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Foundation Fieldbus Interview Questions and Answers,Communication FoundationFieldbus January24,2016

ManufacturingSolutionsIntegratedManufacturingERPSystemFromEpicor.FreeERPInfoPack!

WhatisFOUNDATIONfieldbus(FF)technology?

TheFOUNDATIONfieldbussolution isasystemic technologycomprisedofabidirectionalcommunications protocol used for communications among field devices and to the controlsystem. ItusesaFunctionBlockstructure for truedistributedcontrolandDeviceDescription(DD) technology for parameterisation and integration of data via a network hierarchy forsubsystem integration and a welldefined system management structure for reliability anddeterminismoffunctionalexecution.

ArethereregisteredFFdevices?

Yes. The Fieldbus Foundation registers a wide range of FFcompliant products fromtransmittersandmeterstovalvepositioners,actuators,controllersandlinkingdevices.Theseproducts are available from a wide variety of the worlds leading automation equipmentsuppliers.Theonlineproductcatalogue(www.fieldbus.org)givesregistereddeviceinformationincluding which standard blocks were tested for interoperability, the presence of untestedFunctionBlocks(ifany)andadditionalusefulinformationaboutthedevice.

WhatisFFH1?

TheH1Fieldbusisabidirectionalcommunicationsprotocolusedforcommunicationsamongfielddevicesandtothecontrolsystem.

WhatarethebenefitsofFieldbus?

From a business viewpoint FF technology delivers savings in total installation costs. H1Fieldbusreducesinstrumentwiringwhichinturnmeanslessterminationandfewerscrewdriverturns. The technology reduces hardware requirements and lowers capital expenditures(CAPEX) whilst it also reduces operating expenses (OPEX) through improved plantefficiencies,betterassetmanagementandreducedmaintenancerequirements.

SpecificbenefitsofFieldbustechnologiesincludereducedwiringmultivariableinformationviaasinglemultichannelfieldinstrumentsimplerintegrationandeasiermaintenance.Ultimately,Fieldbustechnologywillbethekeytogreatermanufacturingflexibilityandproductivity,higher

quality products and improved regulatory compliance. This can be achieved by predictivemaintenance scheduling and better upkeep via the embedded diagnostics, performanceanalysisdataandoperationalstatistics.Properlyadjustedandcalibrateddevicesensurelowerprocessvariabilityandhigherplantavailability.

Theisochronousbuscycleenablestighterlooptuningandasaresult,betterprocesscontrol.Firmwaredownloadgivestheabilitytostaveoffobsolescence,givingFieldbusplantslongerlifethanotherplants,withgreaterease.

HowisFOUNDATIONH1unique?

FFH1istheonlydigitalFieldbusprotocoldevelopedtofullymeetwiththeoriginalIEC61158requirements. Unlike other protocols, H1 provides explicit synchronisation of control andcommunication for precisely periodic (isochronous) communication and execution of controlfunctionswithminimised dead time and jitter. It synchronises clocks in Fieldbus devices forsupportofFunctionBlockschedulingandalarmtimestampingatthepointofdetection.

Additionally, H1 provides automatic address setting, eliminating the need to manually setaddresses offline using a tool or DIP switches and avoiding subsequentmistakes. H1 alsouses peertopeer communication where devices communicate directly using apublisher/subscribercommunicationrelationship,enablingdatatobesenttoseveraldevicesinasinglemessageandthusreducingsystemoverhead.H1includesalarmandeventreportingforefficientdiagnosticsandprocessalarms,whilstonlinefirmwaredownloadsmakeitpossibleto upgrade devices in order to stay ahead of the obsolescence curve. Lastly, a rigorousinteroperabilitytestingprogramensuresconnectivityproblemsareminimised.

IsFOUNDATIONH1easiertousethantraditionaltechnology?

H1 has made it possible to mine important information from the plant floor. When usedcorrectly, this information empowers operators and technicians tomake plant operation andmaintenance easier. Some end users report commissioning time savings as high as 75%comparedwithconventionalanaloguetechnologybyswitchingtoFieldbustechnology.

FOUNDATION technology enables improved asset management using device managementsoftware,asmany failurescanbepredictedand faultscanbediagnosed indetail.Together,devicemanagementsoftwareandFieldbusdevicestypicallyenable:

Identification&informationDiagnostics,performanceanalysis&operationalstatisticsParameterisation,ranging,reconciliation&audittrailSimulation&overrideCalibrationtrim&logDocumentaccessDeviceeventcapture&monitoringCommissioningMaintenancelog&servicenotesDevicelistingMaintenance&calibrationschedulingHowdoesH1Fieldbusdifferfromthevariousdevicelevelnetworksnowinuse?

H1wasdesigned specifically for processautomation. It differs significantly fromdevicelevel

networksforfactoryautomationinitsabilitytoprovideintrinsicallysafepowerforfielddevicesused in hazardous locations. Moreover, thanks to scheduling, H1 Fieldbus is isochronous,whichisarequirementforcontinuousregulatoryprocesscontrol.

H1wasdesignedanddevelopedasasystemtechnologyprovidingsignificantlymorestructureandbenefitthanmerecommunicationnetworks.H1technologydeliverstruedistributedcontrolnetworking and provides asset optimisation through a broad range of diagnostic capabilitiesand consistent data structures through the use of Electronic Device Description Language(EDDL).FFtechnologyalsoprovideseasyandextensiveintegrationcapabilitiesthroughopenandconsistent integrationtoHSEinasingleengineeringenvironment.Devicelevelnetworkswithout power and intrinsic safety (and no scheduling) are mostly appropriate for on/offcomponentswithlimiteddataandfordedicatedapplicationssuchasmachinecontrol.

WhatisFOUNDATIONHSE?

FOUNDATION High Speed Ethernet (HSE) is a control network technology specificallydesigned for process automation to connect higherlevel devices such as controllers andremoteI/O,highdensitydatageneratorsetc.,andforhorizontalintegrationofsubsystems.

WhatarethebenefitsofHSE?

FOUNDATIONHSEenjoysafullyfledgedredundancyschemegivingcontrolsystemsgreateravailabilitythansystemsusingsimpleringtopology.UseofunmodifiedEthernetandstandardIP makes HSE systems more costeffective than other Ethernet solutions and proprietarynetworks.SinceHSE isastandard, itenablessimpleand tight integrationbetweenpackageunitsandthemaincontrolsystem.TheHSEnetworkiseasierandcheapertomanagebasedoncommonnetworkknowledgeandstandardSNMPtools.

HowisFOUNDATIONHSEunique?

FOUNDATIONHSEisbasedonunmodifiedIEEE802.3Ethernetandisthereforecompatiblewith standard Ethernet equipment. Unlike ring topologyFOUNDATION HSE providescomplete DCS style redundancy with redundant network switches, redundant devices andredundantcommunicationportsensuringunsurpassedavailability.FOUNDATIONHSEisalsobasedonstandardIP,enablingittocoexistwithotherdevicesandensuringcompatibilitywithstandardtools.Atthehighestlevel,FOUNDATIONHSEincludesastandardapplicationlayerthat provides interoperability between devices beyond the mere coexistence provided byEthernet and TCP/IP. FOUNDATION HSE communication is scheduledriven to minimisedeadtime and jitter with support for peertopeer communication directly between devices.Again,arigorousinteroperabilitytestingprogramensuresminimumconnectivityissues.

IsFOUNDATIONHSEeasiertousethanRS485andcoaxbasednetworks?

Thehubandspoketree topologyofEthernetmakes itveryeasy toaddandremovedeviceswithoutupsettingtheoperatingnetwork.BecauseFOUNDATIONHSEisbasedonunmodifiedEthernet, standard Ethernet tools can be used for installation qualification, testing andtroubleshooting. These tools speed up the resolution of communication problems.FOUNDATIONHSEissupportedbytroubleshootingtoolsthatarenotavailableforRS485orcoax, and since FOUNDATION HSE is based on UDP and TCP, standard networkmanagement tools employing SNMP, RMON, etc., can also be used. Similarly, familiar IPaddressingisusedincludingsupportforDHCP.

WhatmakesFOUNDATIONdevicesinteroperable?

Interoperability ismadepossibleby the fact thatdevicesandsoftwareareconforming to thesame standard. Testing and registration ensures that Fieldbus devices bearing the officialcheckmark seal can be connected on the same bus or network and exchange informationwithout significant integration effort. End users can select the best device for a specificmeasurement or control task, regardless of the manufacturer. A current list of registeredproductsisavailableontheFFwebsite(www.fieldbus.org).

HowisinteroperabilitydefinedinrelationtoFOUNDATIONtechnology?

InteroperabilityreferstotheabilityofanyITKregistereddevicetoworkwithanyHISTanalysedhost. Users want to operate their entire plant from one operator interface and maintain alldevicesonthesystemwithonemaintenanceapplication.

AreFieldbusdevicesinterchangeable?

Yes.EnduserscansubstitutearegisteredFieldbusdevicefromonemanufacturerwiththatofanothermanufacturerontheirnetworkwithoutlossofdegreeofintegration.

WhattypesofprojectsareappropriateforFieldbustechnology?

Fieldbus technology is now being utilised on automation projects of all sizes, including bothnewandretrofitinstallations.AccordingtoanARCAdvisoryGroupsurvey,asignificantnumberofendusersareinstallingsmall,mediumandevenverylargeFieldbussystemsconsistingofover2,000 I/Opoints.Shell, amajoradopterofFF technology, reportedlyhasover120,000installedFieldbusI/Owith31%ofthesedevicesreplacingconventionalinstruments.

WhatapplicationsarebestsuitedtoFieldbus?

BothH1andHSEwerespecificallydesignedforprocesscontrol.BecauseH1andHSEhavecomplementary characteristics covering diverse networking needs, Fiedlbus technology isappropriate for many applications. These include closedloop continuous control, batchsequencing, remoteI/O and legacy system integration. The Fieldbus Foundations SafetyInstrumented Systems (SIS) specification includes an additional safety layer based on IEC61508requirements,makingitsuitableforuseinplantsafetysystems.

Howdoendusersinfluencethedirectionofthetechnology?

TheFieldbusFoundationestablishedanEndUserAdvisoryCouncil(EUAC)togiveendusersa voice in the direction of its technology. The group meets two to three times per year todiscussissuesinvolvingtheirspecificplantsandindustries.

IsFieldbusanopentechnologyavailabletoallsuppliersandendusers?

Yes.A key tenet of both theFieldbusFoundationandPROFIBUSprotocols is that theyarenonproprietaryandavailable toallcompanieswishing to include it in theirproductofferings.BoththeH1FieldbusandtheHSEcontrolnetworkarepartoftheIEC61158protocolstandardandIEC61784profile.

DoesFFtechnologyallowtraditionalcentralisedcontroltobelocatedintheDCS?

Yes.FFtechnologyprovidesenduserswiththefreedomtolocatecontrolinthecontrolsystem

orinthefielddevices.TheFieldbusFoundationdoesnotadvocateaparticularcontrolstrategy.

HowdoesFFsolvethebusinessneedsofenterpriseautomationapplications?

FOUNDATION HSE enables information from intelligent devices to pass through controlsystemswithoutloss,degradationorthedelayassociatedwithproprietarycontrolnetworkstodevicemanagementsoftware.ItalsoallowsthepossibilityofelectronicintegrationwithhigherlevelMISandERPapplications.

HowdoesFOUNDATIONtechnologyhandlediscreteI/O?

FieldbusFoundationmembersprovideavarietyofdiscretedevicesforminiremoteI/O,relay,on/off valves, valve couplers, electric actuators, etc., on H1 as well as large remote I/O onHSE. Indeed, on/off signals in bulk are an integral part of the FOUNDATION solution. Thetechnologyiswellsuitedforhybridcontrol.

IsFFtechnologyarecognisedinternationalstandard?

The IEC voted to include the FOUNDATION HI and HSE specifications in the IEC 61158internationalFieldbusstandard.TheCENELECTechnicalBureauaddedtheFOUNDATIONH1specifications toEN50170, theFieldbusEuronorm. Inaddition,FOUNDATION technology istheonlyimplementationoftheANSI/ISA50.02standard.

Both NAMUR (Germany) and JEMIMA (Japan) have voiced support for FOUNDATIONtechnology, and provided input from the end user community that aided in specificationdevelopment. Approval and support by key international industry bodies gives confidence tousers that their investments in FOUNDATION solutions are based on recognised globalstandards.

WhatarethemostcommonFieldbuswiringerrors?

Therearetwoverycommonwiringerrorsthatwillcauseoperationaldifficultyifnotremedied.The first isashort from theshield tooneof theFieldbusconductors.Although thismaynotimmediatelycausecommunicationerrors, itground references (since theshield isgrounded)theFieldbuspairandmakes itmuchmoresusceptible toelectrical interference.Thesecondmostcommonproblemisan incorrectnumberofTerminators.Thereshouldbe twoandonlytwo Terminators on each Fieldbus segment. Since the Fieldbus installation will likely beinstalledbycontractorswhoareunfamiliarwithFieldbus,werecommendaSegmentCheckoutProcedure BEFORE powering up or commissioning the segment.Many host suppliers havesuch procedures. The Fieldbus Foundation also publishes an engineering document forFieldbusthatcontainssuchaprocedure.

HowisProfibusPAdifferentfromFoundationFieldbus?

BothProfibusPAandFoundationFieldbussharethesamePhysicallayerdefinitions.Thereforethe connectionandpowerequipment thatwillwork identically for both typesof busses.Thedifferenceisintheprotocol.Therefore,FBT3FieldbusMonitorwillnotworkwithProfibusPAasitwasonlydesignedforFoundationFieldbus.

WhatisthedifferencebetweenHARTandFieldbus?

Thatislikeansweringwhatisthedifferencebetweenabicycletireandacar?HARTisonly

meantforcommunicationprotocol,whereasFieldbusisactuallyasystemarchitectureincludingcontrolstrategyetc.

Notmerelybetter,butradicallydifferent.Indeed,mistakingFieldbusforadigital420mAorabetterDCSislikemistakingthecomputerforabettertypewriter.Itcannotevenbecomparedtosmart transmitter protocols. Imagine for a moment: A system that renders obsolete allseparate signal conditioners, isolation amplifiers, input cards, output cards, CPU cards, I/Pconverters,andtheirwebofinterconnectingwires,almostanentireDCS.Asystemcompletelyselfcontainedexpressedsimplyas fielddevicesandamanmachine interface(MMI) likeanoperatorconsole.Asystemwhereallcontrols,alarms,computation,selection,Totalizationandmuchmoreperformedby the fielddevicesmicroprocessors.A systemsopowerful itmayovercomeaprocesscontrolsproblemswithafewclicksofamouse.

Fieldbus is the ultimate. The signal transmission has evolved to completely digital, systemarchitecturehasevolvedtocompletelydistributedtothefield.I.e.Fieldbusnotonlyreplaces420mA,butalsothebynowhopelesslyoutdatedDCSarchitectureoftheseventies.

WhatarethebenefitsofFieldbus?

ThebenefitsofFieldbusaremanyandsomewerealreadydescribedLowercostofpurchaseandownershipProductivityandqualityincreaseHigherintegrityandaccuracyAccesstomoreinformationanddiagnosisEasierinstallationandstartupFreedomofcomponentchoiceEasiertoconfigureEasyforhavingsingleconsistentdatabase

HowdoesFieldbuslowerthecost?

Economicbenefitsspringmainlyfromthefactthatlesshardwareisrequired.OftheexpensiveDCSonlytheoperatorconsoleremains.Duetoitsnoncriticalnature,aPCorIPC(IndustriallyhardenedPC)withofftheshelfsoftwaremaybeused,evenforlargesystems.

Fieldbusdevicesmayperformmultiplemeasurements,controlandcomputations.Thenumberof transmitters may be reduced, single loop controller and computational units will not berequired,againsavingcost.

Though Fieldbus, the device price may initially be higher than conventional or smart, thereduction indevicesandwiringwithassociatedcable traysandmarshallingboxeswillmakethesystemcheaper.Manufacturerscannolongerrelyonproprietarytechnologytokeeppricesup.OpencompetitionenabledbyaplayingfieldlevelledbyFieldbuswillreduceprices.

Cost isnotonlypriceTherearesubstantial longtermsavings resulting from the increasedinformationflowintermsofdiagnosticinformationandeasiercalibrationetc.

ManyFieldbustransmitterswillbeofamultiplevariabletype.Forexample,dualchannelfieldmounted temperature transmitters measuring two temperatures being able to transmit both

variables in real time.Sincea single transmitter can condition several sensors, the cost permeasurement point will be lower. This is the ideal combination of price and performance.Previouslymultipletemperaturessensorswheretypicallyhandledbyconditionersinthecontrolroomwith longwires into the field,havingapenaltyonaccuracyandnoise immunitydue tofactorslikeleadwireresistance,poorornothermocouplecompensationwireandpoorroutingof low level signals. Multiple variable technology makes it economically viable to use fieldmountedtransmittersrightnexttothesensorsforimmediateinterfaceandtransmissioninthehigh integrity digital domain. Added advantage is the capability to perform high and lowselection,redundantsensorvoting,averageanddifferencecalculationinthefield.

Fieldbushelpscompanies increaseproductivity, flexibility,qualityandcomplywithevermorestringentenvironmentalregulations,andatthesametimelowertheiroperatingcost.Fieldbusisthemeanswherebycompanieswillstaycompetitivewellintothe21stcentury.

Whatisallthisdiagnosticsstuffinfieldbus?

Thedigitalcommunicationprovidesthemeansforallthedevicedata,configuration,operationanddiagnosticsdetailstobeaccessedfromthecontrolroom.ThecompleteinteroperabilityofFieldbusenablesthisdatatoalsobeaccessedbyanysoftwarethatdesirestodoso.

ThemultiplevariablenatureofFieldbusalsoallowsnewinnovativedevices.TransmittersthatmeasuremorethanonevariablehavebuiltinflowandHTGcomputersetc.

SchemesforconsistentdiagnosticsinterrogationandfailurealertsweredesignedintoFieldbusfromtheverybeginning.Theselfdiagnosticsoffielddevicesmayreportfailuresandproblemsimmediately,enablingmaintenancepersonnel topinpointerrors instantlyorevenbefore theycancauseanyharm.Moreselfdiagnosticswill thereforebe implemented.Hardware failureslikesensor,actuatorandmemoryproblems,andtheoperatorreportsallsoftwareproblemslikeconfigurationorcalibrationerrorswithout theneedformanual interrogation. In theeventofafaultitisreportedwiththeassociatedinformationlikewhichdevice,whattypeoffault,priorityandthetimestampetc.

Thebenefitfortheoperatortogetthisinformationwithouthavingtobringthetransmitterintoaworkshop for testing isobvious.The timethatcanbesavedbynothaving to testonlya fewtransmittersisenormous.Diagnosticsenablesyoutoquicklydetermineifaprocessproblemisduetothetransmitterornot,withouthavingtodoseveralfieldvisits.Productioncangetbackinoperationinminutes.

Allthisreduceserrorsandmakescalibrationlessofachoirensuringitisdonecorrectlyandonaperiodicbasis.Theendresultisabettermeasurementandimprovedquality.

Whatisatransducerblock?

Thisblocksinterfacesthefunctionblocktothedevicehardwaresuchassensors,actuatorsandswitches.Thisblockisresponsibleforinformationandfunctionalityspecifictoforexamplethemeasurementofaparticularphysicalproperty,suchaspressureortemperature,oraspecificmeasurementtechnology,suchasultrasonicorCoriolis.

Thetransducerblocksinterfacetothefunctionblocksoverhardwarechannel,whichisdifferentfrom functionblock links.Transducerblockhandlenotonlymeasurement,butalsoactuationanddisplay.

Whatisaresourceblock?

The resource block contains information which is common to the whole resource, includingdeviceidentification,hardware,devicefeatures,memoryandCPUavailability,writeprotection,managementoffailsafeandalarms.

Thedataisnotprocessedinthisblock,sothereisnoblockalgorithm.

Whatisautomaticalarmnotification?

AlarmsandeventsarejointlyreferredtoasAlerts.Thefunctionblocksintothedevicesdetectalarms.Whenalarmsandothercriticaleventsoccur, the functionblockautomaticallynotifiestheMMIbysendinganAlert.TheMMIconfirmsreceiptofalertnotificationtothefielddevice.Ifthefielddevicedoesnotreceivetheconfirmation,itwillresendthenotification.Alertsarealsoissuedwhenalarmconditionsdisappear.Thustheoperatorinterfacedoesnothavetoperformperiodic polling to determine if there is an alarm condition, and surely any alarm can bedetected.Informationinthenotificationincludestimeandpriority.

Thecurrentstatusofalarmsandeventsmayalsobecheckedatanytimefromalarmandeventparametersineachblock.

Howdoesdistributedtrendingwork?

The device may do trending itself. This way periodic time critical communication is notnecessary.AMMIonlyneedstopolladevicefordatawhenaparticularparameterisdisplayedonthescreen.Whentheparameterisnotdisplayed,pollingneednotbedone.

Asetofdata,16samples, iscollectedoverashort,userconfigurable,periodof timeand isthenautomaticallysent inasinglecommunicationto theMMIwherethe longtermstorage isdone.

Whatisadevicedescription?

Sinceprofilesofferschoicestowhatcapabilitiesandparametersareimplementedinadevice,a configuration device or other MMI need to know which of these a particular deviceimplements.

The device description (DD) specifies which application area and device profile a devicebelongs to. For certain parameters it also specifies options are valid and more specificmeaningsetc.DDmayalsodefinemethods,e.g.forstepsaHosttofollowwhencalibratingthedevice. The DD file also can describe an user group or vendor specific features on thesedevices.

TheDDispreparedbythedevicemanufacturerandmayeitherbestoredinthedeviceitselforbesuppliedseparately,e.g.onafloppydiskorCDROM.AMMImayusingtheDDcompletelyand correctly configure a new device or new version of a device without a correspondingupgrade.

WhatarethedistinguishingfeaturesofatrueFieldbus,deliveringtheoriginalpromise?

ThemaincharacteristicsofFieldbusare:

Completelydigitalreplacementof420mAControl,alarm,computationandotherfunctionsdistributedtodevicesinthefieldMultivendorinteroperablethroughopenspecificationSpecificallydesignedforprocesscontrolDCS/smarttransmitterandotherprotocolscallingthemselvesFieldbusdonotshowthesecharacteristics

Whatisthebenefitofbeingcompletelydigital?

UnlikethesemidigitalDCSsystem,theFieldbussystemisapuredigitalsystem.Insteadofthesystemspressureortemperaturetransmittersconvertingasenseddigitalprocessvaluetoan analog 420 mA signal before feeding it to the DCS and the rest of the system chain,Fieldbuskeepsthesignalpurelydigitalallthewayfromthetransmittertothedigitalinputofthecontrolvalve.Fieldbusdevicesshouldnotbemixedupwithwhatwetodaycallsmartdeviceswhichareahybridof420mAandslowdigitalcommunication.Keepingthesignaldigitaluntiltheveryendofthechainallowsforinfinitelymorecomplexandprecisesignalprocessing.Andtheindustrystandard,inexpensivecablethatlinksalltheFieldbuscomponentstogethercanbehundreds ofmeters long and remain totally free of the noise pickup and signal degradationassociatedwithanaloguesignaltransmission.

Ina420mAanalogsystem,asinglevalueistransmittedbyaninfinitevariationofacurrent.Asignalerrorjustchangesavalidsignaltoanothervalidsignal,noiseandothersignaldistortioncannot be detected in an analog system. The signal from even the most accurate analogtransmittermaybetotallyinaccuratebythetimeitreachesthecontroller.MuchmorenoiseisrequiredtodistortadigitalsignalsuchasinFieldbus,whichonlyhastwovalidstates,oneandzero.Secondly,errorchecking isalsoused to filteroutscrambledmessagesandmakesuretheyaresentagain.Alldata ischeckedandguaranteed free fromdistortiondue tonoiseorimpedancemismatchthatmayaffectanalogsignalsundetected.

Eachanaloguelinktransmitter,signalconditioner,isolationamplifier,cable,inputcard,outputcard,I/Pconverterwhispersananalogyofwhatithearstothenext,butsomethingisalwayslostoradded.

Whatemergesat theendmaynotevenresemble theoriginalmessage.Nomatterhow longthe chain, at its end digital equipment listens only for patterns of zero and one, which itreassembles into the original message, ignoring all other whispered information as noise.Hencetheintegrityofdigitaltransmission:noaccuracylost,anynoiseadded.

The integrityofadigital signalling results inbetteraccuracyandsecurity.The latter isbeingvery important in theprocess industrywhereexpensiveequipment, life and theenvironmentmay be at stake. Fieldbus has taken this integrity even further. Measurement and controlvariables passed between function blocks have not only a value, but also a status, whichinclude signal quality, limit information and a substatus. The signal quality informs if e.g. ameasurement isGood, totallyBadorperhapsUncertain,e.g.outof rangebya fewpercent.Otherfunctionblocksusethistoe.g.putcontrolinmanualincaseoffault,calculationfunctionblocksmay also take this information into consideration, e.g. exclude itwhen calculating anaverage.

HowisFieldbusmoredistributedthanaDistributedControlSystem?

Unlike the semidistributedDCSsystem, theFieldbussystem isacompletelydistributed. InFieldbus the process control functions are distributed to equipment in the field, while stillallowingoperationandtuningfromthecontrolroomusingthedigitalcommunication.Thefielddevicesaretypicallymultidropped,severalconnectedtoeachotherandtheoperatorconsole,drasticallyreducingwiringwithsubsequentsavingsincostofpurchaseandinstallation.

InthelegacyDCSofthe1970thecontrolfunctionsforseveralloopswascentralisedtooneormoreunitcontrollerswhichcontainscontrol,inputandoutputcards.

Since inFieldbus communication is completely digital no input or output cards are required,andsincethecontrolfunctionsareperformedbythedevicesinthefield,nocontrolcardsarerequiredeither.Fielddevicesmaybeconnecteddirectlytotheoperatorconsole,henceadatahighwayconnectingcontrolcardsisnotrequiredeither.Sincetherearenocardsandnodatahighway, they need not be made redundant either. All that remains of the classic DCSarchitectureisthefielddevicesandtheoperatorconsole.ObviouslyaFieldbussystemmeansatremendoushardwareandsubsequentcostreduction.

WillFieldbusreplaceDCSasweknowit?

Yes.Somecompanieshaveavestedinterestinsayingthatitwont,butwithFieldbusthereisnoneedforanaloganddigitalinputcards(sincecommunicationisalreadydigital)andthereisnoneedforanycontrollerCPUcards(sincecontrolisdoneinthefielddevices),norisadatahighwayrequiredtolinkthesystemtogether.Allthatwillremainistheoperatorconsole.Askyourself,isthataDCSasweknowit?

ThefirstDCSsystemsemergedmorethantwentyyearsago.Thoughamajorimprovementatthetime,thistechnologyofyesteryearhasmanydeficiencies.

EstablishedDCSmanufacturersnowoffering theirownPCbasedcontrolsoftware,orbuyingoutPCsoftwarecompaniesmustbeseenasthefinalnailinthecoffinforDCS.

ManycontrolsoftwarepackagesforPCtodayhaveallthebellsandwhistlesofaDCSandareavailableforsecureandstableoperatingsystems.ThesearemostlikelytoserveasoperatorconsolestomakecompleteFieldbussystems.

Lets lookatwhatwouldhappenifashortsightedcustomerdecidedtogofor the legacyDCSarchitectureofthe1970s,andthenupgradetoFieldbusafewyearslater.Hewouldmostlikelyhave to replace theentire transmitter forallmeasurements,wiringwouldhave tobe redone,power supplies, safety barriers and any interfaceswould have to be replaced, input, output,CPUcardsanddatahighway(mainandredundant)thrownaway,andterminationaddedallthat remains is theoperator console.Upgradability toFieldbus is an important considerationwhenchoosingacontrolsystem.

WhathappenstodatabaseconfigurationinaFieldbussystem?

Fieldbus is using the concept of a completely distributed database.Device and control loopinformationisencapsulatedinthefielddeviceitself,ranges,tuning,wettedmaterialsdataetc.AnyMMIthatdesirestodisplaythatinformationmayaccessit.Thiswaytherearenoduplicateinconsistentdatabases.InDCSsynchronisationbetweentheconsoleanddevicedatabasewasnotguaranteed.

Therewasalwaysariskthatthedevicewascalibratedforoneflowrange,buttheconsoleforanothergivingawrongindication.InFieldbusonlyonedatabaseexists,inthefielddevice,andtheMMIgets itscalingdatafromthere.Ahandheldconfigurationdeviceaccessesthesamedevicedataastheoperatorconsole.

IsntFieldbustooslow,31.25kbit/scomparedtoseveralMbit/sforDCS?

InFieldbustherearetypicallyonly12devicepersegment(onapairofwires)whereasonaDCSallthedeviceoftheentireplantareindirectlyconnectedtoasinglebuswhichthereforemustbefaster.

InordertotransferalllinkandsupervisorydatafastenoughtogettightclosedloopcontrolandfastMMI(ManMachineInterface,suchasanoperatorconsole)screenupdate,Fieldbusimplementsanumberfeaturesforcommunicationoptimisation.

AutomaticdistributedalarmdetectionandnotificationDistributedTrendingParameterviewobjectgroupsSchedulingUpdateeventnotification

Thankstothemechanismsforpassingofconfiguration,alarmandtrenddatathissocalledbackgroundtraffichasbeenreducedtoaminimumleavingmoretimeforoperationaltrafficandagainimprovingcontrolperformance.Afterconfiguration,thesystemresolvestagsandparameternamesintoaformat,whichminimisescommunication.

WhatareViewobjects?

Blockparametershavebeenarrangedinfourgroupsdependingonusageandstorage.Allparametersinagroupmaybeaccessedinasinglecommunication.ThiswayanMMIneednotoccupythebusbyseveralrequestsgettheparametersonebyone.

Whatisanupdateevent?

An event is also a form of Alert. The MMI is automatically informed about configurationchanges through an event issued by the block, which is changed, and is also tracked by arevisionparameter includedintheviewobjects.Eachtimeastaticparameter ischangedthisrevisionparameterisincremented.

ThereforeaMMIonlyneedtoupdate itselfwhenachangehasoccurred, itdoesnothavetomakecontinuouschecks.

Whatisscheduling?

Function block execution and communication is scheduled to optimise control andcommunication efficiency. Itmay be used to ensure that blocks are executed in the correctorder.Thesequentialpassingofdynamic timecritical functionblock inputandoutputdata iscalledoperational traffic.This trafficand theexecutionof the functionblocks isscheduledbythesystemsoastooccuronapreciselyperiodbasiswithaminimumdelaytherebyachievingoptimumclosedloopcontrolperformance.Schedulingallowstheusertocontroltheorderandalsothefrequencyofexecutionofablock.

Without scheduling the loop dead time will most likely be longer than the time it takes tocommunicate the operational traffic on the Fieldbus network. Another reason is that withoutschedulingtheblocksmaybeexecutedinthewrongorder.

IsFieldbuseasy?Itsoundscomplicated?

As the first protocol to bring the full power of microprocessor technology into fieldinstrumentation,Fieldbusdistilcountlesshoursofthoughtintoeveryaspect.Whenyoupressakeyorclicktochangetheconfiguration,theeffectispowerful,yetalltheworkhasbeendoneforyou.There isnoneed tounderstand thewealthof technicalstrengthhidden inyour fielddevice inorder toappreciate it.Fieldbus iseasy touse, there isnoneed tounderstand thelayers and baud rate, it has been taken care of by the best engineers from the leadingcompaniesintransmitters,systemsandactuators.

Configurationbecomeseasierbecause itwillbedone thesameway forbasicallyalldevicesusingthefunctionblockconcept,noneedfortrainingonseveraldevicetypesorprogramminglanguages.Allmanufacturersuse thesameblocks, regardless if theyare ina fielddeviceornot.Fieldbus isbasedonuserdefined tagsandstandardizedparameternames likeSPandPV. He refers to devices by its tag. The user need not think of device address, memoryaddressandbitnumbersetc.ConfigurationmaybeeditedonaPCandthendownloadedtothedevicesinthefield.Ifyouwantaflowtransmittertointegrate,justinstantiatedthefunctionblock, no need to rewire or buy an additional device. Once physically connected, the linksbetween function blocksmay be changed, function blocks can be added and removed etc.Moreadvanceddevicesmayexecuteavirtuallyunlimitednumberoffunctionblocks.

Procedures like calibration, range setting and diagnostics are implemented consistentlybetween manufacturers and device types. I.e. a density and pressure transmitter from twodifferentcompaniesareoperated in thesamebasicway.This reducesconfusionandmakesoperatortrainingeasy,eventhoughyourpreferredvendorforvariousdevicetypesisdifferentandmaychangeovertheyears.

Fieldbus already has blocks for all kinds of process control functionality like input, output,control,calculateandvarioustypesofcomputationsforminganadvancedset.Severalof theblocksimplementalarm.Newblockswillkeepgettingadded.

Connection is a simple task since devices are connected in parallel and terminal numbermatchingwillbeaminimum.Onewirewilltypicallyconnectasmanyastwelvedevices.Cabletrays,conduitsetc.willbedrasticallyreduced.Italsobecomeseasytoadddevices,justhookitupinparallel,noneedtorunanewwire.

Fieldbus has capability to simulate input or output values or statusmaking it possible for asingleperson to fromthecontrol roomsafely test thesystemresponse to faultsandprocessconditionswhichwouldotherwisebedifficultordangeroustotryout.Previouslymakingsuchatest was troublesome. Two persons equippedwith walkietalkies were always required, oneclimbingontanksandpipesinthefieldwithasimulator.Thereisnoneedtoexposeanybodytoanunfriendlyorhazardousenvironment.

Ringingout the transmitters, i.e.match thewires in themarshalling rack to their respectivedevicesisalsomucheasier,applypowerandconnecttheHostandaskforthetagandyouaredone!>

Fieldbusdevicesstoreinformationusefulformaintenanceinthedevice,whereitwillnevergetlost.Youmaystoreacalibrationdatalikewhenandbywhom,descriptionoftheserviceofthedeviceandevenindividualblocksmaybestored,thiscouldbee.g.LevelBoiler1.

A wealth of information is available in the device, which also includes wetted materialsinformationand serial numbers.Thismaynot only beavailablewith theHost, but also fromoperatorconsole.Accessingthedeviceoperatingtemperaturereadingallowsyoutoseeifitisoperatingwithin range.The temperature readingprovesextravaluable inapplicationswhereheat tracing and winterization is used. The temperature reading is an indication of if it isworkingornot,sothattransmitterdoesnotfailorpipesarecloggedduetosolidification.

Instrumentcalibrationandmaintenancedatamaybestoredinthetransmitterdatabasewhereit,unlike ifstoredonpaperoradisk,willnotbemisplacedorseparated fromthe transmitterevenifthedeviceismovedaroundintheplantorevenshelved.Thismayincludeinformationlikeperformedbywhom,where,when,howandatwhatvaluecalibrationlastwasmade.Theinformation isnotonlymore thanseenbefore, the interoperabilityofFieldbusmakes itmoreaccessible than seen for smart transmitters. This instrument management is an importantfeature to help comply with ISO9000 and ISO14000 requirement for updated and traceablecalibrationrecordsagainatoolforbettersensing.

WontFieldbusstifleinnovation?

Sincedigitalcommunicationwasfirstintroducedinprocesscontrol,manufacturershavebeenforcedtoadapttheirproductstoamyriadofprotocolsastheyemerge.AstandardFieldbushasrelieved device manufacturers from this task. Once again they can concentrate on trueinnovationssuchassensingtechniquesgettinghigheraccuracy,reliability,stabilitytoambienteffects,andtransmittersformultiplevariables.

Fieldbusisaperformancespecification.Thismeansproductdevelopershavemaximumchoiceoverhowthey implement it intoproducts.Fieldbusdevicemanufacturerscanselect fromthemicroprocessor, programming language and methods of their choice. This can greatlystreamlinetheproductdevelopmentprocess,andproductioncosts.

AsinglecommonFieldbuswillspurthirdpartysoftwareandotheraccessories,asthemarketfor a given productwould be larger, providing an economy of scale factor justifying productdevelopment.MoresoftwareandaccessorieswillmakeFieldbusmoreattractivecontributingtotheproliferation.

Fieldbus will just be another requirement in an already long list of standards specified byinstrumentbuyers,essentialwhenitcomestoputtingthepiecesofaplanttogether,andbeingabletomakereplacements:

TemperaturesensorsProcessconnectionElectricalconnectionCalibrationFieldbusCleaningforoxygenserviceEnvironmentalconditionstestingIntrinsicsafety

Materialgrades

Somehaveforgottenthebeautyofstandardsandhavetakeallthebenefitsofstandardizationfor granted somuch so that we even argue if they are good or not. The question is if theindustrializedsocietycouldfunctionifitwasnotforthethousandsofstandardsmakingittick.Couldweevenimaginegoingbacktoatimewhereaboltandanutfromdifferentshopsdonotfittogether?Iftheywerenotforstandardsalotofthingsinourliveswouldnotworke.g.boltsandnuts.Standardizationofmeasures,screwsetc.istheveryfoundationonwhichengineeringrests.

The situation we have nowwhen new protocols show up every day is in a way hamperingdevelopmentbecausenobodydare to chose.Lackof standardization,newprotocols comingup,instrumentmanufacturersandcontrolsoftwaremanufacturershasbeentrackingamovingtarget.

Standards are especially in the USA criticized for stifling development. However, standardsactually enables true innovation rather than just coming upwithmany new solutions for thesameproblemalreadysolved.Onceastandardhasbeenlaiddown,enoughpeoplearewillingto buy products based on it providing a large enough market for new ideas based on thetechnologyinthestandard.Manufacturersnowdaretospendmoneydevelopinganewproductmakinguseofthestandard,knowingthestandardwillnotchangesosoon.

Investing in development based on somebody elses proprietary technology is a great risk,manufacturersatthemercyofthetechnologyownerneverknowiftherewillbeanewversionoutthenextmonthrenderingtheirdevelopmenteffortsuseless.

As an example, the technology to make multiple variable transmitters and control andcomputation in the fieldhasbeenaround fora long time,but it hasbasicallybeenpointlessbeforeFieldbusbecauseonlywith the interoperability thatFieldbusprovides is it possible tomakegooduseofit.

AlsoRead:BasicsofFoundationFieldbus

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POSTED BY S BHARADWAJ REDDYInstrumentationProfessionalWorkinginareputedOil&GasCompany.