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How Fieldbus WorksIn analog control systems, instruments produce a 4-20mA outputsignal that travels all the way from the remote distillation column,tank or process unit to the control room, marshalling rack, re-mote I/O concentrator or RTU over twisted pair cables. Similar-ly, 4-20mA control signals travel from the control system to valveactuators, pumps and other control devices. Hundreds, some-times thousands, of cables snake their way through cable trays,termination racks, cabinets, enclosures and conduit (Figure 1).Instead of running individual cables, fieldbus allows multiple in-

struments to use a single cable, called a “trunk” or a “segment,”(Figure 2); each instrument connects to the cable as a “drop.”

A fieldbus trunk or segment—either FOUNDATION fieldbus H1or PROFIBUS PA—is a single twisted pair wire carrying both adigital signal and DC power that connects up to 32 fieldbus de-vices to a DCS or similar control system. Most devices are two-wire bus-powered units requiring 10 to 20mA.The fieldbus segment begins at an interface device at the controlsystem. On a FOUNDATION fieldbus H1 (FF) system, the inter-

Introduction to FieldbusMike O’Neill, Director of Fieldbus at Moore Industries-International, Inc

OCTOBER 2006 Valve��World 39

This article presents the two basic fieldbuses used in process control: Networks and DeviceCouplers. We’ll show how fieldbus works, how to connect instruments, and explain why—in mostcases—you can’t connect all 32 instruments on a single fieldbus segment as all the advertisingclaims. We will also talk about the differences between PROFIBUS and FOUNDATION fieldbus,FISCO vs. Entity intrinsically-safe fieldbus systems, and installing redundant segments.

Figure 1. Traditional 4-20mA field wiring often results in a rat’s nest ofwires, cables and terminations.

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Figure 2. A fieldbus installation substantially simplifies wiring.

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face is called an H1 card; on a PROFIBUS PA system (PA), it is aPROFIBUS DP/PA segment coupler. In terms of signal wiring andpower requirements for the segment, FF and PA are identical:

• Minimum device operating voltage of 9V• Maximum bus voltage of 32V • Maximum cable length of 1900m (shielded twisted pair)

The DC power required by the bus is normally sourced through afieldbus power supply or “power conditioner” which prevents thehigh frequency communications signal from being shorted out bythe DC voltage regulators.Typical power conditioners make 350to 500mA available on the bus and usually incorporate isolationto prevent segment-to-segment cross talk.

For PA, the “segment coupler” usually incorporates the powerconditioning component. In FF segments, the power conditionersare separate from the H1 interface card and are often installed inredundant pairs to improve the overall reliability. Figure 3 showsa typical fieldbus segment.

When calculating how many devices can fit on a fieldbus segment,a user must take into account the maximum current requirementof each device, the length of the segment (because of voltage dropsalong the cable), and other factors.The calculation is a simple

Ohm’s law problem, with the aim of showing that at least 9V canbe delivered at the farthest end of the segment, after taking intoaccount all the voltage drops from the total segment current.

Connecting InstrumentsAs noted, each fieldbus device connects to the segment in parallel,via a “drop” on the fieldbus segment called a spur.The simplestspur connection is a “T.”The problem with simple “T” connections(Figure 4), is that if any one of the devices or cables short out, ittakes down the entire segment.A short can occur during fieldmaintenance of an instrument, from an accident in the field, cor-rosion causing electrical problems, or a host of other possibilities.

Another way to connect fieldbus devices is via junction boxesspecifically designed for fieldbus, often referred to as “device cou-plers” (Figure 5), that allow multiple fieldbus devices to connectat one location.Typically, users will install a device coupler in afield enclosure, and connect nearby instruments to it.The field-bus cable may continue onward to another device coupler.Amulti-instrument segment may have several device couplers.

Two basic types of electronic spur short-circuit protection areused in device couplers: “current limiting” and “foldback.” Bothprevent a spur fault from shorting out the segment and both auto-reset back to normal on removal of the fault. ▼

1900mSegment LengthTerminator

H1 Card or DP/PASegment Coupler

Fieldbus Host

Fieldbus Power Supply

120m

Fieldbus cable can be used but, in general, H1 and PA systems can run on standard instrument grade

twisted pair cable

Maximum number of fielddevices per segment is 32

Terminator

Figure 3. A fieldbus segment starts with an H1 interface card and a powersupply for FOUNDATION fieldbus or a segment coupler for PROFIBUS.Up to 32 devices can be supported on a single segment. The boxes with a“T” in them indicate the location of the segment terminators.

Figure 4. “T” configurations are the simplest fieldbus connection.However, if one device fails or short-circuits, it “takes down” the entiresegment.

T

Fieldbus DCPower Conditioner,350-500mA(Not Required for PA Systems)

T

DC Power Input

FieldbusTermination

FOUNDATION Fieldbus or PROFIBUS PA Network (Twisted Wire Pair)

1,900m MaximumSegment Length

120m Maximum Spur Length

AutomaticSegment

Termination

FieldbusDevices

FieldbusTrunk

Out

TG200 FieldbusDevice Coupler

TG200 FieldbusDevice Coupler

FieldbusTrunk In

FieldbusTrunk Out

FieldbusTrunk In

"Fold-Back"Short Circuit Protectionwith Auto Reset

H1/PA Interface

Figure 5. A device coupler permits multiple instruments to be connectedto a fieldbus segment. Each “spur” has short-circuit protection, so itcannot harm the entire segment. More than one device coupler can beused in a segment.

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The current-limiting technique limits the amount of power theshort circuit can draw to between 40 and 60mA (vendor depend-ent) but it also holds that fault on the segment continuously.Al-though this design protects the segment from the initial short, theadditional current draw can deprive other instruments on thesegment of power, overload the segment power supply, and possi-bly cause catastrophic failures on the segment.

When a short circuit deprives other instruments of power, somemay “drop off ” the segment because they do not have enoughpower to operate properly. Consequently, when current limitingprotection is used in a device coupler, many end users allow asafety margin.That is, they do not install as many instruments asthe segment can theoretically power; instead, they leave a certainnumber of spurs empty.

For example, if a user wants the segment to be able to keepworking with two failures—which can draw up to 120mA of cur-rent—the segment calculations must assume a maximum currentavailability of 350mA minus 120mA for the faults, or 230mA.

Instead of theoretically being able to power 32 devices that draw10mA each, the segment is now only able to support 23 such de-vices. In practice, some users are wary of relying on current lim-iting couplers, and most limit each segment to only 16 devices toprevent large-scale segment failures.

The fold-back technique, disconnects the shorted spur from thesegment, thus preventing loss of an entire segment.The fold-backtechnique has a logic circuit on each spur (Figure 6) that detects ashort in an instrument or spur, disconnects that spur from thesegment, and illuminates a red LED that can be seen by mainte-nance personnel.

With fold-back device couplers, users no longer have to worryabout spur failures and can have confidence about placing moredevices on fieldbus segments. Since the cost of H1 cards ($2,500)and other segment hardware can be cost-prohibitive, being ableto place more devices on a segment can save users a considerableamount.

Segment TerminationEvery fieldbus segment must be terminated at both ends forproper communication. If a segment is not terminated properly,communications errors from signal reflections may occur. Mostdevice couplers use manual on/off DIP switches to terminate cou-plers. In a segment, the last device coupler should contain the ter-minator, and all couplers between the last coupler and the H1 cardshould have their terminator switches set to off.

The boxes with a “T” in Figure 3 and 5 illustrate where a typicalsegment is terminated properly.

A frequent commissioning problem during startup is determiningthat terminators are correctly located. During installation of thefieldbus system, the DIP termination switches sometimes are setincorrectly, creating problems during startup.The instruments canbehave erratically, drop off the segment mysteriously, and general-ly raise havoc—all because the terminations are not set properly.

Diagnosing the problem often requires physically examining eachdevice coupler to determine if the switches are set properlythroughout the segment.Automatic Segment Termination, asfound in TRUNKGUARD Device Couplers, simplifies commis-sioning and startup. It automatically activates when the devicecoupler determines that it is the last fieldbus device coupler inthe segment; if it is, it terminates the segment correctly. If it isnot the last device, it does not terminate the segment, since thedownstream device coupler will assume that responsibility. Noaction—such as setting DIP switches—is necessary by the instal-lation person to terminate a segment properly.

Redundant FieldbusesFieldbus has one major problem: all communications and powerare dependent upon a single twisted pair trunk cable. If the trunkcable fails, it can “take down” all the devices on the segment atone time. Not only is the fieldbus segment lost to the control sys-tem, devices on the segment can no longer talk to each other.Al-though fieldbus instruments can continue to operate if the con-trol system fails, any cable fault (open or short-circuit) couldrender the entire segment inoperable.

This problem is especially serious on plant-critical segments,where the failure of a segment may adversely affect plant orprocess applications, lead to costly process shutdowns, cause ahazardous condition, or release materials into the environment.

No provision is made within either fieldbus standard for redun-dant segment communications.Various fieldbus vendors, includ-ing major process control companies, have developed redundantfieldbus schemes that involve complete duplication of all equip-ment—including H1 or DP/PA interfaces, power supplies, field-bus cables, device couplers, and some critical fieldbus instru-ments (Figure 7)—plus complex software voting schemes.A“voting scheme” is needed because most control systems cannottell when a fieldbus segment fails.They can only detect if the H1or DP/PA interface itself fails, or if a particular fieldbus device

Logic

FAULT

Device

Trunk

Figure 6. Fold-back short circuit protection has logic that detects a short,removes the shorted circuit from the segment, and lights an LED. Thisprevents a short from affecting the segment.

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fails. If the interface remains powered, the control system has todetermine, by analyzing instrument signals (or the lack thereof),that the entire segment has failed.

The prospect of losing an entire critical segment often makes endusers wary of fieldbus. If a segment contains “process-critical” in-struments, they generally limit the number of instruments persegment to only a few, or spread critical instruments over multi-ple redundant segments.

MooreHawke has solved the redundancy problem for FOUNDA-TION fieldbus systems.

The TRUNKSAFE™ Fault-Tolerant Fieldbus System maintainscontinuous communications between field devices and a DCS inthe event of any single point failure on a FOUNDATION fieldbussegment.TRUNKSAFE (Figure 8), consists of dual, redundantTPS200 Advanced Fieldbus Power Conditioners (one for each legof the segment), two fieldbus cables, and a TRUNKSAFE TS200Device Coupler.

In a typical TRUNKSAFE application, two redundant H1 interfacecards are connected to two legs of a fieldbus segment, and wiredout into the field.The power on each leg is properly conditioned bythe TRUNKSAFE Power Conditioner, and run to the TRUNKSAFEDevice Coupler, which are connected to multiple fieldbus devices.

If a fault occurs on either cable, the power conditioner on the af-fected leg immediately cuts power to that leg and its H1 inter-face, which forces the DCS to switch to the alternate H1 card.The device couplers can be driven by either trunk; if one trunkloses power, the device coupler automatically takes its powerfrom the remaining trunk and applies the segment terminator.This all takes place automatically within a few milliseconds.

With TRUNKSAFE, it may not be necessary to duplicate all field-bus instruments in a critical segment.An end user may decide toduplicate an instrument if the device itself is prone to failure, butit is no longer necessary to duplicate instruments for protectionagainst a trunk failure.The cost of a fully-redundant TRUNK-SAFE Fieldbus System is only slightly more than a standard field-bus system, and far less than that of a fully-duplicated system. Infact, the availability of an inexpensive, fully fault-tolerant FF sys-tem now makes it possible for end users to provide redundancyon more than just processcritical loops.

Intrinsically-Safe Operation:Entity vs. FISCOFieldbus systems are suitable for use in many hazardous areas;that is, wherever standard 4-20mA instruments can be used.Allforms of electrical protection (non-incendive, flameproof, intrin-sically-safe) are available to suit the requirements of any site pref-erence or experience.

Entity solutions have been around since the 1950s.The Entityconcept is based on using barriers and power supplies to limit theamount of energy that can enter a hazardous area (Figure 9). Ingeneral, Entity systems are highly reliable, especially when basedon simple resistive current-limiting.

Intrinsically-safe fieldbus was originally based on the FOUNDA-TION fieldbus FF816 specification, which allowed Entity param-

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Device Coupler

FieldbusInstruments

Device Coupler

Distributed Process Control System

Figure 7. Some fieldbus redundancy techniques require completeduplication of the segment; in some cases, this means duplicate fieldinstruments. When a segment fails, logic in the DCS determines that afailure has occurred, and switches from one H1 card to the other.

On-Board Diagnostics Module (Optional) Provides Comprehensive Fieldbus Physical Layer Diagnostics

Digital Diagnostic Communications

RedundantTPS200

Fault TolerantPower

Supply/Conditioners

DCS/H1Interface

TS200 Fault TolerantDevice Coupler

RedundantFOUNDATION Fieldbus H1

Interfaces

BREAK!Uninterrupted

Fieldbus SegmentCommunications

DCS/H1Interface

T

Up to 350mA of Isolated, Redundantand ConditionedPower per Segment

Fieldbus Termination isAutomatically Switched to

the Side of the Trunk that isMaintaining Normal Communications

Auto-Detects Field Cable Failure and Then Prevents

Communications on that Side of the Trunk

TPS200

Segment Length Can Be Up to 1900m. Cable Lengths on Each Side Do Not Need to Be Balanced

T

T

Disconnects power to H1 card when short circuit detected

With TRUNKSAFE, it may not be necessary to duplicate all fieldbus instruments.

Figure 8. The TRUNKSAFE Fault-Tolerant Fieldbus System only requiresan extra power conditioner and a trunk cable. If one segment fails, itinstantly switches over to the backup segment.

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eters for field devices to be at least 24V/250mA/1.2W.The bar-riers that were available initially only allowed around 80mA forGas Groups A, B, C, D (NEC)/II (IEC), or four devices per seg-ment, which is an unacceptably low number of devices in a prac-tical application.

FISCO stands for Fieldbus Intrinsically Safe Concept. It is basedon work done by Physikalisch-Technische Bundesanstalt (PTB),the national metrology institute in Germany, and involves physi-cally testing complete systems to ensure safety and to derive anenvelope specification into which every component has to fit.Toprovide a FISCO solution, every part of that system, includingdevices, power conditioners and cable, has to comply with strictlimits. FISCO power suppliesgenerate up to 110mA inGroup A&B applications, and250mA in Group C&D.

FISCO power supplies arebased on sophisticated elec-tronic current-limiting, andtherefore tend to be expen-sive, quite complex, and typi-cally have lower MTBFs thanconventional fieldbus powerconditioners.With only110mA available in GroupA&B, a FISCO-type systemcan support only four to fivedevices per segment (allow-ing a maximum of 20mA perinstrument plus power forthe device coupler pluspower losses over distance).In a plant with 250 instru-ments, this would require 50to 60 FISCO segments. Con-sidering that each segmentrequires an H1 card ($2,500)plus a power supply, plus adevice coupler, plus a field-bus cable, the total cost forFISCO hardware alonewould be about $5,000 persegment times 50-60 seg-ments, or $250,000 to$300,000.

This does not count the timeand labor involved in running50 to 60 fieldbus segmentsinto the plant, installing theconsoles, enclosures and de-vice couplers, and wiring allthe components.This couldcost an additional $100,000.

The ROUTE-MASTER™ Entity Fieldbus System is based on a“split-architecture” approach to the barrier. Part of the barrier isin an isolator (back-of-panel) and part of it is in each of the spursof a field-mounted device coupler (Figure 10). By splitting the in-trinsically safe current-limiting method in this way, the ROUTE-MASTER can put a full 350mA on the trunk that leads into haz-ardous areas with Gas Groups C&D, and still haveintrinsically-safe spurs that match FF816 Group A&B approveddevices.This overcomes both the FISCO and conventional Entityrestrictions on available current. Instead, up to 16 devices can beput on a segment, nearly four times as many as a FISCO system,with a greatly reduced cost for hardware and labor.

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SAFE AREASAFE AREA

H1 Interface

Fieldbus Power Supply

DCS

I.S.Barrier

FieldbusTerminator

FOUNDATION Fieldbus Network(Twisted Wire Pair)

FieldbusTerminator

Fieldbus Devices

T

T

HAZARDOUS AREAHAZARDOUS AREA

Figure 9. Entity-type systems use barriers and electronic techniques to limit the amount of current entering ahazardous area. This technique has been used since the 1950s, and is the basis for most fieldbus intrinsically-safesystems, but it only allows 4-5 instruments on a fieldbus segment.

SAFE AREA

Spur Wiring120m

Maximum

TRUNKIN

++

AdditionalFieldbusDevices

HAZARDOUS AREAClass 1, Division 1,Groups A, B, C, D

Zone 0, IIC

Spur Wiring120m

Maximum

TRUNKIN

++

AdditionalFieldbus Devices

AdditionalFieldbus Devices

HAZARDOUS AREAClass 1, Division 1,

Groups C, DZone 0, IIB

SAFE AREA

ROUTE-MASTERDevice Coupler

FOUNDATION Fieldbus1900m Maximum Segment Length

including Spur Lengths

"Split Architecture" Dual Trunk Outputs Deliver 350mA per Segment

H1 FieldbusConnection

ROUTE-MASTERFieldbus Power

Supply/Conditioner

ROUTE-MASTERDevice Coupler

Auto FieldbusTerminator

T

Auto FieldbusTerminator

T

HAZARDOUS AREAClass 1, Division 1,

Groups C, DZone 0, IIB

FieldbusDevice

FieldbusDevice

HAZARDOUS AREAClass 1, Division 1,Groups A, B, C, D

Zone 0, IIC

RedundantAC Power Input

RedundantDC Power Output

Figure 10. The split-architecture Entity approach used in ROUTE-MASTER puts part of the barrier in the powersupply, and the other part of the barrier in the field. This allows up to 16 instruments to be powered on anintrinsically-safe fieldbus segment.

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There is a segment calculator (Figure 11), that helps calculate thenumber of devices per segment, based on the current and voltagerequirements of each device, length of the segment, and otherfactors. It also bases its calculations on a requirement that eachdevice receives at least 9V.

Most people are concerned with the amount of current each de-vice draws from the fieldbus segment. However, the amount ofvoltage available at the field device is also a critical variable. Forexample, if devices on the segment draw too much current, andthe segment has a long run, the increased current draw may re-sult in too much voltage drop; consequently, some devices mayget less than 9V.Although the devices get enough current, they donot get enough voltage.

The ROUTEMASTER system can support two trunk cables foreach segment.The two trunks run separately from the powerconditioner to the field devices, but they are part of the same seg-ment. Instead of one long segment, it uses two shorter trunks.Therefore, the total segment distance on each trunk leg is mini-mized, reducing the amount of voltage drop, and providing morethan 9V to each device.

FOUNDATION Fieldbus H1 vs. PROFIBUS PAFrom a field wiring perspective, FOUNDATION fieldbus andPROFIBUS are physically identical.They use the same twisted-pair cables and device couplers, and require the same segmentterminators. Both handle up to 32 devices per segment.

One primary difference is that PROFIBUS is a polling system,while FOUNDATION fieldbus utilizes cyclic transmission. Other

differences in-clude:

• FF devices havescheduled timesat which theytransmit theirinformation,whereas PA de-vices submittheir data atrandom times.In PA, slavepolling involvesthe bus masterasking for infor-mation fromthe devices.Thelink activescheduler in FFhas a timetable, so it determines when devices communicate onthe segment.

• Address allocation in PA has to be done by communication witheach device individually, whereas FF devices will announcethemselves to the bus master.

• Each PA device will go back to the bus master with its info,which will then transmit to other devices the relevant data. FFdevices can talk to each other and bypass the bus master, henceproviding peer-to-peer communication.

• FF devices can have built-in function blocks that allow them totalk to each other peer-to-peer, perform control functions, andcontinue to operate if communications are lost to the controlsystem. PROFIBUS systems do not have function block capabili-ty. PROFIBUS instrumentation reports to and takes directionsfrom the PA master; if communications to the PA Master(s) arelost, the instruments must go to a fail-safe position or maintaintheir last settings until directed otherwise.

• FF and PA differ in the way that the segment control cards con-nect to the DCS or control system. FF uses an HSE (high speedEthernet) network to connect remote H1 cards to the DCS; PAuses PROFIBUS DP, which is an RS-485 network, orPROFINET, an Ethernet-based network, to connect its PA de-vices to the DCS.

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Figure 11. This segment calculator determines current and voltagerequirements for a segment, and makes sure that all devices receive atleast 9V of power.

Mike O’Neill is Director for Fieldbus at Moore Industries-International Inc, and is based in the UK. He actively con-tributes to Fieldbus FOUNDATION physical layer standardsand test specifications and regularly presents on fieldbus appli-cations for EPC’s and end-users. A graduate of the Universityof Wales in Cardiff, UK, he is a Chartered Member of the In-stitute of Measurement & Control

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