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RTUs vs PLCsWhat is the Difference?

A

common question we hear frompeople just becoming involved in aut-omation is What is the difference be-

tween a PLC and an RTU? Before answering I tryto find out if they actually know what the acro-nymsPLCandRTUreally represent (see the re-lated article aboutacronyms in gen-eral on page 3).

PLC meansProgrammableLogic Control-ler. RTU canrepresent Remote

Telemetry Unit or Remote TerminalUnit (likeTetragenics offers). These two kinds of RTUs arecompared against PLCs across the industry.

But, knowing what PLC and RTU stand for isnot enough. You need to know what each pro-vides. In this article I discuss the differences be-tween a PLC and the two types of RTUs, startingwith the least sophisticated and working towardsthe most sophisticated device.

Remote Telemetry UnitsRemote Telemetry Units are usually nothing

more than a multiplexed addressable I/O devicewith communications. They have input and out-put points, and they are connected to a more in-telligent controller. The controller is responsiblefor the control algorithm. This kind ofRTUhasvery little computing power and is specified foruse in installations like water and wastewater au-tomation.

The Remote Telemetry Unit is strictly a slavedevice. It is not programmable and cannot be

used as a stand-alone controller, but it is usu-ally addressable. You can use it to relay statuand values both from the remote site to a controller and from the controller down. But itcannot communicate with other devices at orbelow its own level.

ProgrammableLogic Controller

PLCs use whathe industry calls

Ladder Logic. [Lader Logic is arepresentation

of relay logic an

consists of two vertical lines with contact symbols along the rungin between (hence, the ladder look - see theexample below).]

The first PLCs were just a software repre-sentation of relay logic. In other words, theywere designed to duplicate the functionality oa rack of interconnected relays. In the last fewyears higher end models have been supple-mented with analog inputs and outputs. Awide range of pricing is available from low

end PLCs at $150, to high end models that sefor around $100,000.The low end PLCs are not even addressab

(they cannot be used as a slave to another de-vice or as acomponentin a controlsystem).

byS

teveShannon

RTUs vs PLCs

Acronym Soup -What is

that TLA?

Open House a Success!

Come See Us!

Tech Tips

Did You Know?

I N THIS

I SSUE

Putting You in Control!

Continued

on page 2...

RTUs! PLCs!

R8

[RPM INPUT]X200

C8C8-1C8-2

R9

[PRESS INPUT]

X201C9 C8-1

R10C101 [R1]

MODUA1B1

R11C101 [R3]

MODUA1B1

L.O. TEMP ALARM

L.O. PRESS ALARM

PRESS LOCKOUT ALARM

RPM LOCKOUT ALARM

LOCKOUTCOIL

LOCKOUTCOIL

Y101

Y102Example Ladder Logic

Tetragenics Company Newsletter

Control SCADAMonitoringAutomation Communications

HydroelectricVIEWSVIEWSVolume 2 May 1997Putting You in Control!

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Anaviga-

tor on the

USS NavalShip George

told his captainthey were on

a collisioncourse.

The captain radioed theblip on the screen. Youare on a collision course,please alter your course.

Roger came the returnvoice. Suggest YOU altercourse.

Surprised, the captainsaid again, I repeat, youare on a collision course,alter your route.

Roger that, we suggest

you alter course.Loosing his patience the

captain boomed into theradio, This is CaptainJohnson of the USSGeorge, I order you toalter your course! After amoment a calm voicecame over the radio.

This is alighthouse. Itsyour call!

QUALITY PERFORMANCE / QUALITY PRODUCT

RTUs vs PLC - Face Off

Tetragenics 406-782-9161 www.tetragenics.com

electrically reading each I/Opoint. This is done quickly,but in a system with lots ofI/O points it can take sometime to completely scan allthe points.

PLCs can be usedas stand-alone devices

but they are diffi-cult to configure.You must useladder logic to

program them.Normally,they are not

usable as the mastercontroller in a control system.If your system contains lots ofI/O that must be monitoredor controlled, PLCs are usu-ally not the best choice. Theyare not appropriate for use asprotocol convertors or for

controlling other intelligentelectronics devices (IEDs).

Remote Terminal Units

The most sophisticated ofthe three devices is the secondkind of RTU the RemoteTerminalUnit. This is thekind of RTU that Tetragenicshelped pioneer.

These RTUs actually havethe intelligence needed to con-

trol a process (or multipleprocesses) without interven-tion from a more intelligentcontroller or master. Tetra-genics RTUs offer SCADAcapabilities and features thatlesser RTUs cannot begin tooffer, such as interrupt drivendigital inputs, time stamped

ming training, PLCs can be

difficult to configure. Andbecause of the wide pricerange, you get the functionsyou pay for.

If you need an intelligentunit to control multiple processes without interventionfrom a controller or master,you need a Tetragenics Re-mote TerminalUnit. Theseintelligent units also provideadvanced control functions

and are suited for expansion

Know Before You Buy

Whatever unit you selectmake sure it meets your re-quirements. And remember,

just because you think RTUstands for Remote TerminalUnit, someone else thinks itstands forReally Tuff Um-brella.

PLCs scan their I/O by sequence of events, datalog-

ging, intelligent communica-tions, multitasking sequentialcontrol, PID control, alarmlogging, modular construc-tion, easy programming; well,the list just goes on and on.

The Remote TerminalUnit is a fast and flexible solu-tion to most control needs. Itserves both as the master con-troller or a slave controller. Infact, it can be used as both a

slave and master simulta-neously in a vertically de-ployed control system. Also,it is easy to use as a protocolconverter or for controllingIEDs. And it can be expandedas the control system grows.

Face OffThe face off between PLCs

and RTUs really depends onwhat you need and can afford

now and what you need inthe future. Sometimes a quicklook at where you want yoursystem to be 2, 5, or even 10years from now can help youdecide what you want to buytoday.

If you have a small systemand need only a slave devicewith little computing power,a Remote Telemetry Unit willprobably work. But if youneed to interface with otherdevices, you will have to lookto PLCs or Remote TerminalUnits.

If you need a stand-alonedevice that has power, a PLCmight work for you. But beprepared for some program-

... Page 1

Continued

PLC

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Published by the Tetragenics Com-pany three times a year to provideuseful information about products,corporate news, and industry eventsPlease send comments to the editor.

Editor: Ruthmeri Gleason

Newsletter Staff: Don Scott, StevShannon, June Tangaro, Rita Spear

1997 Tetragenics Company

Tetragenics is a registered trademarkof Tetragenics Company. All producnames are the registered trademarksor trademarks of their respectivemanufacturers.

TETRAVIEWS

Putting You in Control!

130 North Main, Butte , MT 59701

Website: http:\\www.tetragenics.comPh: (406) 782-9161 Fax: (406) 496-5330

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Printed on Recycled PapHelp Keep the Earth Alive.

Tetragenics 406-782-9161 www.tetragenics.com

Acronym Soup - Its a Mouthful

AcronymsA/D - Analog to DigitalAC - Alternating CurrentADC - Analog to Digital ConverterAFRC - Air-Fuel Ratio Controller

AGA - American Gas AssociationAI, ANI -Analog InputANSI - American Nation Standards

InstituteAO, ANO - Analog OutputASAP - As Soon As PossibleASCII - American Standard Code

for Information InterchangeBCD - Binary Coded DecimalBios - Basic input/output systembps - bits per secondByte - Eight bitsCAD - Computer Aided DesignCD-R - Recordable CD-ROMs

CD-RW -Rewriteable CD-ROMsCDPD - Cellular Digital Packet Datacent - centuryCI - Counter InputCMOS - Complex Metal Oxide

SemiconductorCMS - Communications Monitoring

SystemCPU - Central Processing UnitCRT - Cathode Ray TubeD/A - Digital to AnalogDAC - Digital to Analog ConverterDC - Direct CurrentDI - Digital InputDIMM - Dual inline memory moduleDO - Digital OutputDOS - Disk Operating SystemECL - Emiter Coupled LogicEEO - equal employment opportunityEFM - Electronic Flow MeasurementEIA - Electronic Industries Associa-

tionsEIT - Engineer in TrainingEPROM - Erasable Programmable

Read Only MemoryFAQ - Frequently Asked Questions

FAT - Factory Acceptance TestFET - Field Effect TransistorFubar - Fowled up beyond all

recognitionFYI - For Your InformationFYA - For Your AmusementGUI - Graphical User InterfaceHMI - Human Machine InterfaceIDE - Independent drive electronicsIED - Intelligent electronic deviceIEEE - The Institute of Electrical and

Electronics EngineersI/O - Input/Output (connections

from a computing device to theoutside world)

IRS - Internal Revenue ServiceISA - Instrument Society of America, or

Industry Standards Architecture

LAN - Local Area NetworkLED- Light Emitting Diode

MMI - Man-Machine InterfaceMOSFET - Metal Oxide Semiconduc-

tor Field Effect TransistorMRI - Magnetic Resonance ImaginingMTBF - Mean Time Between FailureMUX - MultiplexorNEC - National Electric CodeNEMA - National Electrical

Manufacturers AssociationNFPA - National Fire Protection

AssociationOSHA - Occupational Safety and

health AdministrationPAL - Programmable Array Logic

PCA - Printed Circuit Assembly (aPCB with components)

PCB - Printed Circuit BoardPCS - Plant Control SystemPE - Professional EngineerPEEL - Programmable Electrically

Erasable LogicPID - Proportional-Integral-

Differential controlPLC - Programmable Logic ControllerRAM - Random Access Memory

ROM - Read Only Memoryrpm - Revolutions per minuteRTD - Resistance Temperature

DetectorRTU - Remote TerminalUnit,

Remote Telemetry UnitSCADA - Supervisory Control And

Data AcquisitionSCSI - Small computer systems

interfaceSIMM - Single In-Line Memory

ModuleSMTP - Simple Mail Transfer

ProtocolSOE - Sequence of EventsTBOS - Telemetry Byte Oriented

Serial ProtocolTCP/IP - Transmission control

protocol/Internet protocolTG - Tetragenics

TGR - Tetragenics Graphics terminalTLA - Three Letter AcronymTTL - Transistor Transistor LogicUL - Underwriter Laboratories IncVLB - Vesa Local BusVRTX - Virtual Real-Time Executive

SystemWAN - Wide Area NetworkWhatzit - A bacon cheeseburger at the

Butte M&MWORM - Write once read manyWTV - Windows TetraVision

As you can see, the list only gets

longer. There is also a whole newslew oflanguage shortcuts being cre-ated in Internet chat rooms: gbh(great big hug), atm (at the moment),bfn (by for now), brb (be right back),or ttfn (ta ta for now). English profes-sors are even now mourning the lossof the language.

ttfn .

OPEN

HOUSE A

HI T

Thanks to everyone who came toour Open Houseto celebrate our

25th AnniversarIt was a greatsuccess. Westopped countinafter 300!

We enjoyed greafood, fun demonstrations, andgood company.

TEAM TETRAGENICSTGTGTGTGTGTG NEWSNEWS

Have you noticed that every product nowadays has to have its own little acronym (a word made upof the initial characters of a grouping of words). One of the first shortcuts people look for is intheir speech or writing. Consequently, our lives are riddled with acronyms. Perhaps as a measure

of the intensely busy lives they face, acronyms are especially prevalent among engineers. But keeping trackof industry and company acronyms and exactly what they mean (and what function the product canperform) can be confusing and sometimes downright misleading. We have no acronym police to tell uswe are wrong. Like it or not, acronyms are here to stay. So, we decided to ask around and find out whatTLAs people use the most. Good grammar notwithstanding, heres some of what we found. If you have adifferent definition for any of these (and I am sure some of you do), let us know. We will have a face off.

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Putting You in Control!

130 North Main Street

Butte, MT 59701

www.tetragenics.com Tetragenics Company 406-782-9161 Fax: 406-496-5330

Did You?Know? . . .

Summer Issue

BULK RATE

U.S. POSTAGE

PAIDPermit No. 96

Butte, Mont. 59701

SPOTLIGHT

New board option lets you add multiple TG332 CPUs to your system for greater

serial expansion. The new option on the TG332 CPU board provides more port capabilitieand processing power. You simply add the new board to a chassis containing another CPUboard (either a TG332 or Z80). This allows the new CPU to operate independently of theinput or output boards in the chassis. The original CPU handles the I/O functions and itsown COM ports. Each additional CPU provides another three open, serial ports for connection to other devices (such as expansion units, JEM meters or Schweitzerrelays).

You can increase functionality at a site without disturbing the exist-ing database or adding a new chassis and/or cabinet. Just add the TG332CPU to the database as a new RTU with its own name, ID, and database.You do not need an added communication link because each additionalCPU is connected to the previous TG332s port B. See the example diagram.

The new option number for the TG332 CPU is 43743-1740S. Call formore information.

by June Tangaro

More Ports, Multiple Functions

AskOur

Panel of Experts

TechTips

Master

A

B

A

B

43743-1740P 43743-17TG332 TG33

P S

Multiple TG332s

We have a Website. Visit us at www.tetragenics.com. We are addinginformation all the time. Data sheet information is available and we hopto get the newsletter online soon. Let us know what you would like tohave available!

We support a growing line of protocols. If your system

needs to talk to existing or new equipment, call us. Also, weoffer a wide range of system integration services and engi-neering support.

We have two new engineers joining our team, Kirt Mayson andTodd Dvorak. Kirt and Todd bring years of experience and educationto our company. Youll hear more about these guys in later issues.

Your Own Copy Multiple Copies . . .Are you reading your coworkers copy ofTetraViews? Or receiving multiple copies? Or

just too swamped to read it? If you want to beadded to or taken off our mailing list, call ourmarketing department or e-mail us.

COME SEE US

We are always available atour offices in Butte, MT.

Also, we will be at the showsbelow. Hope to see you there.

Metro East Trade Centre: May 13-15,

1997, Pickering, OntarioWater Power 97: August 5-8, 1997, AtlantaGeorgia

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