a safety factor—will save headaches in the panelfabrication stage. Panels should always have plenty of expan-sion room left over, because I/O is invariably added as thejob progresses and the operators see the advantages of PLCs.The designer should refer to the layout considerations pro-vided by the manufacturer. Extra space should be left toprovide access to the boards and connectors of the PLC. Thediagnostic and status indicators should all be visible. The

designer should leave room between I/O racks for wirewaysand large hands.

One good technique for ensuring efficient panel layoutsis to involve maintenance personnel in the design procedure.This not only optimizes the layout but also introduces thestaff to the hardware (Figure 5.4u).

In general, the best defense against creating a tangledmess when designing a PLC system is to follow proper doc-umentation techniques. A little more time spent documentingpanel layout, I/O counts, and wiring diagrams results in a lotless time spent starting up the system. PLCs can handle largeamounts of I/O points with varying electrical characteristics,so things can get pretty confusing in a hurry. Cable require-ments between hardware boxes vary from one type of PLCto another, so this is an important consideration in panellayout.

Enclosure Enclosures should nearly always be provided forthe PLCs themselves. This protects the electronics frommoisture, oil, dust particles, and unwanted tampering. Most

FIG. 5.4tGrounded AC power-distribution system with master-control relay.

Notes:1. To minimize EMI generation, connect a suppressor across an inductive load.2. In many applications, a second transformer provides power to the input circuits and power supplies for isolation from the output circuits.3. Connect a suppressor here to minimize EMI generation from the net inductive load switched by the CRM contacts. In some installations, a 1 µf

220 ohm suppressor or 2 µf 100 ohm suppressor has been effective. (Courtesy of Allen-Bradley)

Suppressor1

Suppressor1

Suppressor3

User DCsupply

CRM

CRM

IncomingAC

L1L2L3

Disc. 1 FU

2 FU3 FU

H1H3 H2

H4

X1 X2

L1L2

L3

To motorstarters

Step-down2

transformer

Enclosurewall

Back-panelground bus

FuseGrounded conductor

Multiple E-stop switch Start Equipment-groundingconductors

Grounding-electrodeconductor togrounding-electrodesystem

Connect whenapplicable

Controllerpower supply

e I/O circuits form a netinductive load switched by theCRM contacts. erefore, asuppressor is neededacross the line at the load sideof the CRM contacts.

L1GNDN or L2

CRM

Inputsensor

Inputmodulewiring

armOutput module

wiring arm

Outputactuator CRM

To DC I/Oactuators/

sensors

+ −

© 2006 by Béla Lipták

930 PLCs and Other Logic Devices

manufacturers recommend a NEMA 12 enclosure for thestandard industrial environment or a NEMA 4X for outdooror corrosive environments. A NEMA 3R is not recommendedbecause it is not sealed against dust and moisture. This typeof enclosure is readily available in a variety of sizes and, infact, may be already included with a new system.

PLCs are designed to be located close to the machine orthe process under control. This keeps the wiring runs shortand aids in the troubleshooting procedure. At times, however,mounting the PLC directly on the machine or too close tothe process is not advisable, such as in cases of vibrationinherent in the machine, electrical noise interference, orexcessive heat problems. In these situations, the PLC mustbe either moved away or successfully protected against theseenvironmental conditions.

Temperature Considerations Installing any solid-state devicerequires paying attention to ambient temperatures, radiant heatbombardment, and the heat generated by the device itself.PLCs are typically designed for operation over a broad range

FIG. 5.4uTypical enclosure layout. (Courtesy of Allen-Bradley.)

51 mm (2'')

120 mm(4'')

153 mm (6'')

Wiring duct

102 mm(4'')

153 mm(6'')

51 mm(2'')

Area reserved for disconnect transformer,

control relays, motorstarters, or other devices.

13081

FIG. 5.4vGround connection details. (Courtesy of Allen-Bradley.)

Mounting bracketor ground bus

Flatwasher

Flatwasher

Nut

Starwasher

Back panel

Welded stud

Scrape paint

If the mounting bracket is coatedwith a non-conductive material(anodized, painted, etc.), scrape

the material around the mountinghole.

Stud mounting of a ground bus or chassis to the back panel Stud mounting of the back panel to the enclosure back wall

Alternative bolt mounting of chassis to the back panelBolt mounting of a ground bus or chassis to the back panel

17666 17664

17665

Ground bus ormounting bracket

Flatwasher

Nut

Starwasher

Nut

BoltBack panel

Tappedhole

Flatwasher

Scrape paint on paneland use star washers.

If the mounting bracket is coatedwith a non-conductive material(anodized, painted, etc.), scrape

the material around the mounting hole.

Back panel

Nut

Weldedstud

Use a wire brush to removepaint from threads to allow a

ground connection.

Back wall of enclosure

Scrape paint on panel anduse a star washer.

Mounting bracket

Flatwasher

Starwasher

Bolt

Flatwasher

Scrape paint

If the mounting bracket is coated witha non-conductive material (anodized,

painted, etc.), scrape the materialaround the mounting hole.

Back panel

Tapped hole

12342-1

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 931

of temperatures, usually from 0 to 60°C. When analyzing theproposed PLC environment, however, one should rememberthat enclosure temperatures usually run a few degrees higherthan ambient temperatures. Radiant heat on an enclosure fromsurrounding tanks can raise the internal temperature beyondthat specified by the manufacturer.

Heat generated by the PLC is a key issue when the deviceis placed in ambient temperatures close to the extreme men-tioned in the specifications. The temperature rise caused bythe power consumption of the PLC itself is not hard to esti-mate. In addition, most manufacturers will provide a notationof the power consumption of the triacs driving field loads.When designing the hardware layout within the panel, oneshould adhere to the manufacturer’s suggestions regardingways to minimize heating problems. Most PLCs use convec-tion over fins to take heat away from particular areas withinthe hardware. Care must be taken to ensure that no obstructionto air flow over these fins is introduced by placement of thePLC in the enclosure. Wireways are typically provided withholes to allow air to pass through. Generally, one can avoidproblems with PLC enclosures by simply leaving plenty ofair space around the heat producers.

Should all of these factors combine to cause a tempera-ture problem, the panel can be vented, air conditioned, ormoved to another location. Usually, simply blowing filtered

air through the enclosure will resolve minor difficulties. Ifair conditioning is required, small units that are designed forcooling electronic enclosures are readily available.

Noise Noise or unwanted electrical signals can generateproblems for all solid-state circuits, particularly microproces-sors. Each PLC manufacturer suggests methods for designinga noise-immune system. These guidelines should be strictlyfollowed in the design and installation phases, because noiseproblems can be very difficult to isolate after the system is upand running. I/O systems are isolated from the field, but volt-age spikes can still appear within the low-voltage environmentof the PLC if proper grounding practices are not followed.

A well-grounded enclosure can provide a barrier to noisebombardment from outside. Metal-to-metal contact betweenthe PLC and the panel is a must, as is a good connectionfrom the panel to the ground (see Figure 5.4v). Noise pro-ducers within the panel should be noted during the paneldesign phase, and the PLC must not be located too close tothese devices. Wiring within the panel should also be divertedaround noise producers to avoid picking up any stray signals.Often, it is necessary to keep AC and DC wiring bundlesapart, particularly when high-voltage AC is used at the sametime that low-level analog signals are present. Refer toTables 5.4w and 5.4x and Figure 5.4y for recommendations.

TABLE 5.4wFollow These Guidelines for Grouping Conductors with Respect to Noise (Courtesy of Allen-Bradley)

Group Conductor Cables Fitting This Description Into This Category: Examples:

Control and AC power-High-power conductors that are more tolerant of electrical noise than category 2 conductors and may also cause more noise to be picked up by adjacent conductors

Corresponds to IEEE levels 3 (low susceptibility) and 4 (power)

Category 1 AC power lines for power supplies and I/O circuits.High-power digital AC I/O lines—to connect AC I/O modules rated for high power and high noise immunity.

High-power digital DC I/O lines — to connect DC I/O modules rated for high power or with input circuits with long time-constant filters for high noise rejection. They typically connect devices such as hard-contact switches, relays, and solenoids.

Signal and communications—Low-power conductors that are less tolerant of electrical noise than category 1 conductors and should also cause less noise to be picked up by adjacent conductors (they connect to sensors and actuators relatively close to the I/O modules)

Corresponds to IEEE levels 1 (high susceptibility) and 2 (medium susceptibility)

Category 2 Analog I/O lines and DC power lines for analog circuits.Low-power digital AC/DC I/O lines — to connect to I/O modules that are rated for low power such as low-power contact-output modules.

Low-power digital DC I/O lines — to connect to DC I/O modules that are rated for low power and have input circuits with short time-constant filters to detect short pulses. They typically connect to devices such as proximity switches, photo-electric sensors, TTL devices, and encoders.

Communications cables (ControlNet, DeviceNet, Universal remote I/O, extended-local I/O, DH+, DH-485, RS-232-C, RS-422, RS-423 cables) — to connect between processors or to I/O adapter modules, programming terminals, computers, or data terminals.

Intra-enclosure — Interconnect the system components within an enclosure

Corresponds to IEEE levels 1 (high susceptibility) and 2 (medium susceptibility)

Category 3 Low-voltage DC power cables — to provide backplane power to the system components.

Communications cables — to connect between system components within the same enclosure.

© 2006 by Béla Lipták

932 PLCs and Other Logic Devices

Line voltage variations can cause hard-to-trace problemsin the operation of any computer-based system. PLCs are noexception, even though they are designed to operate over amuch larger variation in supply voltage. Large spikes or brown-out conditions can cause errors in program execution. Mostmanufacturers protect against this, enabling the controller tocome up running after a brownout, but these measures maynot be acceptable in all applications. The designer may wishto add an isolation transformer to a proposed PLC system,sized for twice the anticipated load. This is cheap insurance,and PLC manufacturers will help determine the required load.

Triac outputs require some special attention that will benew to relay users. Triacs used for AC loads typically leak asmall amount of current. In the case of triac outputs from aPLC, this leakage may be enough to keep panel lamps glowingor small relays energized. When a triac is used to switch theinput on a PLC, the leakage may be enough to make the PLCs“think” the input is on. A dummy load (shown in Figure 5.4z)can be used to drain this leakage when the input should beoff. Whenever a mechanical contact is used in series with aload energized by a triac (as shown in Figure 5.4aa), a resistance-capacitance (RC) network should be used as shown to protectthe triac from inductive kickback. A varistor should be providedin parallel with a load whenever the load can be “hot-wired”around the triac (Figure 5.4bb). The user should check with thePLC manufacturer for the suggested RC and metal oxide varis-tor (MOV) types for the particular application. Triacs cannotdirectly drive large motor starters and similar devices. PLCmanufacturers provide surge specifications for the various I/Ocards. Sometimes an interposing relay or dry contacts will berequired for large loads.

PLCs are similar to most electrical control systems. To besure, solid-state devices, microprocessors, and triacs requiresome special considerations during the design, installation, andstart-up phases of a project, but these concepts are not too

complex or difficult to assimilate. As always, good design habitsin the beginning will ensure a safe and reliable control system.

Hookup PLC panels can be very neat and orderly if all theterminals are arranged in a logical fashion. The actual resultis a direct function of the time spent during the design process.Interposing terminal blocks between the PLC I/O structureand the field is suggested, because the terminations providedby PLC manufacturers are shrinking in the race to providehigher-density I/O. This also gives the panel designer the abil-ity to place the field termination points where they are easilyaccessed. Wiring ducts keep the panel neat and protect thewire from mishap.

Following good wiring practices can avert many noiseproblems. Low-voltage signal wiring should be kept awayfrom noise sources. Analog signals should be shielded, withthe shield terminated at an isolated ground in the panel only(to prevent shield ground loops). Again, these analog signalsshould be separated from power wiring.

Software (Program) Development

The I/O list mentioned previously will be used to begin theprogram development. Basic control philosophy decisionsneed to be made at this point. Should valves fail open orclosed? What fail-safe provisions are necessary for analogcontrol? These philosophical decisions should be docu-mented and included with the process operational descrip-tions.15 Often this document will be referred to as the softwarefunctional specification. Its purpose is to define, as preciselyas possible, the operation of the controls.43 It also has severalother functions:

1. It communicates the functional requirements of thecontrol system to those writing the PLC code.

TABLE 5.4xFollow These Guidelines for Routing Cables to Guard against Noise (Courtesy of Allen-Bradley)

Route This Categoryof Conductor Cables: According to These Guidelines

Category 1 These conductors can be routed in the same cable tray or raceway with machine power conductors of up to 600 VAC (feeding up to 100 hp devices).

Category 2 If it must cross power feed lines, it should do so at right angles. Route at least 5 ft from high-voltage enclosures, or sources of RF/microwave radiation.If the conductor is in a metal wireway or conduit, each segment of that wireway or conduit must be bonded to each adjacent segment so that it has electrical continuity along its entire length, and must be bonded to the enclosure at the entry point.

Properly shield (where applicable) and route in a raceway separate from category 1 conductors.If in a contiguous metallic wireway or conduit, route at least 0.08 m (3 in) from category 1 conductors of less than 20 A; 0.15 m (6 in) from AC power lines of 20 A or more; but only up to 100 kVA; 0.3 m (1 ft) from AC power lines of greater than 100 kVA.

If not in a contiguous metallic wireway or conduit, route at least 0.15 m (6 in) from category 1 conductors of less than 20 A; 0.3 m (1 ft) from AC power lines of 20 A or more; but only up to 100 kVA; 0.6 m (2 ft) from AC power lines of greater than 100 kVA.

Category 3 Route conductors external to all raceways in the enclosure or in a raceway separate from any category 1 conductors with the same spacing listed for category 2 conductors, where possible.

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 933

2. It records the thought process (regarding control) ofthe system designer to be used in the event of a per-sonnel change. Such information can be invaluable.44

3. It provides a review document for personnel workingin other capacities (mechanical, process, and projectmanagement) to ensure that they understand the oper-ation of the controls.

4. It provides a guide for developing the operationaldescription for the operator’s manual.

After the functional specification has been reviewedand approved, a detailed operational sequence chart, timing

diagram, logic diagram, flowchart, or electrical schematicis developed from it. This schematic is translated or codedinto the appropriate PLC language, cross-referencing I/Owith PLC designator tags. The piping and instrument dia-gram is also cross-referenced with PLC designators. In thisway, future cross-referencing of system drawings and PLCcodes is facilitated.

As the code is entered, a memory map or register indexis kept by the programmer (Table 5.4cc). This map is usefulfor organizing program data in logical arrangements and willprove invaluable during start-up, when the programmer mayneed to locate available blocks of memory quickly for pro-gram revisions. Most PLC programming software can gen-erate a cross-reference; however, certain types of instructionsare difficult for the program to cross-reference, so a list isstill desirable.

The use and understanding of PLC programming dependson knowledge of the process to be controlled, an understand-ing of electrical schematics, and an appreciation for logicoperations and for various types of logic and relay devices.Other sections of this chapter provide information on eachof these topics. A review of those topics might be useful atthis point.

Although the programming style and language used is,to some extent, dictated by the size of the PLC used, thereare fundamental programming elements, including logicoperations, timers, counters, and arithmetic capabilities, thatare provided in all models. Some of the characteristics ofthese important elements are briefly discussed below (seeSections 5.5 and 5.6 for more information). Ladder logiccontinues to be popular because it was purposely created tolook like hardwired relay logic drawings already in use. It isthis similarity between ladder diagrams used for relays andthe programming version that eased the change from usingrelays to using PLCs. In addition, ladder logic is relativelyeasy to learn.9,45

Boolean Logic Relay type instructions are the most basicof all PLC instructions. Boolean logic functions (AND, OR,and NOT) are diagrammed as combinations of normally openand normally closed contacts. A coil symbol represents theresult of the logic function. In ladder logic, contacts can beshown in series or parallel to achieve the “AND” or “OR”functions, respectively. A normally closed contact representsa “NOT” function.45

FIG. 5.4yMounting and wiring details. (Courtesy of Allen-Bradley.)

Category-2conductors

Conduit

Conduit

Category-1conductors

(AC power lines)

Tighter spacingallowed

with conduit

Enclosure wall

TransformerUse greater

spacing withoutconduit

Tighter spacing allowedwhere forced by spacing

of connection points

Category-2conductors

I/O block

1771 I/O chassis

Place modules tocomply with spacingguidelines if possible

FIG. 5.4zDummy load for leakage. If a “leaky” triac, such as a proximityswitch, is used to switch the hot side of the line, then a pull-downresistor (R) is required to counteract the leakage.

Input

Common

L1

N

R

© 2006 by Béla Lipták

934 PLCs and Other Logic Devices

One popular programming technique involves defining thesequential logic in electrical schematic format, using actualtag numbers, and then translating this diagram into the appro-priate programming language. Figure 5.4dd shows the trans-lation of some examples of typical circuits to ladder diagrams,Boolean algebra, and mnemonics. Because this translation isrelatively simple, maintenance and engineering personnelhave accepted PLCs, although they have not accepted com-puters as readily. The unknown has been replaced with thefamiliar.

Timing and Counting Figure 5.4ee is a schematic repre-sentation of a timer and a counter. Although their formats

differ, the principles are the same. The legs of the timerrepresent start/stop and reset. Timers can be on-delay or off-delay and can be cascaded (that is, linked together in series).Counters can be up or down and have a count leg (in whichthe number of switch closures is the count) and an up/downleg (in which the position of the switch determines up countor down count). A PLC with arithmetic capability can use acombination timer and counter as an integrator. Figure 5.4ffshows a turbine meter pulse counter turned into a low-costintegrator within the PLC. Obviously, the scan rate of thePLC (scans per second) must be twice the pulse rate of theturbine (pulses per second).

Arithmetic Capabilities Figure 5.4ff shows an arithmeticprogram that permits the rapid addition of pulse counts fromtwo counters hooked to two electric meters. The resultingsum is displayed through a panel meter. This logical additionis performed using integer mathematics (that is, no decimalcalculations can be performed). Most PLCs use an approxi-mation technique called “double-precision integer mathemat-ics” to do calculations of greater complexity (such as PID).Some PLCs have true floating-point mathematics capability.

Floating-point mathematics is a powerful tool for processapplications. For example, in the integrator example in

FIG. 5.4aaExamples of where to use suppression. (Courtesy of Allen-Bradley.)

AC output module

Solid-stateswitch

Solid-stateswitch

L1

1MS

L2 1MS

1MS

1MS

L1

L2

L1

Suppressor

Suppressor

SuppressorSuppressor

Suppressor

Supp

ress

or 1M

Example 1:An AC output modulecontrols a motor starterwhose contacts control themotor.

Although the motor starter is aninductive load, it does not need asuppressor because it is switchedby a solid-state device.

e motor needs supressorsbecause it is an inductive loadswitched by hard contacts.

AC output module L1

1CR1CR

L2 L1

1S

L2

e interposing relay needs asuppressor because it is an inductiveload switched by hard contacts.

e solenoid needs a suppressorbecause it is an inductive loadswitched by hard contacts.

Example 2:An AC output modulecontrols an interposing relay, but the circuit can beopened by hard contacts.e relay contacts controla solenoid.

Example 3:A contact output modulecontrols an inductive load.

Contact output module L1 Pilot light with built-instep-down transformer

L2

e pilot light needs a suppressorbecause it is an inductive loadswitched by hard contacts.

FIG. 5.4bbSurge suppression for inductive AC load devices. (Courtesy of Allen-Bradley.)

Output device Output device Output device

Varistor RC Network Diode (A surge suppressorcan also be used.)

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 935

Figure 5.4gg, the division of pulses by elapsed time can beexpressed as a decimal number rather than as a truncatedinteger. Feedforward calculations and PID can be performedin double-precision integer mathematics but are more memory-intense than in floating-point mathematics. Floating-pointmathematics may require the use of a separate microproces-sor within the CPU and usually involves two adjacent mem-ory locations to store the mantissa and the abscissa in a formof scientific notation. The programmer automatically trans-lates when the memory location is followed by a period (.),indicating a floating point number.

Programming Documentation The following technique canbe used for PLC programming applications. There are manyadvantages to this approach.

1. Develop detailed I/O lists. Table 5.4p shows an I/Ocross-reference relating tag numbers to I/O points.This list should be used extensively; starting withoutit will cause confusion and errors resulting from inev-itable changes.

2. Develop a detailed descriptive operational sequenceof events. Figure 5.4ii shows a sample sequence usinga process batch application.

TABLE 5.4ccExample of Memory Map for Milling Machine

Coil Definition Used in Rung(s)

CR0 R.H. Head Retract 13 15 19 – 20 – 33 – 46

CR1 L.H. Head Retract 14 15 21 – 33 – 38 – 47

CR2 Clamp Part 4 5 – 34

CR3 Cycle Part Unclamp 2 2 25 25

CR4 Shot Pins Out 5 5 6 27 – 28

CR5 Machine Slide Adv Milling 6 6 7 22 24 29

30 32 35

CR6 Machine Slide Retract 7 7 8 – 22 39

CR7 Milling Spindles Off 9 9 10 – 24 – 24 – 32

CR8 Start Boring Spindles 10 10 11 12 20 23

CR9 R.H. Bore Complete 1131

1133

1338

13 46

CR11 Unclamp Swing Clamps 15 15 16 – 23 28 –29

– 30 – 31 – 31

CR12 Unclamp Pull Clamps – 3 16 16 17 26

CR13 Hold Light On 3 3 4 4 – 26 34

CR15 Milling Complete Shot Pins In 8 8 9 – 27 – 39

CR17 Pump #1 Pressure 25 26 28 52

CR18 Unclamp Pull Clamp 26 49

CR19 Retract Shot Pins 27 50

CR20 Unclamp Swing Clamps 28 51

CR29 L.H. Bore Complete 12 12 14 14 47

CR31 Pressure Off Pump Time Start 17 17 18 18

CR32 Pressure Off Pump #1 18 – 52

CR101 Auto Lube Cycle 36– 43

3648

40 40 41 41

CR102 Lube Off 41 42 42 – 43 – 44 45

– 48

CR103 Lube On – 41 – 41 42

CR104 60 Min. Lube Restart 43 44

CR105 Start/ Disable – 40 45 45

CR106 Contin. Lube Restart 40 44

Source: Xcel

© 2006 by Béla Lipták

936 PLCs and Other Logic Devices

FIG. 5.4ddLadder translation. Here is a comparison of programming languagesthat are used with various PLCs. The most popular is still the relayladder diagram because plant personnel are more familiar with it.54

FIG. 5.4eeTimer/counter schematic. A key part of any PLC programming isits capability to do timer/counter functions. The instructions areentered either horizontally or vertically, depending on the make.Horizontal programming, however, is more commonly used.54

FIG. 5.4ffPulse counter totalizer.

Relay ladder diagram1 PB 2 CR 4 CR 5 CR SOL A

1 PB 2 CR 4 CR 5 CR SOL AFree format equivalent PLC diagram

3 LS

3 LS

Boolean Statement((1PB • 2CR) + 3LS) • 4CR • 5CR = SOL A

Code or mnemonic languageLoad 1PBAnd 2CROr 3LSAnd 4CRNand 5CRStore SOL A

Preset Currenttime

Timer: Horizontal

Output

ResetStart/stop

Preset Currenttime

Counter:

Output

ResetCount/signal

Vertical

PresetStart/stop

Reset Currenttime

Output1

Output2

PresetCount/signalReset Current

count

Output1

Output2

CTRC100V101

CTRC101V102

ADDV101V102V103

Meterpulse

Meterpulse

@0X5

@6X6

@12X5

X6

X7

X7

Count meter 1pulses CR1

CR2

CR3Totalize pulses

Count meter 2pulses

FIG. 5.4ggSample ladder diagram—annotated. (Courtesy of Xcel.)

Rung10

@ 52

Rung11

@ 57

Rung12

@ 61

Rung13

@ 65

Rung14

@ 71

Rung15

@ 77

Left millspindleDV5BY6 : 24

Rt. millspindleDV15AY15 : 32

Millingspindles

offCR7 : 9

Startboring

spindlesCR8 : 10

Startboring

spindlesCR8 : 10

Startboring

spindlesCR8 : 10

Startboring

spindlesCR8

Rt. boreslide RET

L. S. 7X7

Left boreslide RET

L. S. 8X8

10122333

11203138

1113

1346

1214

1447

15−20−46

19−33

15−33−47

21−38

15−23−29−31

1628

−30−31

R. H. borecomplete

CR9

R. H. borecompleteCR9 : 11

L. H. borecompleteCR29 : 12

L. H. borecomplete

CR29

R. H. borecompleteCR9 : 11

R. H. borecompleteCR9 : 11

R. H. headretract

CR0

R. H. headretract

CR0 : 13

L. H. headretract

CR1

TMR

2

2

TMR

2

2

L. H. borecompleteCR29 : 12

L. H. borecompleteCR29 : 12

L. H. headretract

CR1 : 14

RT. boreslide ADV

L. S. 9X9

Left boreslide ADV

L. S. 10X10

Unclampswing

clampsCR11

Unclampswing

clampsCR11 : 15

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 937

3. Develop electrical schematics or ladder diagrams forsequential control.

4. Develop piping and instrumentation drawings (or alogic diagram) for process control. Figure 5.4jj showsa diagram that will allow maintenance personnel tofind important activities quickly. Note the I/O matrixindex, showing what happens inside the PLC soft-ware, and the cross-referenced I/O memory locationsand tag numbers.

5. Translate the drawings in steps 3 and 4 to the PLClanguage.

6. Enter the program code using a memory map (seeTable 5.4cc).

7. Debug the program at the programmer’s facility. Usea simulator to debug the program. Run through theoperational sequence defined in step 2. If it haschanged, be sure to ascertain how that change hasaffected other parts of the program. Rewrite thesequence description to reflect current operations, ifnecessary.

8. Save and document the program. Reproduce the pro-gram on transportable media, such as floppy disks.(Do this daily.) Document programming changesusing printouts.

9. Enter and debug the program in the field. It is essen-tial to note all changes made on the documentation.Some of the biggest problems with relay systems areundocumented field modifications.

10. Redocument and reproduce the final program.14,21,46,47

The final documentation package should include the fol-lowing: I/O list and cross-reference, descriptive operationalsequence, electrical schematics, process schematics, programlisting (see Figure 5.4gg for annotated final document), mem-ory maps (showing the memory areas that have been usedand those that are available), and notes for future programchanges or additions.

One important word about the documentation package:A major advantage of the PLC is its ability to be repro-grammed as plant requirements change. Without proper doc-umentation, previous programming efforts will have to bereproduced in order to make the changes that are required.Poor documentation results in wasteful efforts at reconstruc-tion. The PLC, like all engineering tools, requires good con-trol systems engineering practices in order for its full poten-tial to be realized. Good documentation is an essentialelement of any PLC project.22,48

Programming and Documentation Tools Both PLC and after-market parties offer programming and documentation tools forthe system designer or user. Programming software is typicallyprovided by the PLC manufacturer and is designed to programa specific machine or family of machines. Some third partiesare offering universal programming software. These softwarepackages vary greatly in price and capabilities but offer on- andoff-line programming to many different types of PLCs, real-time status, and some very sophisticated annotation. Commu-nications to different PLCs are usually supported by differentsoftware packages. Each vendor’s product offers different typesand amounts of ladder and contact comments. Again, manytypes of cross-references are available to be printed out. Often,other PLC design documentation problems may be solved, suchas the generation of panel configuration drawings, point-to-point wiring diagrams, and I/O layout. One system even printsout the wire labels.

HMI Software (Program) Configuration The operator inter-face needs to be designed and implemented. Generally, thesame process is followed whether the interface is a local panelor the SCADA server in the control room. Sketches of thedisplays should be made and reviewed with operations staff.These should emphasize operator-friendly graphics for theplant operators, who will spend the most time with them.49

Most of the time, operators need to view an operations-orientedview of the process. Only when an abnormal event occurs willthey go to the detail screens to find the cause.50

Once a consensus is reached on how the system should beoperated, the screens can be configured on the interface equip-ment. “Linking” is the term used for defining how the screenwill change based on the contents of PLC memory locations.

FIG. 5.4hhFloating-point integrator.

CTR

V200V201

TMR

V202V203

SF

V400

@0X10

CR12

@6X12

CR12

@12Y10

CR11

@16CR11 Calculate gallons/min

Latch on calculation request

Time pulse integrating period

Count turbine meter pulses CR10

Y10

CR11

CR12

CR12

Special functionuser math

Start address: V400 Next address : V406Error output ? . . . . . . . . . . . . . . . . . . . . . . . NError output designator . . . . . . . . . . . . . . . . . Entry format:(Math term: operator, math term: operator, . . . = result)V201 V202 C200 =V205No. time const. gal/minPulses period

© 2006 by Béla Lipták

938 PLCs and Other Logic Devices

Reports Configuration Some simple reports can be directlyset up in the operator interface software. More extensivereports usually require separate reporting or database soft-ware. These reports should be set up early so that they canbe checked before installation of the equipment.

Software/Hardware Integration

Once the program is entered, a simulation is recommended,and the program checkout process begins “on the bench.” Thisprocess uses the functional specification to prove the softwareis acceptable. A large percentage of the program can be provedin this manner. Program debugging can be completed beforefield installation. Field corrections will then be minimized,and high-salaried electrical and installation personnel will notbe standing around waiting. The savings that can be realizedare quite significant.51 In addition, it’s important to keep inmind that there’s no substitute for a bench-simulated programcheck. The software simulation proves the program and allowsacceptance by the customer. The program should be repro-duced and documented after it has been checked.

Once the panels are built, and all configuration and pro-gramming are complete, as much of the system as possibleshould be set up in a staging area for a complete checkout.Final control elements (valves, motor starters) function can

be checked for proper outputs. Analog sensors (pressure,level, flow, and so on) are usually simulated with a 4–20 mAcalibrator. Using the PLC and programming aids, the panelwiring can be “rung out” (that is, checked point by point)through the PLC. Each I/O point should be activated sepa-rately to the terminal block or from the panel controls (but-tons, lights, switches). In this manner, the electrical integrityof the panel from the terminal blocks inward is ensured. Ifany continuity problems exist thereafter, they will be locatedin the field wiring. All remote I/O should be connected. Thedata highway or Ethernet networks should be temporarilyconnected. All displays should also be checked to make surethat the I/O points trigger the correct part of the screen oractivate the correct alarm.

Some organizations prefer to perform a simulated oper-ation checkout at this time. This is a highly useful approachand can be implemented if the simulators and the I/O pointarrangement are organized to simulate the process outsidethe panel. Some I/O points may need to be jumpered forsimulation. For instance, if run contacts are required toclose a few seconds after the motor is called to run, theycan often be temporarily connected to the output for thetest. It is generally felt that each hour spent troubleshootingat this point will save two or three hours during systemstart-up. Occasionally, operations personnel are brought in

FIG. 5.4iiBatch sequence. (A) Graphic representation of a simple batch process reactor vessel, with inputs and outputs. (B) Outputs for each step inthe process are described in the “Output Table.” The advance conditions for each step are shown along with input interlocks (bottom),which affect certain outputs. (C) The “Input Table” correlates the connection terminal number with the word description of each input.29

Output tableOutputs

Step Description

Home Start

2

1

3

4

5

6

Fill

Heat

Cool

Agitate

Empty

End

Inputconditionsto advance

Auto select andstart

Tank full(Level switch 1)

Temperaturesetpoint or timer

Timer

Timer

Tank empty(Level switch 2)Automatic reset

Conditionalinterlocks:

Valv

e 130

0Va

lve 2

301

Valv

e 330

2Pu

mp

303

Stea

m30

4A

gita

te30

5Va

lve 4

306

X X X X

XXX

X

X

Valv

e 2 o

pen

and

Valv

e 1 o

r 4 o

pen

Tem

pera

ture

not

at se

tpoi

ntor

tank

not

empt

yTa

nk n

ot em

pty

(Lev

el sw

itch

2 no

t set

)

X = Output energized

M D

Valve 1

Valve 2Pump

SteamvalveLevel switch 1

Level switch 3Level switch 4

Temperaturesensor

Level switch 2Agitator

Valve 3

Input table

Recipeselector

−1−2−3

MA

Autoselect

StartManual

step

(A)

Input no. Description Input no. Description001 Auto select switch002 Start pushbutton003 Level switch 1004 Temperature sensor005 Level switch 2006 Valve 1 open limit switch007 Valve 2 open limit switch008 Valve 3 open limit switch009 Valve 4 open limit switch

010 Recipe 1 select011 Recipe 2 select012 Recipe 3 select013 Level switch 3014 Level switch 4015 Manual step pushbutton

(C) (B)

Valve 4

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 939

to demonstrate this process. This is an effective way forthe operations personnel to become familiar with the newsystem, especially in new automation projects in whichthere may be some “fear of the unknown” to overcome.Occasionally, operations personnel find operational flawsor make suggestions for improvements that can be easilyimplemented in the panel shop but would be difficult orimpossible in the field.

Test software intelligently by trying to break it. Testingall aspects of the project during start-up will save time andmoney in the long run. Also it is recommended to wait untilafter major milestones in the project to perform software andhardware upgrades. This makes upgrade troubleshooting eas-ier.49 At the conclusion of this exercise the panel is acceptedby the customer and is shipped to the job site.

System Checkout and Start-Up

After electrical interconnections are made and point-to-pointwiring is completed (mechanical completion), the system isready for start-up. The ability of the PLC to operate step bystep through the start-up becomes very useful at this stage.

Experienced PLC personnel may provide temporaryswitches in the back of the panel in order to facilitate the start-up procedures. These switches can be key-locked, software-locked, or disconnected for normal operation. They are alsovery useful as future maintenance and troubleshooting toolsto diagnose future problems.

Unanticipated circumstances are always a factor duringstart-up. Wiring errors, program errors, and mismatches betweenPLC and HMI databases are common problems at this stage.For this reason it is not uncommon to have electrical and pro-gramming personnel available at this time for implementationof any changes that might be necessary. Quite often, programchanges can be accomplished over long-distance telephone linesusing modems. These changes are not easily implemented with-out adequate documentation. After successful start-up, the plantis signed off.

After Start-Up

Once again, it is imperative for future successful plant oper-ation that complete, current documentation be available. Thisdocumentation should include the items discussed previously.Especially useful for future changes or additions are the start-up notes and notes pertaining to future modifications.

Training of operations, maintenance, and engineering per-sonnel should be timely and “hands-on.” It is useful to videotapethese training sessions for future reference (e.g., for training ofnew personnel). Suggested programs for PLC training can beobtained from the PLC vendor. This is an important functionprovided locally at the job site, at a nearby metropolitan area,or at the PLC vendor factory.

Troubleshooting When troubleshooting a PLC system it isgood to remember that almost 80% of the time the problemis either outside the PLC entirely or in a broken I/O module.Program errors and wiring errors and mismatch of PLC/HMIdatabases can also cause these problems, but these are usuallyresolved during start-up. Some simple troubleshooting tipscan be followed to decide if a problem is in PLC or elsewhere.

Discrete Inputs: Because DIs are usually high imped-ance, check the input voltages with a high impedance mul-timeter. Using a low impedance solenoid voltage checker ondigital inputs can sometimes cause confusing results. For adiscrete input, an indicator on the module shows the state ofthe input. This should match what is observed on the HMIscreen or PLC programmer monitor. The commons for theDI card may be grouped or individual. Check the voltagebetween common for that point on the I/O card terminal block(or swing arm) and the point in question. If possible actuatethe field device and verify that the PLC logic status and

FIG. 5.4jjProcess and logic diagrams. This logic diagram is for a sump pump.40

HS-101-2In stop

position

HS-101-2In run

position

HS-101-2In autoposition

A

A

OR*

* No. 1pump run

M-101

GR

17DT2

Sec

OR41

OR40

A40

1001

1002

1005

1006

1003

A, 2

B, 2

Level greater

than 6 ft.(1.8 m)

LSL-103-2

Level greater

than 3 ft.(0.9 m)

LSLL-103-1YNo. 1 pump

in autoposition

HS-101-1

No. 1 pumpin manualposition

HS-101-1

5

6

5 7 17 41

2

R

UAHH100-1

Levelgreater

than 12 ft.(3.6 m)alarm

LSHH-103-4

Symbols defined for programmable logic controller logic diagram1004 1008, etc. indicates PLC input

, , , etc. indicates PLC output

*OR Indicates field wired “OR” gate

A40 Indicates “AND” gate in PLC line 40

*

© 2006 by Béla Lipták

940 PLCs and Other Logic Devices

voltage change together. If the module shows the correct I/Ostatus, but the PLC monitor or HMI does not, the I/O moduleor its configuration is suspect and may need replacing.

Discrete Outputs: Because triacs-type outputs “leak” asmall amount of current, they may show a voltage on amultimeter if they are not connected to a load. For discreteoutputs, an indicator on the module shows the state of theoutput. This should match what is observed on the HMIscreen or PLC programmer monitor. The power to the DOcard may be grouped or individual. Check the voltage forthat point on the I/O card terminal block (or swing arm).Then check the voltage for the point in question. If possiblecause the output to actuate and verify that the PLC logicstatus and output voltage change together. If the moduleshows the correct I/O status, but the measured output voltagedoes not, the I/O module or its configuration is suspect andmay need replacing.

Analog 4–20 mA Inputs: For analog I/O, the input valuesare not displayed on the module. Temporarily remove fieldwiring and place a 4–20 mA calibrator directly on the pointand select either passive or sourced as appropriate for the inputconfiguration. Verify that 4 mA gives the minimum value, that20 mA gives the maximum value, and that 12 mA producesthe value exactly between the min and the max values at theHMI or the PLC. If this works, then the problem is outsidethe PLC hardware.

Analog 4–20 mA Outputs: For analog I/O, the outputvalues are not displayed on the module. Some modulesrequire an external loop power supply. Verify its operation.Put a multimeter in current mode in series with the point inquestion. Have an operator set the output to 0%, 100%, and50%. This should produce readings of 4 mA, 20 mA, 12 mA,respectively. If this works, then the problem is outside of thePLC hardware.

Erratic Problems: To troubleshoot erratic PLC opera-tions that affect more than a single I/O module, check PLCgrounding, power to PLC power supply, DC power output ofthe power supply, and batteries. Electromagnetic interference(EMI) or radio frequency interference (RFI) generated bylarge motors starting, arc welding, lightning strikes, handheldradios, or transmitters (a common issue) can produce erraticoperation. Improvements in power conditioning, grounding,and shielding can usually resolve these issues. If the programhas been affected, it may need to be reloaded.52

CONCLUSION

PLCs are durable, delivering real-time control in a ruggedand dependable package with no moving parts.7 They can beinstalled on either the factory floor or outdoors, withstandingtemperature swings up to 60°C PLCs have the critical abilityto process sequential logic without experiencing faults in theoperating system. Moreover, users continue to use PLCsbecause they know how to support them and understand thesimple ladder logic language they use.10

The sheer number of PLC applications is enormous.According to a recent Control Engineering magazine poll,“The major applications for PLCs include machine control(87%), process control (58%), motion control (40%), batchcontrol (26%), diagnostic (18%), and other (3%).”53 Theresults don’t add up to 100% because a single control systemgenerally has multiple applications. Many sources document-ing various PLC applications are listed in the Bibliographyof this section.

References

1. Dummermuth, E., “A Bit Twiddler’s History of the PLC,” A-B Journal,November 2002.

2. Strothman, J., “More than a Century of Measuring and ControllingIndustrial Processes,” InTech, June 1995.

3. “The History of the PLC as Told to Howard Hendricks by DickMorley,” R. Morley Inc., 2002, http://www.barn.org/FILES/historyofplc.html.

4. Morley, R., “PLCs According to Morley,” Industry.Net Report, Sep-tember 27, 1995.

5. Strothman, J., “Leaders of the Pack,” InTech, August 2003.6. Hart, D., “The PLC-5, The Evolution of an Industry-Leading Solu-

tion,” A-B Journal, November 2001.7. “PLCs Provide Plenty of Possibilities,” Control Magazine, November

2000.8. Mintchell, G. A., “Distributed Control Goes Discrete,” Control Engi-

neering, October 2000.9. Johnson, D., “Nano Devices Lead Assault on Traditional PLC Appli-

cations,” Control Engineering, August 2002.10. Merritt, R., “PLCs Persist and Prosper,” Control Magazine, April

2001.11. Cleaveland, P., “PLC Manufacturers Make Their Solutions a Lot More

Open,” Instruments and Control Systems, April 1999.12. Korarek, L. A., “Sizing Criteria for the Evaluation of Programmable

Controllers,” Instruments and Control Systems Seminar Workshop(available from Allen-Bradley).

13. Hordeski, M. F., Microprocessor Cookbook, Blue Ridge Summit: TabBooks Inc., 1979.

14. Intel Corporation, “2816 E2PROM Data Sheet,” Santa Clara, CA,1981.

15. “The PC User Buyer’s Guide,” PC User.16. “Fast Growing PC Market Encourages Wide Range of Product Offer-

ings,” Control Engineering, January 1983.17. Waterbury, B., “Distributed Intelligence Goes to Work,” Control Mag-

azine, August 2000.18. Eckard, M., “Tackling the Interconnect Dilemma,” Instruments and

Control Systems, May 1982.19. ESD Vendor Workshop, “Control Networks vs. Data Acquisition (A

Case for Dividing the Tasks),” 12th Annual Conference and Equip-ment Display, ESD, March 1983 (available from ISSC).

20. Hoag, D. S., “Programmable Controllers in Distributed Process Con-trol Systems,” International Conference on Power Transmissions,Houston, TX, June 1982 (sponsored by Power Magazine).

21. Jannotta, K. L., “Factory Communications: How to Talk to theMachines,” International Conference on Power Transmissions, Hous-ton, TX, June 1982 (sponsored by Power Magazine).

22. Whitehouse, R. A., “The Future of Programmable Controllers,” ISA1976 Annual Conference, Advances in Instrumentation, Vol. 33, PartI, Instrument Society of America, October 15–19, 1976.

23. Wallace, D., “How to Put SCADA on the Internet,” Control Engi-neering, September 2003.

© 2006 by Béla Lipták

5.4 PLCs: Programmable Logic Controllers 941

24. Laduminsky, A. J., “Peripherals Enhance PC Performance,” ControlEngineering, January 1983.

25. Lutz-Nagy, R., “The Dawn of Intelligent Motion,” Power Transmis-sion Design, February 1980.

26. “PC Users Guide,” Instruments and Control Systems.27. Londerville, S., “Programmable Controllers in Boiler Control Appli-

cations,” Instrument Society of America, Northern California Section,ISA Days, June 2, 1982.

28. Bennett, L. E. and Lockert, C. F., “Evaluating Controls for BatchProcessing,” InTech, July 1976.

29. Dietz, R. O., “Programmable Controllers in Batch Processing,”InTech, May 1975.

30. Larsen, G. R., “A Distributed Programmable Controller System forBatch Control,” InTech, March 1983.

31. Melville, H. J., “PLC Applications in Batch Digester Control,” ISA1980 Annual Conference, Advances in Instrumentation, Vol. 35,Part II, Instrument Society of America, October 20–23, 1980.

32. Hawkinson, G. M., “Selecting a Programmable Controller for a Ret-rofit,” 9th Annual Programmable Controllers Conference Proceed-ings, ESD, March 1980.

33. King, J. and Ptak, J., “Toothpaste Processing Using a PC Mini Micro-computer Network,” 12th Annual Conference and Equipment DisplayProceedings, ESD, March 1983.

34. Harrold, D., “Controllers —Heartbeat of Production,” Control Engi-neering, January 2002.

35. Larson, K., “The Shifting Sands of DCS, PLC, & PC Technology,”Control, May 1995.

36. Stafford, T., “Taking Back Control,” InTech, July 2002.37. Keskar, P., “Sailing to the Islands —Islands of Automation among

Goals in Budget-Strapped Upgrades,” InTech, February 2003.38. Harrold, D., “Figure Out When Enough Is Enough,” Control Engi-

neering, June 2002.39. Bryant, J. A., “Is Your Plant’s Control System Safe?” Power, August

1979.40. Klostermeyer, W. H. and Thurston, C. W., “PCs Prove Reliable in

Chemicals and Plastics,” Control Engineering, April 1976.41. Hoffman, E. L., “Redundant Control Systems,” 10th Annual Confer-

ence and Equipment Display Proceedings, ESD, March 1981.42. Sykora, M. R., “The Design and Application of Redundant Program-

mable Controllers,” Control Engineering, July 1982.43. Penz, D. A., “Organizing PC Software Development,” Instruments

and Control Systems, February 1982 and March 1982.44. Heider, R. L., “The Programmable Controller as a Process Link to a

Distributed Computer Network,” ISA 1980 Annual Conference,Advances in Instrumentation, Vol. 35, Part II, Instrument Society ofAmerica, October 20–23, 1980.

45. Rosandich, R. G., “What to Know About PLC Ladder Diagram Pro-gramming,” EC&M, June 1996.

46. Conrad, W., “Document Your PC Design,” 9th Annual ProgrammableControllers Conference Proceedings, ESD, March 1980.

47. Stockweather, J. W., “Proper PC Documentation for the User,” 11thAnnual Conference and Equipment Display Proceedings, ESD, March1982.

48. Langhans, J. D., “The Programmable Controller in a ContinuousProcess,” Control Engineering, July 1972.

49. Dunlap, R. A., “8 Ways to Improve Control System Projects,” ControlEngineering, April 2003.

50. Harrold, D., “Find the Right Engineering Tools,” Control Engineer-ing, September 2001.

51. Kraemer, W. P., “Testing and Start-up of Programmable ControllerSystems,” Paper PC 179–14, IEEE Transactions on Industry Appli-cations, Vol. 1A-16, No. 5, September/October 1980.

52. Rosandich, R. G., “Troubleshooting PLCs,” EC&M, May 1996.53. McBride, A. E., “Programmable Logic Controllers—Get More than

What You See,” Control Engineering, February 2001.54. Programmable Controller Course, Instruments and Control Systems,

Radnor, PA: Chilton Co., 1981.

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© 2006 by Béla Lipták