foundation fieldbus tutorial

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Tutorial onTutorial onTutorial onTutorial on

Foundation Fieldbus

Contents: Page

a) Introduction on Foundation Fieldbus 2b) Comparison with conventional wiring 4c) Fieldbus Terminologies 8

d) Network wiring fundamentals 12e) Network wiring options 15f) Segment design 18g) Overview of Fieldbus network 22h) Fieldbus communication mode 24i) Fieldbus blocks 28j) Loop scheduling 30k) Interoperability 32l) Reliability and Redundancy 35m) Fieldbus signals 40n) Fieldbus segment length calculation 43o) Fieldbus in Hazardous area 45p) Intrinsic safety 47q) Choosing a host system 51r) Control design 55s) Project engg. standard 58t) Merits of Fieldbus 62u) Commissioning 63v) Checkout and troubleshooting 66

w) Diagnostics with fieldbus 69


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Foundation Fieldbus

INTRODUCTION

Fieldbus is a digital, two way, multidrop communication link amongintelligent measurement and control devices. Fieldbus is gradually replacing 4 to20 mA standard instrumentation signals used to transfer measurement andcontrol between control room and plant floor. It is one of several local areanetworks dedicated for industrial automation.

Two related implementations of Foundation Fieldbus have beenintroduced to meet different needs within the process automation environment.These two implementations use different physical media and communicationspeed.

H1 works at 31.25 Kbits/sec and generally connects to fielddevices. It provides communication and power over standardtwisted pair wiring. H1 is the most commonly used implementation.

HSE ( High speed Ethernet) works at 100Mbit/sec and generallyconnects input/output subsystems, host systems, linking devices,gateways and field devices using standard Ethernet cabling.

Conventional analog and discrete field instruments use point to pointwiring: one wire pair per device. They are also limited to carry only one piece ofinformation, usually a process variable or control output over those wires.

As a digital bus, Foundation Fieldbus doesnt have those limitations.

a) Multidrop wiring: - Foundation fieldbus supports upto 32 devices on a singlepair of wires (called a segment), more if repeaters are used. In actual practice,considerations such as power, process modularity, and loop execution speedmake 4 to 16 devices per H1 segment more typical.

That means if you have 1000 devices, which would require 1000 wirepairs with traditional technology, you need only 60 to 250 wire pairs withFoundation Fieldbus, that is lot of wiring and installation saving.

b) Multivariable instruments:That same wire pair can handle multiple variables

from one field device. For example, one temperature transmitter mightcommunicate inputs from as many as eight sensors, reducing both wiring andinstrument costs.

c) Two way communication: - In addition the information flow can now be twoway. A valve controller can accept a control output from a host system or othersource and send back the actual valve position for more precise control. Inanalog world, that would take another pair of wire.


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d) New type of information:-Traditional analog and digital devices have no wayto tell you if they are operating correctly, or if the process information they aresending is valid. As a consequence, technicians spend a lot of time verifyingdevice operation. But Foundation fieldbus devices can tell you if they are

operating correctly, and if the information they are sending is good, bad oruncertain. This eliminates the need for most routing checks and helps you detectfailure conditions before they cause a major process problem.

Their predictive diagnostics can also help increase plant uptime andperformance by detecting or predicting deteriorating performance and failureconditions before they cause problems.

e) Control in the field:-Foundation fieldbus also offers the option of executingsome or all control algorithms in field devices rather than a central host system.Depending on the application, control in the field may provide lower cost andbetter performance, while enabling automatic control to continue if there is a host

related problem.

f) Safe and effective process control:- Foundation fieldbus H1 was developedspecially to meet the needs of the process industry.

It can withstand the harsh and hazardous environment of the processplant.

It delivers power and communications over the same pair of wires.

It can use existing plant wiring.

It supports intrinsic safety.

f.i) Control you can count on:- Foundation fieldbus also provides

deterministic process control : control communication happen on schedule,without delays caused by other traffic on the bus. If a message doesnt getthrough, it tries again.

Control reliability doesnt stop there. If fieldbus devices lose theirconnection to the host system, they are capable of maintaining safe and effectivecontrol across the bus.


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COMPARISON BETWEEN CONVENTIONAL WIRING AND FOUNDATION

FIELDBUS NETWORK

In a conventional Distributed Control System, two wires are used to

connect to a field instrument. The wires carry electrical power to a device. Thedevice signals its measured values to the DCS controller by varying the current ituses between 4 and 20 mAmps. The controller gathers the data from number ofdevices, makes the necessary calculations and sends commands by varying thecurrent to the actuator. Refer Figure: 1

Following figures shows the wiring in a panel using conventional DCSwiring as compared to one using Foundation Fieldbus network.


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Figure1: Wiring Schematic using Conventional Wiring

Fieldbus also uses two wires to carry power to the devices. A number ofdevices share the same Field bus wires. Because the devices share the wires,the devices can send data to each other without a DCS controller. A twisted paircable called as a trunk connects the field instruments with the controller. ReferFigure: 2


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Figure: 2 Wiring Schematic using Fieldbus Network

Comparing a traditional DCS installation with Foundation Fieldbusillustrates that there is a significant decrease in terminations, number of I/O cardsrequired, home run wiring and control room panel space when FoundationFieldbus network is used.

Conventional Wiring Foundation Fieldbus

Terminations 16 devices require 240 terminationsa) 16 devices to JB (3 terminals

each) = 48 terminalsb) JB to Marshalling rack =

48 terminals

c) Marshalling rack to I/O cards =48 terminals

16 devices require 60

terminations

a) 16 devices to Megablock (3terminals each) = 48 terminals

b) Megablock to Marshalling rack

= 6 terminalsc) Marshalling to I/O cards =

6 terminals

Topology One to One Multi-drop

Transmission

Method4 to 20 mA DC analog signal Digital Signal

Transmission

DirectionOne-way Bidirectional

Type of signal Single signal Multiplex signal


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MultivariableDetection and

Transmission

Requires one cable for each measuredvariable.

e.g. In a control valve, control and

signals are transferred throughdifferent cables.

Supports Multivariable transmission.

e.g. In a control valve using

Foundation Fieldbus network all the

control and signals are transferred viasame cable.

Errors in dataTransmission

and Accuracy

When data in transmitted usingconventional wiring, it is first

converted to analog in the field

devices and then the signal istransmitted to the system where it is

again converted to digital form. This

conversion is prone to errors and so isthe analog data transmission which

thus reduces the accuracy.

Fieldbus transmits data using digitalsignals. Signal transmission errors

rarely occur in digital signal

transmission, unlike analog signaltransmission. In addition, Fieldbus

does not need A/D and D/A

conversions because data is alwaystransmitted digitally. Fieldbus

removes these errors, improving

transmission accuracy.

Interoperability

Information exchange betweendevices of different manufacturers is

difficult because each device uses its

manufacturers protocol

Foundation fieldbus devices allow

digital data to be exchanged betweendevices from different manufacturers.

Therefore, the freedom to configure

the process control system increases

since there is no need to choose onedevice manufacturer.


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VARIOUS TERMINOLOGIES USED IN FOUNDATION FIELDBUS NETWORK

SYSTEM

Trunk: It is the link between the Host controller (or the H1 card in the controller)to the megablock in the field. It is a twisted pair cable.

Figure 3: Typical Foundation Fieldbus Cable

(Pen shown for size comparison)

CableType

Distance(metres/feet)

CharacteristicImpedance

(Ohms)

Resistance(Ohms/Km)

Attenuation(DB/Km)

Description

Type A 1900/6270 100 22 3 Each twistedpair has shield

Type B 1200/3960 100 56 5 Multiple twistedpair with

overall shieldType C 400/1320 Unknown 132 8 Multiple twisted

pairs, no shieldType D 200/660 Unknown 20 8 Multiple

conductorcable, nopairing of wires

Spur:It is the link between the megablock and the field devices. It is same as thetrunk cable and connects the field instruments to the megablock.

Splice:The spur cable less than 1 meter (3.28 ft.)is termed as splice.

Megablock:The Megablock is a wiring connection block that allows termination

of two trunk cables and number of spurs to devices. For a large number ofdevices, multiple megablocks can be cascaded by connecting trunk of onemegablock to the other trunk. Refer Figure.4


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Figure: 4 Typical connections made in a Megablock.

Terminator: - A Terminator is an impedance-matching module used at or neareach end of a transmission line. Onlytwo Terminators can be used on asingle H1 segment. Generally, oneterminator is at the control room endof the cable and the other terminatoris in the junction box in the field. Theterminator can be a separate part ormay be part of a wiring block or part ofa Fieldbus power supply. Theterminator should be clearly markedso that it can be identified in aninstalled system.

Figure 5: Types of Terminators1) separate2) in power supply3) in megablock

Fieldbus Power supply (Power conditioner): An ordinary constant voltagepower supply cannot be used directly to power a Fieldbus. A power conditioner(PC) needs to be used to provide a filter between the network and the powersource so that the power source does not absorb the signals on the network. The

12

3


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voltage needs to be between 9 and 32 Volts. Generally,Fieldbus power supplies provide about 24 Volts. Atypical Fieldbus device uses about 20 mAmps ofcurrent. Generally, the number of devices on a Fieldbusnetwork segment is less than 16. A power supply with a

16 x 20 = 320 mAmp current rating is sufficient for mostapplications.

Figure: 6 A typical Power supply Module

Surge Protectors: Surge Protectors are devices that divert a surge current toearth, and control voltage to a level, which will not damage the connectedequipment. Once the surge current has subsided the Surge Protector Deviceautomatically restores normal operation and reset to a state ready to receive thenext surge. While in operation the surge protector device does not adverselyaffect the performance of the fieldbus or connected equipment, it allows signalsto pass with very little attenuation while diverting surge currents safely to earthand clamping output voltages to safe levels.

For Trunks and spurs For Field Instruments and Devices

Figure:7 Surge Protectors

Current to Fieldbus converter: It is a device that it is used to convert any

normal 4 to 20 mAmps (or 0 to 20 mApms) field transmitter signal into a Fieldbuscompatible signal.


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Foundation Fieldbus Host: The Host or the H1 device is usually located in thecontrol room. Its function is to see the operation of the control system made up ofdevices connected by the Fieldbus network.

Current Limiter:A current limiter is a device which overcomes the problem ofspur shorts developed during installation of a new instrument, servicing etc.which otherwise may result in the total break down of the entire segment whosespur is short. The current limiter allows only a given amount of current to be usedby each device. If a spur is shorted, the current will be limited within a fewmicroseconds. Only the shorted device is affected and the rest of the devicescontinue to operate.

Repeaters:Repeaters are optional components used either to extend the lengthof a fieldbus segment or to increase the number of devices on a segment. Theyprovide power and a clean communication signal for the extended part of the

segment.A segment can have as many as four repeaters dividing the segment intofive pieces. Electrically, each piece acts as a separate segment, but devices cancommunicate with each other as though they were on the same segment, even ifthere are upto two repeaters between the devices.

Although a fieldbus segment can have upto 32 devices without repeaters,H1 segments typically dont have more than 12-16 devices even if repeaters areused.

Intrinsic safety barriers:Foundation fieldbus was designed to support intrinsicsafety, and to do so with more flexibility and lower cost than traditional analogintrinsic safety.

In the analog world, each input and output has a dedicated barrier. But inthe fieldbus world, with its single cable supporting multiple devices, one barriercan serve several devices. That is tremendous saving on barriers and its

installation.Depending on the requirement, it is

optional to put several barriers on a singlefieldbus segment. Also both intrinsically safe andconventional points can be attached on thesame segment.NOTE: While converting existing analog wiringin to fieldbus, the analog barriers are also to bechanged with barriers for Foundation fieldbus asthe latter is certified for fieldbus and existingbarriers will not support fieldbus wiring.

Using Foundation fieldbus repeaterbarrier, the combined function of repeater andan intrinsic safety barrier can be made available.Figure: 8 An intrinsic barrier


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NETWORK WIRING FUNDAMENTALS

Many aspects of a Foundation fieldbus network are similar to a traditionalanalog control network. It still requires wire, power, field devices I/O cards, and

possibly intrinsic safety barriers. There are a few new components such asterminators. And there are differences how they are put together.

A fieldbus advantage:Wire isnt expensive, if connecting a couple of instruments a few yards

away. But put in a few miles of wire to connect hundreds or even thousands ofindividual devices across a plant, it leads to a major expense. Especially whenlabor required to install the wire, as well as conduits and cable trays is included.And also the documentation every wire and connection, then keeping up inchanges.

These are the costs that Foundation fieldbus are designed to reduce. Digital communication enables several devices to share the same cable,vastly reducing the total amount of wire required.

Foundation fieldbus H1 can work on standard plant wiring, so you caneven use wires that are already available.

If required to add wiring, either for new construction or for a capacityincrease, available wiring and cable options make the job easier and theresults more reliability. And all that translates into a lot of less cost.

Basic segment design:Conventional analog installations have a dedicated pair of wires

connecting each field device to a host system. Foundation fieldbus installationsuse a single twisted-pair cable also called a bus or trunk to connect multipledevices. The cable, connected devices, and supporting components are called asegment.

Devices connect to the fieldbus either individually or in groups. It theyconnect through individual spurs branching off the main trunk, the result is calleda branch layout or topology.

Figure: 9 Branch topology Figure: 10 Tree Topology


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A bus with spurs connected to the trunk in closed groups is called a tree layout.A single segment can have both branches and trees, as long as a few

rules are followed for total segment length, length of drops, total device count,

and segment current draw.

Key segment limits Typical valuesMaximum of 32 devices per segmentwithout a repeater.Maximum of 240 devices per segmentwith a repeater.

4 to 16 devices per segment.

Each device must draw at least 8 mAfrom the segment.

15 to 25 mA power consumption for atwo-wire device.8.5 mA for a four wire device400 mA typical segment limit.

Voltage range 9-32 Vdc 24 Vdc

Total segment length:Total segment length is determined by adding the length of all the sections

of the segment. The total segment length must be within the maximum allowedfor the wire type used.

The total segment length is the sum of the lengths of all the spurs (S1through S7), plus the lengths of the main cables, or trunks (T1 and T2). For typeA wire, the total must be less than 1900 meters.


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Mixing wire types on a segment:Different types of wires can be used on same fieldbus segment, as long as

the rules about how much of each type can co-exist on the segment.To find the maximum length of each wire type on a segment, first calculate

the following ratio for each wire.

Length of individual wireMax. length for wire type(For max length of each wire type refer the table of fieldbus cables page 8)

Then add the ratios for all the individual wires in the segment. If the sum of theratios is less than 1.0 (or 100%), the wire combinations and lengths areacceptable.

Example:A segment has 2000 feet (610 meters) of type A shielded twisted pair wire

and 400 feet (122 meters) of type D non-shielded, non-twisted pair.

For type A cable2000 ft = 0.326270 ft max

For type D cable400 ft = 0.61660 ft max

0.32+0.61 = 0.93 or 93%The sum of 0.93 or 93% is less than 1.0 or 100%. So this is an acceptable wire

combination.

Spur length:The maximum length of a spur depends on

The total number of devices on the spur, and

The total number of devices on the segment.

Note that a spur can have up to three devices.

Devices per spurTotal devices onsegment 1 2 3

1-12 120 m 90 m 60 m13-14 90 m 60 m 30 m

15-18 60 m 30 m 1 mFor example, if a segment consist of eight devices, then reading across the 1-12 row in the table indicates that the spur with one device can each be 120meters long, the spur with two devices can be 90 meters, and the spur with threedevices can be 60 meters.


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NETWORK WIRING OPTIONS

A fieldbus network should be designed with the location of the fielddevices in mind. Thats especially true in an existing plant with wiring, conduit,

junction boxes, field devices, and related equipment already installed.Some device will be located by themselves, others in groups. Foundation

fieldbus accommodates both situations through branch and tree network layouts(also called topologies).

Branch: Like its name, a branch is a single limb or spur off the main trunk of afieldbus segment. A branch layout makes sense when the devices on a segmentare geographically separated from each other.Refer figure:12

Tree: A tree layout also called a chicken foot has a number of branches, or

spurs, that connects to the main trunk in one location. This layout works wellwhen several devices are located near each other.Refer Figure:11

Either of these network layouts can be used with wire in conduit or not,with a combination of conduit and armored cable, and with existing wiring and

junction boxes.

Conduit option: TreeMany plants have existing wire in conduit. This setup can easily be used in

Foundation fieldbus network with either tree or branch layout.

A tree layout connects several spurs to the main fieldbus trunk (also calleda home run cable) at a single point. Standard shielded, twisted pair wire for thehome run cable and for the spurs that connect to the devices. Or you can useconduit for the home run cable and the trunk, and armored exposed cable for thespurs.

The connection to the main cable is often made with a junction box or spurblock. A spur block takes in a segment and passes it out to other spur blocks orremote devices.

Another connection option is to use standard field terminal blocks. Thestyle of terminal block showncalled a disconnect

block, reduces the risk of a short circuit byeliminating the need to physically unscrew a devicein order to remove it from a segment.

Figure: 11 Tree Conduit (Disconnect block)


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Conduit options: BranchThere are also several options for using conduit with a branch layout. One

option is called as condulet.As the figure shows, a condulet is designed so

that a segment comes into the box on one side (the

top side in the figure) and extends through theopposite side, if necessary. The branch line to thedevice is attached on the third side.

Figure:12 Branch Conduit option

Non-conduit options: TreeConduit is expensive, especially if you need a lot of it. So rather thaninstalling miles of conduit, many plants use cable trays or other ways of routingsignal wires.

One option for use in a tree layout is a pre-assembled fieldbus junctionbox like the one in the figure. The junction box, sometimes called a megablock,combines a set of cable connectors.

These junction boxes come with connectors for four,eight or more devices plus connectors for segment-in andsegment-out. Each connector is labeled to help preventincorrect assembly. Caps protect unused connectors fromthe environment. This type of junction box is typicallymounted to a plant structural member close to the devices itserves.

Figure: 13 Non conduit: Tree

Some junction boxes are available with short circuit protection. If there is ashort circuit on a spur, the short is isolated to just that spur, and therefore justone device. This reduces the risk of a wiring problem affecting several devices.

When installing a fieldbus segment, avoid filling a junction box completely.Allow room for growth by leaving at least one location open for either a fieldbusterminator or an extension of the segment trunk.


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Non-conduit options: BranchFor non-conduit installations with a branch layout, where each device is

attached to an individual drop off the main trunk, a T connector offers low costand easy installation.

The T connector has a segment-in and asegment-out connection, plus a connection for the singlespur or branch to the individual field device.

Figure: 14 Non Conduit: Branch

Like the fieldbus megablock the T connector can be mounted on aplant structural member close to the segment trunk. And, like the megablock, it ismade to withstand typical plant environment.

Using existing wiring and junction boxes:Foundation fieldbus is designed to work with existing instrument-grade

wires. With just a few wiring changes to a junction box, you can convert point topoint analog wiring to a Foundation fieldbus trunk with spurs.

As the figure shows, the positivewires are jumpered together, as are thenegative wires, on the home run (or thehost) side of the junction box. The individualplus minus terminals on the field sideconnect to the spurs that run to groups of 1to 3 devices per spur.

The figure below shows a conventional junction box that has beenconverted to a Foundation fieldbus H1 junction box.

Figure: 15 Conventional JB converted in FFJB


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SEGMENT DESIGN

Using proven segment design tools and good installation practices, it iseasy to design and implement a highly reliable and functional fieldbus segment.

However, loops and processes differ in criticality and functional requirements.

Designing for loop criticality:Designing a fieldbus segment that is both reliable and easy to maintain

depends to a large degree on segmentation. This means putting commonelements on the same fieldbus segment and dissimilar elements on separatesegments.

One of the most important criteria for segmentation is loop criticality: howmuch impact a loop failure would have on the process or on the entire plant.

With mission-critical loops, the loss of automatic control will result in a

shutdown. In highly important loops, a loss of automatic control will require a near-

superhuman effort from the operator to sustain operations. For normal-importance loops, loss of automatic control or operator

visibility could be tolerated during normal mean-time-to-repair.

Finally, loss of automatic control or the operators ability to view a loopwould not have any detrimental effect on view-only or data acquisitionloops.

Mission-critical loop:Initially, a mission-critical loop may be the only loop on a segment. That

way problems with another loop, such as accidental shorting the segment duringmaintenance on the non-critical loop, cant cause loss of the segment with themission-critical loop.

If two mission-critical loops are interacting or cascading, both the loopscan be put on the same segment. But if the process can be kept running withonly one of the loops active, consider separate segments.

Redundancy: On segments with mission-critical loop, it is a good idea to useredundant segment infrastructure. That includes using redundant H1 interfacecards and power supplies. One of the devices on the loop should also have abackup LAS. (Dont use redundant terminators on the same segment. Doing so

can cause signal problems.)If loops also include redundant field devices and process piping, put theseredundant components and loops on separate segments and bring the twosegments into separate H1 field interface cards.

Control in the field: Consider using control in the field for critical loops. As longas the segment retains power (and one device has backup LAS), automatic


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control can be maintained in the field devices even if the H1 interface card and allother host components are lost.

Remember that critical loops may have supervisory control, operatorvisibility, or regulatory reporting requirements that require the host to beconnected to the loop for the loop to remain operational. Control in the field will

not address these needs.

Non-fieldbus control: although Foundation fieldbus is quite capable of handlingcritical loops, sometimes plant practices dictate that specific loops be controlledusing other technologies. There is no harm in using both traditional and fieldbustechnologies on the same project.

Highly important loops:If a highly important loop loses automatic control, an operator will typically

go to the loops physical location and operate the valve manually, receivinginstructions by radio from the control room.

Grouping and loading of such loops is therefore guided by how manyloops an operator can control this way, without control room visibility to the finalcontrol element.

Loops and devices: As a rule of thumb, two related loops will share a segment.You can also put critical operator monitoring information on the same segment.

Consider six devices per segment as a maximum for these loops. Theright number for any plant will be determined by the nature of the process.

System interface: The number of highly important loops that should be broughtinto a single H1 field interface card depends on how many loops an operator canhandle via manual control at the valve, without visibility through the host system,if the card fails. If redundant H1 cards are available, it is possible to bring moreloops on a segment.

Control in the field: If it is okay for a loop to operate in automatic controlwithout operator control, then control in the field can be a good idea for highlyimportant loops. Even if the H1 card and all the other host components are lost,automatic control can continue as long as the segment has power and one of thedevices has backup LAS.

That is why you should not run power through the H1 card if removing thecard would interrupt power to the segment.

Normal-importance loops:Although it is tempting to use higher loading on normal-importance loops,

there are still practical limits. Generally, no more than 4 to 6 normal-importancecontrol loops, and 12 to 16 devices, should be on a single segment.

These limits will help ensure a stable, reliable operation. Heres why;


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Communication: As the total number of devices and loops increases,communication loading also increases. If you have several fast loops, the totalamount of communication on the segment might be more than the segment canreliably handled.

A properly optimized communication schedule can give devices plenty of

time to communicate. But automatic scheduling tools dont necessary produceoptimized schedules. If such tools are used care should be taken for the totalcommunication loading for the segment.

Power: Different devices take different amounts of power. Make sure the totalcurrent draw for all devices on the segment is well within the capacity of thesegment power supply; typically about 400mA.

View-only or data acquisition loops:These kinds of loops are generally not very fast, nor do they place a

tremendous load on the segment.

In general, for view-only or data acquisition applications, a segment canbe loaded with sixteen or more devices.However, you still need to make sure that total current draw of all devices

on a segment is well within the power supplys capacity.In some cases, the number of devices and blocks host system can

support will also be a limiting factor.

Power modularity:Its generally a good idea to group devices, and segments along process

lines. Besides providing structure for the design, this modular approach offersboth maintenance and performance benefits.

Use separate segments for unrelated equipment units or process area. Thatis why, maintenance of the devices or network for one unit during shutdown, forexample, wont affect the operation of other units.

Use separate segments for parallel process streams. That way one processstream can be shut down while parallel streams remain online.

Put all devices for the same loop on the same segment. This includes closelyintegrated or cascaded loops. Although multi-segment loop will work, theyincrease maintenance complexity and the number of components required toclose the loop.

Timing of control execution and communications also becomes a bit lessprecise. For fast or time critical loops, this can degrade performance.

Leave room for growth. You may decide to add more devices to a loop in thefuture. When that happens, the extra capacity built helps to keep all devices onthe same segment.


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Know your comfort level. Traditional analog input or output cards typicallyhave at least 8 to 16 points and are usually non-redundant. Since loops integrityof a Foundation fieldbus implementation is comparable to or better than atraditional analog solution, it is reasonable to use similar risk levels for a firstfieldbus implementation.

Multivariable devices:Multivariable devices can make segment design easier and more cost-

effective by letting you acquire multiple measurements with a single instrument.For example, one mass flow device may provide values for real-time mass

flow, total mass flow, process temperature, density and viscosity. You get thefunctionality of five or six instruments, without the maintenance and reliabilityissues that can come with adding that many devices to the segment.

Cost is also much lower for multivariable devices than for multipleindividual devices, especially when cost for design and for multiple processpenetrations are considered.

Capacity restrictions in some host systems may limit the number ofmultivariable devices you can put on one segment. And if the inputs from amultivariable device are used in controlling valves or other final control elementson more than one segment, it may be better to use separate measurementdevices on each segment.

Host system considerations:Different host provide different levels of support for Foundation fieldbus.

Those can affect the segment design.

Capacity limits:All host have capacity limits of some sort. Some have limits on the total

number of devices for a segment. Some limit the number of devices or functionblocks for a H1 interface card. Some even have a fixed capacity for parameters.

Outages for repairs:Ideally, a failed system component, such as an H1 card, can be removed

under power, its replacement installed, and the configuration of the carddownloaded automatically, all without affecting segment power.

There are host implementations, however, that require the H1 card, oreven the entire controller card cage, to be shut down for repair, affecting a largepart of the process. And there are instances where a partial download isntpossible. In this case, the entire card cage must be downloaded.

In either of these circumstances, a failure generally requires a shut down.Keep that in mind as it can be designed how many segments, which ones will beconnected to each H1 card. More important, select a host that doesnt havethese limitations.


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OVERVIEW OF FIELDBUS NETWORK/WIRING SCHEMATIC

The figure illustrates the typical wiring method for Foundation Fieldbus network. The diagram sare connected to a Fieldbus segment.


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Figure:16 A typical format showing the segment length calculation d


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FIELDBUS COMMUNICATION

One of the most important aspects of Foundation fieldbus is its ability to collectand deliver vast amount of information; not only process variables and control signals,

but other types of instrument and process data as well.

It does this consistently and reliably, while also providing interoperability betweendevices from different manufacturers and compatibility with existing wiring.

The Communication Model:The Foundation fieldbus communication model has three parts:

The physical layer The data link and application layer

The user layer

The physical layer and data layer and application layers makeup the communication stack. The user layer sits on top of thestack and enables you to interact with other applications in yoursystem.

Physical Layer: The first functional layer of the Foundation fieldbus communicationmodel is the physical layer, which deals with the translating messages into physicalsignals on the wire and vice versa.

The physical layer also provides the common electrical interface for allFoundation fieldbus devices. Foundation fieldbus H1 segments require 9-31 volts DC

power and approximately 15-20 mAmps of current per device. They operate at acommunication speed of 31.25Kbaud.The Foundation fieldbus physical layer is defined by approved standards

(IEC61158-2 and ANSI/ISA 50.02, part2). It can run on the existing field wiring over longdistances, supports two wire devices, and offers intrinsic safety as an option.

Data link and application layer: The second part of the communication modelcombines several technologies that control transmission of data on the fieldbus.

The data link and application layers provide a standard way of packaging thedata, as well as managing the schedule for communication and function-blockexecution. They enable process control while providing standardization and

interoperability.

User layer:The user layer sits on top of the communication stack, where it enables youto interact with the other layers and with other applications.

The user layer contains resource blocks, transducer blocks and functional blocksthat describe and execute device capabilities such as control and diagnostics. Devicedescription enables the host system to interact with and understand these blockswithout custom programming.


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Scheduled communications:

All the devices and functional blocks on a Foundation fieldbus segment executeand communicate process control information on regular, repeating cycle.

Timing for this type of communication is determined by a master schedule in a

Link Active Scheduler, which is a function residing in the host system or one of thedevices on the segment.These scheduled (also called cyclic) communications use a

publisher/subscriber method. This means data is send on the bus or published once,and all devices that need the data listen to or subscribe to the same transmission. Aspecific parameter can therefore be used by as many different devices or functions asyou want, without increasing traffic on the bus or potentially affecting controlperformance.

These communications are also deterministic. This means that they always occuron a pre-determined schedule. So information is certain to be broadcast and receivedprecisely when its needed.

The result is regular and precise execution of communication and control, whichhelps reduce process variability. For fast or time-critical control loops, control onfieldbus can improve plant performance.

Unscheduled communication:Foundation fieldbus supports a great deal of information beyond process loop

control data. These other types include.

Configuration information sent to devices or a central database

Alarm, event and trend data Information for operator display

Diagnostic and status information.

This information is important, but not as time-critical as loop control information. Ifit is transmitted 1/8 second early in one communication cycle and1/8 second late in nextcycle, there is no impact on process control or plant operation.

Flexible timing:- Foundation fieldbus gives this information a lower priority on thesegment than scheduled control-loop-related communications. However, a certainamount of time in the communication cycle is reserved for these unscheduled (oracyclic) communications to ensure that the segment is not too loaded to carry theinformation.

During this time, a token-passing method gives each device on the segment the

opportunity to transmit messages until it has finished or an allotted time has expired.

Parameter status:Foundation fieldbus supports a variety of data redundancy checks to avoid

message-bit errors. Two additional features that help ensure data reliability are anapplication clock and status associated with every parameter.


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Each device is designated to check for problems and label the data it sendsaccordingly. This status label shows whether the quality of the data is good, bad oruncertain.For example, a bad status signal could indicate a device failure, such as a failed sensoron a temperature transmitter.

An uncertain status indicates that the quality of the data is unknown. For example, apressure transmitter reading that is 110% if the devices upper limit may be accurate; orit may be inaccurate because the device has saturated high and the actual pressure iseven higher.

Note: Device status is made available to the host system but not all host use thisinformation.

Application clock:Every device on a Foundation fieldbus segment shares the sametime. A system management function called the application clock periodicallybroadcasts the time, either local or universal coordinated time to all devices. Each

device uses an internal clock to keep time between these synchronization broadcast.Alarms and events are time-stamped at the device where they occur, when theyoccur; not later when they are received by a historian, alarm log, or other application ona host system.

Because of this approach, Foundation fieldbus provides superior time resolutionand accuracy for activities such as sequence of events recording and analysis.

Link active scheduler:The link active scheduler (LAS) function maintains the central, deterministic

schedule for communication between devices on a segment. It improves overallcommunication reliability by compelling each device to transmit cyclic data when it isscheduled to do so.

Message retries also increase communication reliability. If a device doesntrespond to the LAS compel data message; for example, if a momentary electricaltransient at a device prevents it from communicating, then the LAS will resend themessage to compel the device to publish its information.

The LAS resides in a device or host system component (such as an H1 interfacecard) on the segment. If the LAS fails, then backup LAS in another device or hostsystem component takes over as master scheduler.

There can be more than one back up LAS on a segment. If the first backup fails,the second backup takes over, and so on. This means that Foundation fieldbus isdesigned to degrade gracefully, further increasing reliability.

Device address assignment:As a digital, multidrop bus, Foundation fieldbus carries signals to and from

several devices over the same cable. To identify which information is associated withwhich device, each device is assigned an address.

Depending on the communication protocol, addresses can be assigned inseveral ways, from dip switches or off-line addressing to automatic online assignment.


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Methods such as using dip switches or offline addressing carry the risk of humanerrors, such as inadvertently assigning an address to more than one device. Theseaddressing errors can cause communication problems, or in extreme cases prevent thebus from working. Thats why Foundation fieldbus doesnt allow these methods ofaddress assignment.

Online addressing helps avoiding problems such as duplicate devices with thesame address, but by itself it does not guarantee there are no addressing errors. Youcan avoid this risk if addresses are assigned automatically by a configuration tool orhost system as each device is connected to the segment.

Finally, you can override the default addresses and assign specific addresses tocertain devices when necessary.

Find tag service:Many communication protocols require the user to identify devices and

parameter, and then link them by address and/or register assignment. This can be adifficult and error prone process.

Foundation fieldbus, on other hand, is a tag-based bus. Instead of requiring ahardware or register address, it can find devices or variables by tag (such as FT-101)To find a specific tag, a find tag query is sent out on the bus. As each device

receives the query, it searches itself for the requested tag. When a device finds the tag,it sends back complete path information and all necessary parameters and descriptorsassociated with the tag. The host or maintenance tool can then use the path to accessthe data for the tag. This feature also helps avoid duplicate tag assignments.


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FIELDBUS BLOCKS

Fieldbus blocks are small, sealed software modules. Each block has a definedset of inputs and/or outputs for a specific function or type of information.

Foundation fieldbus uses three types of blocks.

Resource block

Transducer block Function block

Resource and transducer blocks provide valuable information about devices,sensors and actuators, and their performance. Function blocks are engines of open,interoperable, device-independent control. Together, these three types of blocksmake it easier to improve equipment performance and process control.

Resource block:The resource block deals with the overall device. It contains information such asmanufacturer, device type and serial number. Each device has one resource block.

In addition, the resource block also often provides information about the health orstatus of the device as a whole. Access to this additional information may be one of themost important features of Foundation fieldbus because it can enable you to detectpotential device problems before they affect the process.

During project execution, the resource block is used to identify a device, tag it,and commission it. During ongoing operations, it is used by maintenance technicians toobtain overall device configuration and status information, and to run some types ofdevice specific diagnostics.

Transducer block:The transducer block deals with the wetted parts of a device. It provides the

local input/output functions needed to read sensors and to command actuators,displays, or other output hardware. Its the link between the physical world of sensorsand actuators and the data world of process control.

The transducer block may contain information such as calibration data, sensortype, materials of construction, and in many cases the health and operating status ofactuators and sensors.

Special transducer blocks are also used to provide statistical process monitoring,predict sensor life, detect plugged impulse legs, and similar functions.

During project executions, transducer blocks are used for calibrating the device,setting units, and other tasks related to providing an accurate and reliable input oroutput. During ongoing operations, maintenance technicians use these blocks totroubleshoot and calibrate devices, to perform diagnostic checks, and to carry out othertasks to maintain device health and performance.

There may be several transducer blocks in a single device. For example, onetransducer block may deal with the sensor or actuator, another with the local display,and third with diagnostics.


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Function block:Function blocks provide control-system behavior within the fieldbus environment.

Analog and discrete input and output blocks, and a wide variety of control algorithmssuch as characterizer, splitter, or PID, can be linked across the fieldbus to performprocess control. Its even possible, in many cases advantageous to run a control loop

completely in field devices, without involving the host system.A simple device may have only single input or output function block. Morecomplex devices may have several input and output blocks, as well as blocks formonitoring and control.

During project execution, control engineers use function blocks to implement thecontrol strategy. During ongoing operations, the function blocks provide the process-control information and functions the operators use to run the plant.


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LOOP SCHEDULING

Good process control is time-dependant. If control actions dont happen whenthey should, the resulting process variability can increase energy and feedstock use,reduce yields, and reduce product quality.

Fieldbus solves this problem by executing control on a deterministic, real timeschedule. The technology is designed to accommodate the full range of controlsituations likely to be faced.

Basic scheduling:In Foundation fieldbus, control related communication and function blocks

execute at precisely defined intervals, and in proper scheduled order for processcontrol.

The overall schedule is called a macrocycle. The macrocycle for all devices on asegment are precisely scheduled and all use the same absolute start time. Functionblocks and communications execute at specified offsets from this absolute start time.

Figure shows the schedule for a typical loop where the PID function is in thevalve controller (Device 2). Each activity occurs at a defined offset from the absolutestart time.

Figure:17 Typical loop Scheduling

This cycle repeats on an exact, ongoing schedule. Unscheduled (acyclic)messages can be communicated anytime scheduled (cyclic) messages are not beingsent.

Care must be taken in scheduling loops. Functions will execute in the orderspecified, even if that order is incorrect. Scheduling the AO first, the PID next, and theAI last will add a large and needless delay to overall loop processing.


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Multiple loops on the same segment:This figure shows that you can have several blocks executing at the same time

on the same segment, provided that theyre in different devices and have different starttimes. The example has three loops, with PID in the valve controller.

However, one more than one device cannot communicate on the same bus at

the same time. The example schedule prevents communication overlap by staggeringthe function block communication start times so one block doesnt start until theprevious one has finished.

For the sake of simplicity, the diagram shows blocks executing in sequence, withno processing overlap. In reality, multiple blocks can execute at the same time as longas theyre in different devices, and data can be communicated as soon as theprocessing is complete. Multiple devices cannot communicate at the same time.

Figure:18 Multiple loop scheduling


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FIELDBUS INTEROPERABILITY

The Fieldbus Foundation defines interoperability as the ability to operatemultiple devices, independent of manufacturer, in the same system, without loss offunctionality.

The term multiple devices refers to a set of fieldbus products that may include amix of field devices such as valves and transmitters, and host devices such as controlsystem.

Independent manufacturer means vendor independence. That is, having thefreedom to choose the best technology for the task, regardless of which vendor makesthe product.

In the same system means within the mix of control equipment that operates as asingle automation solution. There are, of course, guidelines for the number and type ofdevices that should be combined together within individual segments of the fieldbusnetwork, primarily for electrical and intrinsic safety purposes.

Without loss of functionality means the device operate without the loss of any of

their designated features. That is, being part of an interoperable network doesntinterfere with any of their functions.

Field-device interoperability:Interoperability between field devices basically means that field devices from

different manufacturers can work together, sending and receiving information related totheir specific function in the process.

The Foundation fieldbus has established guidelines for interoperability betweenfield devices on a fieldbus segment. These guidelines address different aspects ofdevice interoperability such as physical characteristics, communication and softwarefunctionality.

To be truly interoperable, devices must Be physically and electrically compatible with the fieldbus segment (as defined by

the ISA 50.02-2 Physical Layer Specification).

Include a communication stack that passes the Foundation fieldbus StackConformance Test.

Correctly implement the Function Block Application Process Model defined in theFoundation fieldbus specification. This means a devices function block mustinterconnect and interoperate with the function blocks of other devices on thenetwork.

Testing devices for interoperability:

Interoperability testing, using a prescribed set of consistent and rigorous testprocedures, helps ensure that all devices will operate together.

The Foundation fieldbus has established two tests for this purpose; the StackConformance Test and the Device Interoperability Test.

The Stack Conformance Test ensures that the device interfaces correctly withthe bus, that is, electrical characteristics and bus access are consistent with the fieldbusspecification.


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The Device Interoperability Test ensures that the devices function blocks willinteract with other blocks correctly, and will provide accurate information and modebehaviour.

Device capability evolvement:

Foundation fieldbus allows manufacturers to enhance and differentiate theirproducts while maintaining the interoperability users want.

Adding blocks:- A device is registered for the defined set of blocks tested during theFoundation fieldbus interoperability test. If a manufacturer adds additional blocks to thesame device, the device may be retested and reregistered for the additional blocks.

Alternatively, the manufacturer can offer the blocks as unregistered functionality.In some cases, that may be the only option; no functional block type can be registeredunless at least two manufacturers offer the block in their products, and both productspass interoperability testing for that block type.

Devices may also be retested and registered following other changes, such as

firmware revisions.

Different capabilities:- Interoperability test determines interoperability, notfunctionality. The internal operation of a devices control algorithms is determined by themanufacturer. Registered devices can work quite efficiently with each other on thenetwork, but exhibit different behavior due to varying control algorithm characteristics.

Host-system interoperability:In most cases, a host system is used to configure fieldbus devices, set up the

control strategy, and display all information available from the field devices. The hostsystem may also participate with field devices in providing process control.

To do all this, the host system must be able to access, use, and displayFoundation fieldbus data from all devices involved. The Host Interoperability SupportTest (HIST), consists of 18 separate tests, shows how well a host system interoperateswith specific standard capabilities of Foundation fieldbus devices.

Although field device testing is mandatory, host testing is optional. A host canundergo none, some, or all of these tests to demonstrate its support for specificfunctions.

Understandably, the HIST doesnt cover proprietary capabilities thatmanufacturers may add to their products. However, its still possible for a host to accessthose capabilities if the device manufacturer provides a Device Description (DD) and ifthe host includes DD Services to read it.

In short, the HIST ensures that the host is a good citizen on the fieldbussegment, but not that it will access, display, or use device information completely or toits best advantage.


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Off-line interoperability:In on-line interoperability, where field devices are physically connected to the

host system as they are being configured. Quite often, however, field devices are notavailable at the time configuration is being done by the host system.

Capabilities files:- To help solve this problem, the Foundation fieldbus has issued aCommon File Format specification which defines a Capabilities file that can be used todescribe information about a fieldbus device that would normally only be available byreading it from the device itself.

An interoperable host system that supports off-line configuration uses thisCapabilities File, along with the Device Description, to build an offline configuration ofthe field devices.

Anytime configuration:-Offline interoperability allows those doing configuration, suchas engineering and consulting firms, the capability to configure an entire fieldbusnetworkoffline. This means that much of the engineering for a Foundation fieldbus

network, including configuration of the devices and control strategy, can beaccomplished prior to acquisition of actual devices.


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RELIABILITY AND REDUNDANCY

With good design and installation practices, Foundation fieldbus actually offerssignificant advantages in total system reliability. How much redundancy to have in theplant, and how to provide it, depends on the situation. It is based on things like mean

time between failure, system availability and experience. It is also based on how criticalparticular devices, loops, and processes are to safe and effective plant operation.

Wiring reliability:The major concern with wiring is not failure of the media itself, but external

factors that affect the wiring.

Fewer wires mean faster repairs:- Consider the damage if a physical event affects anentire wire bundle. In the world of analog point to point wiring, this catastrophe couldinvolve hundreds, may be thousands of severed wires.

In the digital fieldbus world, however, where many devices can be connected to

the same set of wires, the same number of I/O points would be on far fewer wires.Service would be interrupted in either case. But the time to repair would besignificantly less in the fieldbus scenario because there are fewer wires, and wiringcheckout is faster for each wire pair. And the faster the repair, the sooner productionresumes.

Reasons for assurance:- Excluding external events, wire reliability is determined bythe reliability of the physical wire itself, and wire has the lowest complexity level of thesystem and generally the lowest failure rate.

The reliability of he wire can be greatly enhanced by following installation andmaintenance procedures that avoid accidental shorting or grounding. Those are the

most common causes of wiring failures.Reliability can also be enhanced by selecting the wire, cable routes, andconnectors that shield exposed media from physical contact with electricaldiscontinuities. In addition, fieldbus junction boxes are available that isolate short circuitto a single drop on a segment.

Segment reliability:The total fieldbus network is divided into segments for the purpose of aligning

sections of the network with process, hazardous, or geographic areas, or with specificdevice combinations.

From the reliability standpoint, each segment can be treated as a separate entity,

and thus can be handled separately. If a host H1 interface card connects to more thanone segment, and represents a failure point that could impact more than one segment,then all segments attached to the interface card should be considered as a whole.

Segment reliability depends upon several factors such as

Segment power and power conditioners Segment terminators The segment wire itself


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Various connectors

Field devices connected to the segment

The segment host (if present)The greatest threat to overall segment reliability is loss of power, which

affects the entire segment. One way to counter this threat is redundant segment

power, coming from different sources.

Another threat to segment power involves electrical transients such as

Lightning Solar storms Electrical noise

Good installation practices, backup power with uninterruptible power suppliers(UPSs), and surge protectors minimize disruption from these electrical transients

Proper installation can also reduce the chances of improper grounding; anothermajor cause of reliability problems.

Total system reliability:A systems reliability is only as good as the reliability of each of its parts. So it

makes sense that the fewer the parts, the higher the potential reliability of the system.Fieldbus allows the control system to have fewer parts because control can now

be done in the field.That is, control does not have to go through all the host systems terminations,

input cards, controllers, output cards and so on; each a potential failure point.With the control in the host system, all these parts must be working properly for

the control loop to be working. Failure of any of these components in a non-redundantsystem will cause loop failure. The number of loops affected can range from 8-16 for an

I/O card failure to hundreds or even more if a controller or controller power fails.In Foundation fieldbus environment using control in the field, however, the entire

host system can fail without loss of control. Thats because the control is being done inthe field devices. The host system is being used as the interface to a truly distributedfield control system. Closing the loop in the field can be much more reliable thanthrough the host.

Transmitter redundancy:Transmitter redundancy in a fieldbus environment is implemented basically the

same way as in a traditional, analog environment. The primary difference is thatFoundation fieldbus provides additional information that improves the reliability of the

measurement.

Analog transmitter redundancy:- Analog transmitter redundant schemes oftenrequire triple redundancy. When two of the transmitters report different values, the valuefrom the third transmitter breaks the tie. All three measurements are sent to an inputselector which chooses the input that gets sent to the PID. Sometimes the operatorreceives all three values and manually chooses the value that looks best.


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The Foundation fieldbus input selector block available in some transmittersupport a broad range of input selection criteria; from selecting the high, low or middlevalue, to calculating the average of the three inputs, to eliminating the reading with thegreatest deviation from the others.

Foundation fieldbus provides status information that helps automatically identify ifa measurement is good, bad, or uncertain. A bad or uncertain quality reading can beeliminated from consideration before it is presented to the operator.

This capability may even eliminate the need for triple redundancy. Since the thirddevice is no longer needed to determine which signal is the bad one.

Valve and piping redundancy:Like transmitter redundancy, valve redundancy in a traditional, analog

environment involves installing two valves on parallel pipes, which eventually increasesthe installation cost to double.

In fieldbus, unlike the traditional method, the advantage is in the information the

fieldbus valve instrument provides. Because of this feature it can predictively andproactively indicate if it is having health problems so that it can be dealt before theyresult in a failure.

Since valve are mechanical devices they are subjected to a harsh environmentsand wear of moving parts are thus points of maintenance and potential failure in theprocess loop. Because an analog valve controller has no way of determining valvehealth, it may fail without warning.

Control redundancy:Control redundancy is probably the most important aspect of any total

redundancy scheme because typical DCS and PLC control system failures can affect alarge number of loops. The loss of control, and possible equipment failure or plantshutdown, can carry an extremely high price.

The traditional method of providing control redundancy involves duplicating partsof the host control system. This means potentially a lot of extra equipment; input andoutput cards, terminations, power, controllers, etc.; at a lot of extra cost.

Fieldbus provides a lower cost alternative to the traditional control redundancyschemes. It does this primarily by moving the redundant control loop from the hostsystem to the field devices.

In this scenario, the primary PID loop remains in the host system while thebackup PID loop resides in a field device. If the host is lost, then the field devices ownoutput takes over.

Limitations: Putting redundant control in the field can eliminate the need for costlyredundant host components.

However, when the host is lost, the operator can no longer see what ishappening or control it manually from the operator console. Data will not be available toalarm and event logs and historians. Also, the PID block in a host may offer features(such as autotuning) not available in the devices PID function block. And although


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regulatory control is maintained in the field devices, host resident advanced control islost until the host connection is re-established.

Host redundancy options:Control in the field will provide regulatory control in the event the host or host

connection is lost. But it wont provide operator visibility, host based advanced control,or alarm, alert, or historian data. To ensure these are available, host redundancy isneeded.

Many plants have standard practices for redundancy. These frequently includeredundant communications, operator interface, power, controllers, and I/O. specificimplementations of redundancy depend on the requirement of the process. Foundationfieldbus redundancy should conform to these practices.

Redundant host H1 interface cards:- Although the fieldbus specification does notrequire H1 interface card redundancy, a backup H1 card will allow the operatorcontinued visual access to the process should the primarily H1 card fails. It will also

provide process information needed for functions such as validation or quality systems,plus uninterrupted advanced control. If your plant or process requires these things,redundant H1 cards should be used.

Another common criterion is that redundant I/O is required if I/O modularityexceeds a certain level for example, 8 points per card. If redundant H1 card arentavailable, plant practices may require that the loading of the H1 segment be reduced toa level below the threshold required for redundancy.

Finally if no device on the segment is a link master, capable of taking over thefunction of link active scheduler, redundant H1 interface cards may provide thiscapability.

Custom redundancy block:This software option is a custom function block, residing in the valve, designed

specifically for redundancy. The valve function block passes an output from the primary(host) PID to the valves analog output. If the primary PID fails, the backup PID (in thevalve) sends its output to the valves AO.

Redundant air and power:Since actuators, transmitters, valves and control systems all depend on air or

electrical power to operate, making these sources redundant, or having a reliablebackup, will go a long way towards ensuring a safe plant.

Foundation fieldbus power redundancy includes redundant, isolated bulk power,and redundant power conditioners to the segment. This level of power redundancyprovides reliable power even if a power failure occurs.

Redundant media (wire):The wire in general is the most reliable part of the control architecture. Adding a

backup wire segment make sense only if it is part of a completely redundant processstream with redundant instruments, valves, process piping, and host elements. This isimplemented by having one set of valves and instruments on one segment, and the


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second set on a second segment. Each device is connected to only one segment andone set of physical media. In this case, a link must exist between the two segment toensure status information is continually exchanged.

Link active scheduler:

In a host control system, the control strategy generally dictates the execution offunction blocks as well as communication between the blocks themselves. On a fieldbussegment, this task is the responsibility of the Link Active Scheduler, or LAS.

As the name implies, the LAS actively schedules communication and functionblock execution on the segment. If there is no LAS running on the segment, functionblock execution and communication on the segment ceases.

Because the LAS often resides in the host often resides in the host system, themost probable cause of an inactive LAS is the loss of the host. A host-based LAS isalso unavailable in the case of stand-alone loops, where a host is used for configurationand then disconnected.

Back up Link active scheduler:A back up LAS, usually not residing in the host, coordinates block execution andcommunication on the running segment when the primary LAS is lost or unavailable.

A backup LAS should be used in the host, that is, no control in the field, then theloss of host means loss of control, even if a back up LAS is present. The exceptionoccurs when the host has redundant controllers and Foundation fieldbus H1 interfacecards, configured to take over control if the primary control fail. In this case, the backupLAS would usually be in the host system rather than a field device.

Regardless of where control resides, it is still important to make sure final controlelements are selected to fail to the proper failsafe positions if automatic control is lost.


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FIELDBUS SIGNALS

The twisted pair cables, terminators, and power conditioner work together as awiring system that can carry signals between Fieldbus devices. When a device isenabled to signal, it varies the amount of current it draws from the network.

When not transmitting, a device draws power from the cable for its internaloperation. It also draws an additional 10mAmps that it wastes. When the devicetransmits a high signal, it turns off this 10mAmps. This increases the voltage betweenthe wires. When the device transmits a low signal, it draws an extra 10mAmps from thewires, resulting the voltage decrease. The signal waveform is shown below. Note thatthe signal is above and below the 24 volt non-transmitting level on the network.

Figure 19: Fieldbus signal

Digital data is sent on the Fieldbus at a rate of 31.25 kbits/second. Thus, each bitcell is 32 microseconds long. The digital data, ones and zeros, is represented as aManchester code. A zero is a positive signal transition in the middle of a bit cell; a one isa negative transition in the middle of a bit cell. A sequence of Manchester encoded onesand zeros would look like this

Figure 20: Manchester codesignal


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When a device begins transmitting, it puts out a preamble, an 8-bit sequencewith alternating ones and zeros. This pattern is used by the receiving devices tosynchronize themselves to bit cell boundaries. Besides ones and zeros, there are alsotwo non-data symbols. These non-data symbols are N+, a high level during the wholebit cell, and N-, a low level during the whole bit cell. These symbols are used to make

an 8-bit start delimiter that shows where real data starts and an 8-bit end delimiterthat shows where data transmission stops.

When a device transmits, the different parts are combined to form a data frame:

The Data portion of the frame contains information such as the address of the device forwhich the frame is intended, identification of the type of frame, measurement values,etc. The delimiters are very different from any signal pattern that might be in the Dataportion of the frame. This difference allows the Data portion of the frame to beunambiguously identified and allows Data corrupted by noise to be detected using a

Frame Check Sequence (FCS). The FCS is the very last part of the Data portion of theframe. This feature makes Fieldbus much more robust than many other controlnetworks. Because all devices share the cable, only one device should transmit at anygiven time. Otherwise, there would be chaos on the cable with all the transmittedsignals interfering with one another. A special device, called the Link Active Scheduler(LAS), selects which single device can transmit. The LAS allows each device to transmitby sending out a special frame to each device in turn. A frame might be: the LAS askinga device to transmit data, a device broadcasting its data to other devices, a device


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reporting an error condition, etc. The link active scheduler enabled device can continueto perform the control function even if the connection between the instrument and theHost is lost or disconnected. This is because the LAS allows the data to be transmittedbetween the devices in the field itself thus avoiding the breakdown of the control in theentire segment.

Bus SchedulingThe LAS addresses devices on the logical bus in a cyclic pattern. A complete

cycle is called the Macro cycle. This cycle is sub divided into two addressingsequences. The first addressing sequence is the addressing of prescheduled devices(function blocks), while the second addressing sequence is unscheduled date (eventdata). The length of the macro cycle is determined by the LAS and recalculated eachtime a new device is active on the bus. Execution times per function block in thescheduled sequence of the macro cycle may vary in the range of 20 ms to 50 ms. Thiscommunication, which has fixed repetition rate represents the process relatedsynchronous data sampling. By adding about 400 ms of processing time for

unscheduled activity, the total macro cycle time will be 1000 ms. (Note: As a generalrule, unscheduled time should not be less than 50% of the total macro cycle).

The cyclic update time per tag is one second. It is important to be aware of thefact that process variable update times for a single tag is identical to the update time fora group of tags (in this case 8instrument values). It will be misleading to convert a groupupdate time into an average update time per tag. In special cases, one might want toincrease the update time of a certain tag. In order to do so, one need to segregateinstruments into cycle time groups. The general rule is that, if the update rate isdoubled, only half of the instruments can be present on the segment. The followingsample calculation will illustrate this point.

Seven instruments (each with 38 ms bus processing time) requires a scheduledcycle time of 266 milliseconds. Adding 234 ms of unscheduled time will produce amacro cycle of 500 ms. The instrument values will be updated two times per second,and the total unscheduled time is about 468 ms. per second. It is good engineeringpractice to perform Clustering design when distributing instruments on segments. Inpractical sense, three types of clustering may be relevant:

Geographical - Instruments that are graphically close should be grouped in onesegment or adjacent segments.Process - Instruments that are process related should be grouped on one segment.Reliability - Instruments that are part of a common reliability concept should not be onthe same segment.


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SEGMENT LENGTH CALCULATION CRITERIA

Theoretically, if Foundation Fieldbus supply is 24 V to the network; then as eachdevice uses 9 V as a minimum, there are 24-9=15 volts that are available to be used bythe cable resistance. The total current that can be applied at each trunk is

Voltage / Resistance = Current available

15 Volts / 44*Ohms/km = 340 mAmps(* Considering the trunk cable is 1km and the resistance of both wires is 44ohms.)

Since each device draws 20mAmps, the maximum devices that can beconnected to each trunk are 17 devices.

Generally there are less than 16 devices on any single Fieldbus segment.

Attenuation:As signals travel on a Fieldbus cable, they are attenuated, that is, they are

reduced in amplitude. The longer the cable, the greater the attenuation. The Fieldbusstandard requires that a Fieldbus device transmits a signal at least 0.75 Volts peak-to-peak and that a receiver must be able to detect a signal of as little as 0.15 Volts peak-to-peak. (In electrical engineering talk, this is 14 dB of attenuation). If standard Fieldbuscable is used (attenuation of 3 dB/ km), then the cable can be

14 dB / 3 dB/km = 4.6 km long.

However, there is additional attenuation that needs to be considered. Signals are alsoattenuated by the spur cables that branch off the trunk cable. This attenuation is largely

caused by cable capacitance. Standard Fieldbus cable capacitance is about 0.15 nF/meter and the attenuation caused by capacitance is about 0.035 dB/nF. As an example,if the lengths of all the spurs is 500 meters, then the attenuation will be

500 meters x 0.15 nF/meter x 0.035 dB/nF = 2.6 dB.

As an example, assume that the trunk cable is 800 meters long. The trunk attenuation is

3 dB/ km x 0.8 km = 2.4 dB.

The total signal attenuation is

2.6 dB + 2.4 dB = 5 dB.

This is well within the 14 dB available.


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Signal Distortion:Fieldbus cable is limited to less than the theoretically possible length. Signals

also get distorted by various cable characteristics, spur reflections, etc. Shown below onthe left is a transmitted signal and on the right a received signal at the end of a 900meter long cable with 16 120-meter spurs at the chickenfoot.

Figure 21: Transmitted and received signals

Although it is not possible here to provide a definitive analysis of cable distortion,here are two recommendations to minimize distortion:

If the trunk cable is more than 250 meters long, put a terminator on each end.

Keep each spur length below 120 meters.

These recommendations are a result of testing Fieldbus signal fidelity on a 1 kmlongtrunk cable with 16 spurs 120-meter long at the chickenfoot.


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FIELDBUS IN HAZARDOUS AREA

Equipments and products in hazardous areas are required to meet stringentcriteria. They must be protected to avoid the possibility of them becoming a source ofignition.Fieldbus devices used in hazardous areas can only be supplied with a limitedamount of power to be Intrinsically Safe (IS). Using fieldbus entity isolators, only about 3or 4 Fieldbus devices can be on an IS fieldbus trunk.

Fieldbus Intrinsically Safe Concept (FISCO):

A new Fieldbus powering method called FISCO (Fieldbus Intrinsic SafetyConcept) increases the vailable power to a level that allows about 12 devices to be onan IS fieldbus segment in Gas Group IIB (Groups C,D in North America). This typicallyrepresents more than of 80% of the IS applications. In Gas Group IIC (A,B) the FISCOpower supply will typically power 5 or 6 devices on an IS fieldbus trunk which is a smallnumber compared to non-IS installations.

A process to be controlled in a IIC (A,B) Gas Group may require more devices towork together. One way to do this is to have the H1 host controller relay messagesbetween devices on independent segments. This is problematic because an H1 hostcontroller only has a limited number of Fieldbus connection ports. Repeaters can beused to solve this problem. Repeaters are devices that interconnect Fieldbus segmentsinto a single network. A repeater takes signals from one segment, reconstructs them tothe proper waveshape and retransmits them on to the other segment. The devices onthe separate segments think they are on the same segment. Fieldbus power must beseparately provided to each segment.

Repeaters can also be used to extend the length of a Fieldbus network. This isnot generally necessary. See Fieldbus Limitations on page 22. Repeaters are moreuseful in hazardous area applications to combine electrically separate segments to looklike a single logical segment. The example below shows this arrangement. Note thateach segment requires a pair of terminators. Repeaters often have terminators as partof their assembly.

Suppose Fieldbus segments 2, 3, and 4 are in a hazardous area. Each segmenthas 5 devices that need to communicate with each other and with devices on the othersegments. Three repeaters with built-in FISCO power supplies are used as shown. Allthe data packets sent by any one of the Fieldbus devices appear on all segments

including segment 1. The H1 host controller needs to have only one Fieldbus port. Tothe host and to each of the devices, data transmission and reception appears to be on asingle segment. In reality, they are on separate physical segments. Each segment isindividually powered and terminated in the same way as any other Fieldbus segment.


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Figure 22: Repeaters with FISCO power suppliesAlso referMultiple barrier

Fieldbus Non Incendive Concept (FNICO):

FNICO is the Fieldbus Non-Incendive Concept. FNICO builds on theexperimental work carried out for FISCO, byapplying the same priciples to the non-incendive fieldbus circuits in zone-2 and division

2 hazardous areas. FNICO networks may be live worked in the hazardous areawithout the need for gas clearance procedures, and support even greater number offield devices than FISCO. To assemble a FNICO fieldbus system, the power supply,field devices, cables and wiring components need to comply with FNICO design rules. Awide choice of components is available since any intrinsically safe Entity or FISCOcertified devices and most non-incendive devices can be used.

The key benefits of FNICO system are as follows:

a) Flexible low cost alternatives to flameproof/ explosion-proof technique.b) Live-workable field network, just like intrinsic safety.c) Relaxed installation requirements.d) Simple safety documentation just a list of devices.


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INTRINSIC SAFETY

Devices and barriers for intrinsically safe areas are designed so that the energyreleased by an electrical fault is not enough to cause ignition. The ignition point isfunction of power, determined by voltage and current.

Amount of current allowed on an intrinsically safe segment as well as segmentvoltage, choice of barrier and device count on each barrier depends only on the type ofhazardous atmosphere in which the devices are located, but also on which intrinsicsafety model on use.

For fieldbuses, there are two models:

The Entity model assumes the electrical parameters that represents thecharacteristics of the wire are all concentrated at the point of fault. In thismodel, a wire is considered a source of stored energy. This conservativeapproach leads to a maximum DC current of 83mA permitted in the wireand a maximum voltage of 18.4 V.

This model is well known and recognized worldwide.

The Fieldbus Intrinsically Safe Concept or FISCO model considers theelectrical wiring parameters to be distributed along its entire length. Thisreduces the energy at the fault, resulting in a maximum current of 110mA.This model permits more devices on a wire in a hazardous area.FISCO is not a world wide standard.

Intrinsic safety barriers are certified on the basis of one model or the other. Fielddevices can be certified for both. Despite the differences between the two models, thebasic concepts for designing an intrinsically safe segment are similar.

The ignition curve:Each type of atmosphere requires a certain minimum power for ignition. The plot

of the voltage and current points that provide that power is called the ignition curve.Because power is voltage times current, as voltage increases, the maximum

amount of current required for ignition decreases. And, conversely, as voltagedecreases, the maximum amount of current required for ignition increases.

In a segment using the FISCO model,the maximum current allowed is 110 mA in aClass IIC environment. This means that thetotal current draw for all devices on this

barrier is 110 mA.


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Designing an intrinsically safe segment:To calculate how many devices a single barrier can support, you add the

individual current draws of each devicesince each device type has a potentiallydifferent current draw.

For the FISCO model, as long as the total current draw is under 110 mA for gas

groups A and B, and under 235 mA for gas groups C and D, the segment on thehazardous side on the barrier is intrinsically safe. You must also consider the electricalparameters of each device and be below the amounts permitted for the hazardous areaclassification.

In the example shownin the figure, a single barrieris placed on a segmentbetween the segment powerconditioner and the fileddevices. There is a terminatorin the safe area and in the

hazardous area.

Here are some example calculations to determine the number of field devicesallowable in this example. The current consumptions listed are for illustration purposesonly and do not reflect the actual current consumption of specific devices or devicestypes:

Temperature monitoring.If a temperature transmitter uses 16 mA of current, a maximum of six (6 x 16

=96) transmitters could be placed in hazardous environment on a single barrier. ForClass IIB gases, the maximum current is 240mA, allowing 15 devices per barrier.

Temperature and pressure compensated mass flow.In this case, the temperature transmitter uses 16 mA, a pressure and DP

transmitter each use 20mA, and a control valve uses 25 mA. All four of these devicescould be placed on the same barrier in a Class IIC hazardous environment(16+20+20+25 = 81)


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Combining safe and hazardous areas:There may be occasions when it is desirable to have both safe and hazardous

areas on the same fiel

foundation fieldbus tutorial

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