© 2005 Siemens Milltronics Process Instruments Inc. www.siemens.com/processinstrumentation 1

Trends in Level Measurement Technology

By James Powell, P.Eng.

Of the various networks vying for the fieldbus market, PROFIBUS is emerging as

the network with the most advantages for totally integrated automation.

Currently, there are three mainnetworks vying for the instru-mentation fieldbus market:

HART®, PROFIBUS, and FOUNDATIONTM

Fieldbus. All three have some nice fea-tures; however, in a detailed compari-son, PROFIBUS stands out as the bestsolution for total plant automation. Itoffers a more complete solution, helpsensure high plant availability, and hasadvanced features that save money.

Aspects of PROFIBUSPROFIBUS PA is part of the PROFIBUSprotocol and is specifically tailored forthe needs of the process industry (PA =Process Automation). It allows fieldinstruments to be both monitored andconfigured over the bus. Instrumen-tation can be bus-powered andIntrinsically Safe. PROFIBUS PA can beused in any application and, because itcan be Intrinsically Safe, it is well-suitedfor even hazardous environments inchemical and petrochemical plants.

In a process plant, you need morethan an instrument fieldbus for thepressure transmitters, level devicesand flowmeters. You also need to beable to connect digital I/O, discrete andmotion control aspects such as drives,analyzers, and other electrical gear inmotor control centers (MCC).PROFIBUS PA is just one chapter in thePROFIBUS book. The PROFIBUS proto-col also offers PROFIBUS DP,PROFIdrive and PROFIsafe. These addi-tional chapters let you connect differ-

ent types of devices in a wide variety ofapplications for a complete, totallyintegrated solution. No other fieldbusprotocol can offer such a completesolution. This has been noted by ARCin a recent white paper that stated“PROFIBUS’ unique value propositionis the ability to seamlessly integrateprocess instrumentation like pressuretransmitters and flow meters withdevices for the discrete and motioncontrol side of the application likedrives and sensors.”1

When a plant is down because of a

failed sensor, it is not making money.Maximum uptime is key to profitabili-ty. PROFIBUS helps prevent downtimeby having diagnostics and preventativemaintenance information built intothe protocol specification. A device onPROFIBUS will let you know if themeasurement value is valid with eachupdate. It also indicates when aninstrument should be checked forpotential impending problems. Thiscan really reduce operating costs bypreventing downtime. It lets you opti-mize the maintenance schedule with a

Why use PROFIBUS forProcess Automation?

This example of a PROFIBUS network shows the seamless integration of PROFIBUS PA,PROFIBUS DP and PROFINET.

predictive approach that keeps thingsrunning smoothly and without processupset while reducing your annual oper-ating cost.

There are a number of other fea-tures that cut costs over all stages of theplant life cycle. PROFIBUS PA is buspowered, so it saves on installationwiring costs, with only one cable toconnect and maintain. It’s IntrinsicallySafe, saving the cost of any special con-duit or cabinets. Moreover, two majoradvantages are related to device inter-operability and interchangeability.

Interoperability is the ability to havemultiple vendors’ equipment worksmoothly on the same network. This is akey PROFIBUS strength. You do not haveto worry if a PROFIBUS device from yourfavorite supplier will work with yourDCS system. If the device has gonethrough PROFIBUS International’s con-formance testing, you can be confidentthat it will be compatible with the net-work. You also do not have to worryabout the version of the PROFIBUS stan-dard. If a device is made to DP version 0and you place it on a network with a DPversion 3 device – no problems. Thisfieldbus standard has maintained back-ward compatibility as it has madeadvances with improved features andfunctionalities. Interoperability savesyou a lot of money and headaches dur-ing the commissioning phase.

Interchangeability is the ability toswitch from one vendor’s device toanother vendor’s similar device.PROFIBUS PA instruments have builtinto them a standard called “Profiles.”Profiles standardize the devices fromthe network’s point of view, giving sim-ilar devices an unprecedented level ofinterchangeability. Switching from onevendor’s device to another’s device iseasier than with any other fieldbus onthe market, reducing your reliance on asingle vendor. No other fieldbusdesigned for use in process plants pro-vides this capability. This becomesimportant about one o’clock in themorning when the plant is down due toa device failure and all you have to do isreplace a failed device, e.g. a level

transmitter, with another vendor’slevel transmitter that is available in thestorehouse!2

The Profile standard3 also gives allPROFIBUS PA devices a similarity thatmakes them easier to work with – twodifferent PA devices will have a common“look and feel” because of the standard.This saves time and money in techni-cian training, programming, setup, andinstallation. The HART CommunicationFoundation, PROFIBUS International,and the Fieldbus Foundation all havesub-committees working on Profiles,but PROFIBUS is currently the only onewith a released standard. In fact,PROFIBUS released the third version ofits Profile standard in 1999, and is faradvanced in this area compared to theother networks.

HARTHighway Addressable RemoteTransducer (HART) uses an older tech-nology, and is a combination of digitaland analog. It superimposes a digitalsignal over an analog signal. The mainprocess variable is transmitted via theanalog signal, while configuration isdone over the digital signal. BecauseHART combines digital and analog, it isconsidered a transitional bus. A plantcan move from analog to HART andthen, ultimately, advance to PROFIBUS.In this sense, it competes withPROFIBUS but also complements it.

HART has been around for manyyears and is now common in manyinstallations. It is a proven, valuablestandard for instrument connectivity.PROFIBUS has taken these factors intoaccount and has made integratingHART devices very easy via its HART onPROFIBUS specification.

PROFIBUS PA has a well-establishedProfile standard, whereas HART is justreleasing its Profile standard. It is,therefore, easier to switch similarPROFIBUS PA devices than HARTdevices.

PROFIBUS PA has FISCO, whichmakes intrinsic safety convenientlyplug and play. With HART, you have todo it the old-fashioned way with calcu-

lations and submissions for approvals. HART has diagnostics but they are

more basic and not as frequentlychecked as with PROFIBUS PA.

HART’s digital signal is relativelyslow, running at 1200 bits per secondas compared to PROFIBUS PA, whichruns at 31.25 kilobits per second.HART’s slow baud rate means that,when you are saving parameters to aHART unit or viewing data such as anecho profile, you will have to waitlonger.

Many people cite the wiring costs asa major advantage for PROFIBUS PAover HART. In reality, that will dependon the layout and how you do yourwiring. If the tanks are close to the con-trol room and you are using cabletrays, HART can actually be cheaperthan PROFIBUS PA from an installationpoint of view; however, if there arelonger distances or if you are usingconduit, PROFIBUS PA will likely be lessexpensive.

FOUNDATION FieldbusFOUNDATION Fieldbus is the other pure-ly digital bus. It started its life in thesame industry working group asPROFIBUS. They have a number of thingsin common; for example, the H1 physicallayer of both buses is IEC 61158-2, andthey use the same wires, connectors, andphysical specifications. Beyond that,however, the protocols are quite differ-ent. FOUNDATION Fieldbus is far morecomplex and requires a lot more memo-ry in field devices than PROFIBUS PA.

FOUNDATION Fieldbus scores lowerthan PROFIBUS on interchangeabilitybecause FOUNDATION Fieldbus doesnot have Profiles. It also has weakerdefinitions in its specification that willcontinue to cause problems for sometime to come; for example, FOUNDA-TION Fieldbus defines a PID functionblock with defined inputs and outputsbut it does not define the algorithm. Ifyou switch from one vendor’s device toanother vendor’s device and just moveyour tuning parameters from the onePID to the new PID, they will look thesame, but the algorithm and therefore

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Trends in Level Measurement Technology

the performance will be different.Interoperability was FOUNDATION

Fieldbus’ biggest weakness but theFieldbus Foundation has implementeda Host Interoperability test to fix theproblem. Even with this test, this willcontinue to be an issue for some timeto come. You cannot just buy a FOUN-DATION Fieldbus-based DCS and fielddevices and expect them to worktogether. You have to make sure thatthe DCS supports the devices and thatyou have the correct versions. In otherwords, a FOUNDATION Fieldbus-basedDCS commonly has a “favorites” list offield devices from select vendorswhereas such a constraint is neverimposed by a PROFIBUS-based DCS.

FOUNDATION Fieldbus currentlyhas fewer devices available on the mar-ket than the other buses. It has meth-ods to connect HART devices, but theyare not as clean as the PROFIBUS sys-tem and do not offer the same level ofintegration with engineering tools.

For the purposes of complete plantintegration, a key advantage ofPROFIBUS PA is the easy connection to a

high-speed network through PROFIBUSDP. In response, FOUNDATION Fieldbushas defined an extension called High-Speed Ethernet (HSE), for its so-calledH2 layer. Unfortunately, there are fewdevices for it yet and it cannot accom-modate drives that require a high-speed bus to convey large amounts ofdata at a fast baud rate. If your plantrequires drives which you wish to inte-grate over a fieldbus, you will need touse a bus that supports them – likePROFIBUS. The bottom line is that youcan implement a plant-wide solutionwith PROFIBUS, but you cannot withFOUNDATION Fieldbus.

FOUNDATION Fieldbus promotesthe concept of “control in the field.”The control loops are implementedright in the field devices, independentof central controllers. This is a nice fea-ture. In practice, however, most FOUN-DATION Fieldbus installations end upnot using “control in the field” becauseof the complexity and cost. By contrast,the two-way communications inPROFIBUS PA and its use of Functionblocks make the field instruments part

of the control system. At the end of theday, does it matter whether the controlloop is processed in the field or cen-trally as long as you have a reliablecommunication layer and effectivedevice monitoring and control? Youcould make philosophical argumentseither way but, for most applications,the answer to this is “No.”

ConclusionOver the years, as a specialist in com-munications protocols and networks, Ihave been involved with and support-ed numerous product connectivitymethods such as Modbus®, Allen-Bradley® Remote I/O, DeviceNetTM,FOUNDATION Fieldbus, PROFIBUS andHART. I have observed a growing inter-est in PROFIBUS as an open protocol,with increasing demand from cus-tomers for PROFIBUS-enabled prod-ucts. For that reason, companies likeSiemens, a major industrial automa-tion and instrument supplier, haveintroduced numerous products withPROFIBUS built in, with many moreplanned for future release.

As these various networks havedeveloped, PROFIBUS has moved fasterto incorporate Profiles and other sig-nificant advances. As a result, it hasemerged with numerous advantagesfor plant operators:

• A one-bus solution that can con-nect all layers of plant operationsfor total plant automation

• Bus-powered and IntrinsicallySafe for installation savings

• Advanced, faster diagnostics toreduce downtime

• Interoperability to reduce com-missioning costs

• Interchangeability through thestandard “Profiles” concept

• Easy connection to HART devices• Fast configuration and trou-

bleshooting • A growing number of devices

available• Potentially lower wiring costsPROFIBUS now stands out as the

best choice for total plant automation.PROFIBUS, with its many unique

Trends in Level Measurement Technology

3

Siemens SITRANS LR 200 (left) radar level and Pointek CLS 300 capacitance point levelare among the many recently introduced PROFIBUS devices from Siemens, a major indus-trial automation and instrument supplier.

© 2005 Siemens Milltronics Process Instruments Inc. www.siemens.com/processinstrumentation 4

Trends in Level Measurement Technology

advantages, serves as an integral com-ponent in Totally IntegratedAutomation – an automation infra-structure that allows a variety ofdevices and automation componentsin various parts of a plant to workunder a common environment usingcommon engineering tools and com-munication protocols. For plant opera-tors, it means there will be morePROFIBUS products and even morecapabilities available in the future.■

James Powell, P.Eng, is Product Manager,

Communications Systems, with Siemens

Milltronics Process Instruments Inc.

1 “The Value Proposition of PROFIBUS in the

Hybrid Industries,” ARC White Paper, ARC

Advisory Group, May 2004, p. 14.

2 “The Benefits of the Profile Concept in

Fieldbus Technologies,” by James Powell,

Instrumentation Society of America (ISA),

Expo 2004 Transactions.

3 “PROFIBUS Profile, PROFIBUS-PA, Profile

for Process Control Devices”, PROFIBUS

International publication Order No. 3.042.

PROFIBUS, PROFINET, PROFIsafe, and

PROFIdrive are trademarks of Profibus

International. FOUNDATION Fieldbus is a

trademark of the Fieldbus Foundation. HART

is a registered trademark of the HART

Communication Foundation. Modbus is a

registered trademark of Schneider Electric.

Allen-Bradley is a registered trademark of

Rockwell Automation. DeviceNet is a trade-

mark of Open DeviceNet Vendor Association

(ODVA). SIMATIC is a trademark of Siemens

AG.

A Glossary of PROFIBUS PA Features

Advanced Diagnostics: A device on PROFIBUS PA will let you know if thevalue is valid with each update of the main process variable. It also indi-cates when an instrument should be checked. This reduces operation coststhrough the prevention of downtime.

Advanced Networking: PROFIBUS is one protocol, allowing easy connectionamong PROFIBUS DP and PROFIBUS PA. Once connected, the user can easi-ly communicate over different layers, directly to the field device.

Bus-Powered Device: An instrument that can draw all its power from the busis referred to as “2-wire device.” The bus-powered approach saves on instal-lation costs. (Note that some PA devices are 4-wire devices which are devicesthat get most of their power from an external source.)

Easy Integration of HART Devices: HART devices, common to most installa-tions today, can be easily integrated into a PROFIBUS network via a SiemensET200M or ET200isP.

Intrinsically Safe: The bus can go through hazardous areas. It conforms tothe Fieldbus Intrinsically Safe Concept (FISCO) model which means thatimplementing it in an IS environment is easy.

One-Bus Solution: In a process plant, you need more than a fieldbus for theinstruments. You also need to connect digital I/O, drives, etc. The PROFIBUSprotocol offers PROFIBUS PA and PROFIBUS DP to offer a complete solution.

Profile Concept: Profiles are a standardization of a device from the bus pointof view, giving devices that use Profiles an unprecedented level of inter-changeability. It makes switching from one vendor’s device to another easi-er than it would be with any other bus on the market.

Remote Programming: This is the ability to configure, monitor, and trou-bleshoot a PROFIBUS PA device directly over the network connection fromthe control room. This saves time and money.

SIMATIC Process Device Manager (PDM): PDM is a universal cross-manu-facturer tool for configuration, parameter assignment, commissioning,diagnostics, and maintenance of intelligent process devices and automationcomponents. With one software program, you can configure a number offield devices from different manufacturers using a single user interface.Process device data can be easily set, changed, checked, managed and simu-lated. In addition, you can monitor selected process values, alarms and sta-tus signals of devices online.

Standardized Function Blocks: Related to the Profile concept, each fielddevice is programmed using standardized blocks. The similarity of theblocks makes programming of different instruments easy, and reduces vari-ations of operator training required.

Versatile Wiring: The cable can be multi-drop, trunk or star shaped. Thisallows increased versatility in layout design and minimizes installationcosts.