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Fieldbus Access SublayerCommunications Relationshi s

Fieldbus Message

SpecificationObject Dictionary VirtualField Devices

Application

Layer

AApplication LayerThis corresponds to Layer 7 in the ISO/OSIcommunications model. This is NOT the processapplication see ISO/OSI Model. It is theresponsibility of this layer to provideprotocols/services both internally within andbetween devices on a fieldbus.

For Foundation fieldbus the Application layer isdescribed in two parts, the Fieldbus MessageSpecification Layer (FMS) and the Fieldbus

Access Sublayer (FAS).

The Fieldbus Message Specification Layer

(FMS) allows easy access to data via objectdictionaries and also the structuring of VirtualField Devices (VFDs) allowing access to data via TAGs. The Fieldbus Access Sublayer (FAS) is whererelationships such as Publisher-Subscriber and Client-Server are set up.

Asynchronous communicationDescribes communication where the transmitting device clock is not synchronised to the receiving device clock.Generally it indicates that individual data characters are preceded by a start character and terminated with a stopcharacter. The start and stop characters add extra overhead and reduce the effective transmission speed.

The Fieldbus Jargon Buster, best viewed in Word 97, usesBookmarks and Hyperlinks to aid navigation. Fieldbus terms are

listed alphabetically with hyperlinks (indicated by underlining) torelated terms.


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Home Run or Backbone

Chicken foot

Repeater

SpursSegment 2

BBackbone

Often described as thehome runcable ormaintrunk. It is the main communication highwayacting as a source for other lines (spurs) fromthe control room.

The diagram opposite shows how the home runis connected to the control room via a fieldbus

controller. The home run is in fact just anotherH1 segment which happens to be at the top ofthe tree and connected to the control room. Itmay have other segments connected to it viadevices such as Repeaters, Gateways, Bridgesetc. As a consequence the home run carries allthe communications traffic passed to it.

Balanced Transmission Line

A pair of wires carrying an AC Analogue or DCDigital signal with both sides isolated fromground. Both ends must be terminated. Thisdescribes the transmission for H1 fieldbus.

BandwidthThe difference between the maximum and minimum frequency which can be transmitted along a network. Thisrelates to the maximum data/information carrying capacity of the network.

BlocksBlocks are used to collect together related data in a single structure. Blocks can be interconnected or soft-wired toperform more complex functions. See Function BlocksResource BlocksTransducer Block

9-32

H1FieldbusSignal


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Physical Layer

Datalink Layer

Application Layer

Physical Layer

Datalink Layer

Application Layer

Datalink Layer Datalink Layer

Physical Layer Physical Layer

H1 H1

MessageFiltering

Bridge

Device

BroadcastA communications method where one device transmits onto the network and many devices read the data. Thetransmitting device does not get acknowledgement that all the devices who wished to receive the data actuallyreceived it. SeePublisher/Subscriber& Token Passing

BridgesABridge will allow the connectionbetween different segments thatuse the same media accessmethod i.e. the segments mayoperate at different baud rates.

Unlike repeaters, which simplyrepeat the data on the input portonto the output port, a bridge willallow you to filter only the data thatneeds to be transferred to theother segment, so reducingcommunications traffic.

A Gateway provides a similarfunction but extends it to transferdata between protocols (BetweenFOUNDATION and PROFIBUSPA).

BusThe Trunk/Backbone/Home run with all the devices connected to it. See also Topology.


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Chicken foot

1900m Sum of Chicken foot spurs

Junction Box

Segment

Note: The sum of all the spur lengths and segment length should not exceed

the maximum for the fieldbus cable i.e. Type A cable maximum=1,900m Segment length + All Spur Lengths < 1,900m

Maximum Spur Length is reduced by 30m for each device on the spur

CChicken Foot

Describes a network consisting of anumber of devices connected to oneJunction Box. See alsoSpur, Segment,Topology. Also referred to as a STARconfiguration in Ethernet.

Collision Avoidance/Collision Detection CSMA/CDDescribes a method for controlling data communication on a network. A device wishing to gain access to thenetwork listens, if it hears silence it can then speak, if it then hears a collision it waits for a random time and thenretries. Ethernet uses this type of control. See H2(CSMA/CD = Carrier Sense, Multiple Access with CollisionDetection)


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Coupler

Device

Trunk Trunk

Spur

Coupler

Conditioner (PSU)A normal power supply would not be compatible with afieldbus installation, as it would interfere with the fieldbussignal.

A power supply conditioner can be used to stop thefieldbus signal entering the power supply thereforemaking it compatible. The conditioner is usually anelectronic equivalent of Inductor/Resistor. SeePSU.

CouplerThe Coupler connects devices to the network.It can be simply terminals in a small box,plugs and sockets or specially designed DINrail mounted connection units. It has at leastthree connections: one for the spur and onefor each side of the trunk.

An Active Coupler may contain componentsfor signal amplification and retransmission.SeeSplice

DDDLDevice Descriptions are written in a specialised computer programming language called Device DescriptionLanguage (DDL).

DeterministicA deterministic communications protocol is predictable in the maximum time a device must wait in order to gainaccess to the network to make data transmissions. For real-time control a deterministic protocol is desirable so thatdata transmission deadlines can be met, e.g. for cyclic data transfers.

In practice, even protocols designed to be deterministic, can become non-deterministic when a random erroroccurs in a message. So, in the real world one can only speak about a probability of successfully transmitting amessage when access to the network is gained. The possibility of errors tends to equalise the differences in theEthernet and Central control token approaches.

Traditional

PSU

Conditioner

(Electronic

equivalent of

Inductor/Resistor)


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DeviceA physical entity connected to a fieldbus that is composed of at least one communication element. E.g. Pressuretransmitter. See alsoHost.

Device Description (DD)Device descriptions are provided to host systems so that the host can "interpret" the meaning of the data providedby fieldbus devices. The DD knows the indices in the Object Dictionaryof the device, so the host can use it toaccess variables.

The DD is a clear unambiguous structured text description that precisely describes the field device data and

operations to host systems. This text file is then passed to a tokeniser tool that generated the binary DD used bythe host.

DuplexSimultaneous, two-way independent transmission of data

EEthernet

Network protocol commonly used to link PCs.10BASE-Tx (10 Mbit/sec)100BASE-Tx (100 Mbit/sec)- 100BASE-Tx most popular today Runs over 2-pair (twisted) Category 5 cable- 100BASE-T4 Runs on 4-pair (twisted) Category 3,4 or 5 cable- 100BASE-FX Runs on two strands of 62.5/125 micron fibre

End DelimiterA bit sequence used to signal the end of a frame.See alsoFrame,start delimiter

FFast Ethernet

A term to describe Ethernet at 100 Mbit/sec as opposed to 10 Mbit/sec. SeeH2 &Ethernet


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FDA (Field Device Agent)The Field Device Agent is software code which resides in an H2 device, such as a Link Device. The purpose of theFDA is to provide the Fieldbus Messaging Service (FMS) and system management (SM) services overUDP/TCPto allow field devices to be directly connected to HSE and H1 devices.

FieldbusesA fieldbus is a method to allow digital communications between field devices including control room equipment.There are many fieldbuses available each one providing advantages for particular applications. For processcontrol, including devices such as pressure transmitters, then the most suitable protocols would be PROFIBUSPA/DP and FOUNDATION, as they both supply power on the network. LON and Devicenet are examples of sensorbuses that cater for simple devices such as status switches.

PROFIBUS is common in Europe and exists in three different versions:

FOUNDATION has worldwide usage and it allows for distribution of control to the field, time stamping of alarms atthe field device, has extensive interoperability features and a unique User Layer for advanced functions.

FrameA single set of data transmission from a device. See alsoEnd delimiter, Start delimiter.

FTP:The TCP/IP standard high level protocol for transferring files between machines. FTP uses the Telnet and TCPprotocols SeeSMTP

PROFIBUS DPMainly used to communicate between PLC, systems or Bulk I/O devices.There is no power on the network - in Euronorm

PROFIBUS FMS For PLC- PLC comms - in Euronorm.

PROFIBUS PA For Process Automation For communication between field devices e.g.Pressure transmitters - in Euronorm


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ControlValvePressure

Transmitter

AI AOPID

Physical Layer

Datalink Layer

Application Layer

Physical Layer

Datalink Layer

Application Layer

Application Layer Application Layer

Physical Layer Physical Layer

Datalink Layer Datalink Layer H1 Profibus PA

Message

Filtering

Gateway

Device

Function Block

The Function Block describes data andoperations for self-contained control functions.There are many standard Function Blocks.Some parameters within them are the sameacross all Function Blocks (Universal), someare the same for similar Function Blocks(Function parameters) and some are the samefor similar devices (Profile parameters). SeeResource BlocksTransducer Block

GGateway

A G a t e w a y p r o v i d e scommunications capability betweenieldbuses of different protocols e.g.between FOUNDATION andPROFIBUS PA.

It is different to a Br idge, in that

protocol conversion occurs abovethe application layer rather than inthe datalink layer. There is bound tobe some loss of functionality as aresult of the conversion betweenprotocols (e.g. PROFIBUS PA doesnot timestamp alarms at the fielddevice (Dec98)).


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GSD fileGertestammdaten Device database file, alsocalled device datasheetfor PROFIBUS devices.

The GSD file contains adescription of the deviceand provides a way for anopen configuration tool toread the data sheet (GSDFile) and therefore takeaccount of the devicecharacteristics.

The vendors of thedevices describe thefunctionality of theirdevices by means of aGSD file. The format ofthe GSD file is clearlydefined by the PROFIBUS standard. The configuration tool reads the GSD files into its internal data base and takesall characteristics of the individual devices into account.

The advantage is that the configuration is very simple and can be done by a vendor-independent configuration tool.GSD files are similar to Windows INF files for printer or modem definitions. The GSD files are more limited than

DDLs in the description that they provide.

HH1

This refers to the low speed, low power fieldbus signalling at 31.25 kbit/sec. FOUNDATION fieldbus H1 allows for

- Digital only inter-device communication - Publisher/Subscriber or Peer to Peer.- Power on the bus. A device should be capable of operating within a voltage range of 9 to 32V.

- Time distribution with scheduled events (aids distributed control in the field)H2Refers to High Speed Ethernet (HSE) fieldbus at 100 Mbit/sec. This network is still under development forFOUNDATION and is often referred to as the HSE project. A GatewayorLink Devicewill be used to enable interH1/H2 communications. HSE uses the Collision Detection/Collision Avoidancecommunications control method.SeeFDA

High speed fieldbus

SeeH2

PROFIBUS

configuration toolSystem

configu

ration

Electronic Device Data Sheets (GSD-files)

PLC

PROFIBUS

GSD GSD GSD GSD GSD GSD

PLC


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Home RunThis is the communications cable that returns data to the Control Room. It is the main communication highwayacting as a source for other line (spurs) from the control room. See main trunk-backbone

HostA computer belonging to a user that hosts communication hardware and software that allows connection to thenetwork. In Foundation fieldbus the term is also sometimes applied to a device containing the Link ActiveScheduler. Examples are control room hosts, hand held configuration tools etc. See also Device.

Hub (Ethernet)

A device which repeats the input signal onto all ports andcan be considered to be a Multi-port repeater. It is usedto create STAR network topologies for Ethernetinstallations.

A Hub allows all connected devices to share the samenetwork resources (bandwidth). Switches are nowreplacing hubs as a switch will filter messages betweennetworks allowing improved network designs for optimumnetwork performance.

IIECInternational Electrotechnical Commission; the standardisation body responsible for the agreement of aninternational fieldbus standard. See also Standardisation Organisations.

Inrush CurrentMaximum allowed rate of change of quiescent current after a device powers up (non-transmitting). It should notexceed 1.0 mA/msec after the first 20 msec of connecting a device to a network or after 20 msec after applying

power to the network. SeeMaximum Current, Lift Off Voltage.

InteroperabilityThe ability of implementations of the fieldbus standard in different devices to interwork with one another.

IPInternet Protocol. Includes error checking as integral part.

IP Address: 32-bit address assigned to hosts that want to participate in a TCP/IP internet.

HUB


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NOTE The PNO (PROFIBUS FISCO Model (from PTB)) and FOUNDATION (doc. AG-163)

have different guidelines for IS.

ISO-OSI Model Fieldbus Model

PHYSICAL LAYER

DATA LINK LAYER

FIELDBUS ACCESSSUBLAYER

FIELDBUS MESSAGESPECIFICATION

USER LAYER

SESSION LAYER

PRESENTATION LAYER

APPLICATION LAYER

PHYSICAL LAYER

DATA LINK LAYER

NETWORK LAYER

TRANSPORT LAYER

1

2

3

4

5

6

7

USER LAYER

PHYSICAL LAYER

COMMUNICATION

STACK

M

A

N

A

G

EM

E

N

T

M

A

N

A

GE

M

E

N

T

ISO/OSI model

In ternat ional StandardsOrganisation Open SystemsInterconnection (ISO/OSI).This is a model containingseven layers each describinga function(s) which may beused to produce an opencommunications protocol.

Foundation fieldbus usesthree of these layers plus anextra layer (User Layer).

User LayerHere you can use function

blocks to build processappl icat ions e.g. PIDController or add extra functions.

Application Layer- This is not the process application, its function is to provide a standard order for data containedin the device (Object Directory). It also allows devices to set up communications relationships with each other(publisher/subscriber).

Physical Layer - Defines speed of transmission, wave shapes and amplitude of signals. Also defines the

transmission medium e.g. wire - fibre optic.

Intrinsic SafetyMethod for limiting the amount of electrical power (600 mW) available to field devices. An IS power supply for asegment is likely to provide a voltage of less than 23V (typically, will be 19V) and provide power for about fourdevices (assumes each device is loop-powered and takes about 10 mA) in the hazardous area.

Protection can be via a Barrier or Isolator with the pros and cons of each being the same as for a traditionalinstallation.

ISPInteroperable Systems Project. Historically, slow progress within the IFC (International Fieldbus Consortium)resulted in the formation of the Interoperable Systems Project (ISP), by a set of vendors, who decided to developtheir own specifications based on PROFIBUS DP. A competing organisation, called WorldFIP was immediatelyformed by a different set of vendors basing their protocol on the French FIP standard.However, users then found


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their systems suppliers on one-side and instruments suppliers on the other and pressure from them led to thedissolution of ISP and IFC and the formation of Fieldbus Foundation.

JJabber

Refers to a network problem caused by a device placing corrupted data on the network, transmitting for too long, oran error condition due to an Ethernet node transmitting longer packets than allowed.

KKERMIT

A popular file transfer protocol.

LLatencyThe delay incurred by aswitchingor abridgingdevice between receiving and transmitting data.

LayersRefers to the layeredISO/OSImodel for specifying open communication protocols.

Lift Off Current

SeeInrush Current

Lift Off Voltage

The initial voltage required for a fieldbus device to start operating. This should be 9V or lower. See also InrushCurrent,Maximum Current.

Link Active Scheduler (LAS)

This is the device which controls which device on a Foundation fieldbus network is allowed to transmit at any time.


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1 0 1 0 1 0 1 0

1 N+ N- 1 0 N- N+ 0

PREAMBLE

STARTDELIMITER

1 N+ N- N+ N- 1 0 1

ENDDELIMITER

BITBOUNDARIES

N+ Non data symbol - positive.N- Non data symbol - negative.

Link DeviceAllows foundationH1devices (Pressure transmitters etc.) to link to foundation H2(High Speed Ethernet devices).

MManchester Bi-phase L

Manchester Bi-phase L is a

s i g n a l i n g m e t h o d f o r synchronizing receivers andtransmitters.

It produces a pattern from whichthe information to synchronizethe receiver clock can beextracted. It avoids the need forseparate data and clock circuits.See Physical Layer

Master Slave

Describes a method for controlling data communication on a network. A master device controls which slave devicecan have access to the network to transmit data. An example protocol using this method is MODBUS RTU.

MAUMedia Access Unit. Interfacecircuitry between the bus and

the rest of the communicationscircuitry in a device. The MAUregulates the current andshapes the t rapezoidalwaveform when transmitting.

When receiving, it filters out thesignal and extracts the bits. Italso extracts the current fromthe network and supplies it tothe device components.

Primary

Sensor

A

D

C

Micro

processorFieldbus

Controller

Media

Access

Unit

(MAU)

v+

Vo

+

-

Fieldbus

Wire

Medium

Power

Network powered device


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Maximum CurrentA limit on the maximum device current applies during the interval 500 _s to 20 ms after connection of a device tothe network or after the application of power to the network. Limit is rated quiescent current + 20 mA. See alsoInrush Current,Lift Off Voltage.

Multi-variable device

SeeSensor Fusion

NNodeIs a communications device on the network e.g. a pressure transmitter or valve.

OObject Dictionary(OD)Provides a standard structure for accessing the internal data of a device. It defines the data types e.g. Integer,Boolean, Float, Date, String etc. The OD resides at the Application Layer. The OD is read by systems which thenuse the indices to access the data.

PPhysical Layer

This layer defines everything required to make a physical connection to the fieldbus network. Both FOUNDATIONand PROFIBUS use the same Physical Layer to the IEC 1158-2 standard. It defines:

- Network medium (Wire-Optical fiber etc.)- Wave shapes (square-Trapezoidal etc.)- Wave amplitude- Speed- Startup and synchronizing of communications

(Manchester Bi-phase L).- Intrinsic safety (~ 6 devices).

See alsoISOmodel


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Power SupplyFieldbus power supplies are different totraditional power supplies as they haveextra components to stop the fieldbussignal from entering the power supplywhich would then corrupt it by regulatingout the voltage variation. Traditional PSUcan be upgraded so that they can be usedin fieldbus installations by the use of PSUConditioners.

Redundant PSUs are possible andfacilitated with devices such as PSU

multiplexer from RELCOM.Maximum non-IS output is 32 V DC max.ForISOutput Depends on the barrier used(typically about 19V).

SeeSurge ProtectorsSegmentInrushCurrent Maximum Current

PNO

PROFIBUS Nutzerorganisation e.V. The standardisation organisation driving the PROFIBUS fieldbus

ProfilesA profile defines what must be present (e.g. parameters or services) to provide a useful level ofinteroperabilitybetween devices from different vendors.

Device Profile defines minimum for interoperability between similar devices from different vendors.

Communication Profiles defines minimum for communications interoperability e.g. intelligent devices shouldsupport Publisher/Subscriber and time distribution services. See Token PassingPublisher SubscriberCollision

Avoidance/Collision Detection

Physical Layer profile defines the minimum characteristics for interconnection interoperability e.g. IS and non-IS

power supply current and voltage levels.

Protocol

This is the overall set of rules and formats for data transfer between the sameISO/OSIlayers of different devices.

Redundant Configuration

PSU

Multiplexer

PSU-1

PSU-2

To FieldbusCable

To FieldbusCable

Segment 1 Segment 2


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Publisher SubscriberDescribes a particular transmission/reception method for sharing data on a network. A device can publish data tothe whole network, all devices which wish to subscribe to that data will then read it. See Token PassingApplicationLayer.

RResource Block

The Resource Block describes the characteristics that apply to the whole of the physical device (e.g. manufacturerserial number).

RepeaterA Repeater will allow extension of the length of the fieldbus, with a maximum of four repeaters in any directcommunication path between two devices (maximum number of addresses is 240 for one home run).

Repeaters for Ethernet have the same function as for fieldbus however there are two types of repeater available.

Type I : Converts the input to digital format and then repeats the signal on to the output port. This introduces adelay such that only one of these repeaters can be used on one segment. Could be used to connect differingmedia.

Type II : Instantly repeats input onto the output. Two of these repeaters can be used on one segment.

The use of repeaters has to be balanced with overall network performance. As well as extending the distancerepeaters allow more devices to be connected. Each additional device on the network will use up some of thebandwidth and as more devices are added performance will deteriorate.

Terminators and Power Supplies not shown(Each end of each segment must be terminated)

R R R R

1900 1900 1900 1900

Trunk

Segmentt

H1

R R

R


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Round BoardA Round Board is a protocol converter usually manufactured by National Instruments or SOFTING. It can be usedto make a HART device look like a Fieldbus Foundation device.

SSegment

A Segment is a length of Fieldbus network with a terminator at each end, or a part of a fieldbus network wiring thatis electrically independent from other parts. It may or may not have an associated power supply.

The number of devices on a segment depends upon: -- Cable Type and Length- IS or on-IS application- Power Supply and Device current consumption- Bandwidth available and required response

Sensor FusionSensor Fusion is the addition of intelligence and multi-measurement capabilities to devices such as flowmeters(temperature - DP flow - Pressure)

ServiceServices are operations on communications objects of a defined class. A measured value, for example, is such anobject that belongs to the class ofVariables. Read and Write are the services allowed for this class of objects.

Slow Speed fieldbus

SeeH1

Non-IS application. 1Km segment length(22_/km). 20V PSU. Devices consume10mA.Current = Voltage/Resistance

=20V-9V/44_ =250mA (9v min Voltage)

=25 devices

Note: double the segment length halves thenumber of devices in this case.

IS application. 1Km segment length(22_/km). 20V PSU. Devices consume10mA.Current = Voltage/Resistance=20V-9V/(44+105)_=74mA (9v min Voltage)

=7 devices


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Note:

The sum of all the spur lengths andsegment length should not exceed themaximum for the fieldbus cable i.e.Type A cable maximum =1,900

Segment length + All Spur Lengths

< 1,900m

Maximum Spur Length is reduced by30m for each device on the spur

SMTPSimple Mail Transfer Protocol SMTP specifies how two mail systems interact and the format of control messagesthey exchange to transfer mail.

SpliceA part of the network in which the characteristic impedance of the network is not preserved e.g. connection to aspur using a different wire gauge.

SpurA Spur is a length of fieldbus network between a junction box and a device.A spur can vary in length from 1m (3.3ft) to 120 m (394 feet). A spur does NOT have terminators at either end of the spur cable. The maximum length of aspur depends upon several criteria including cable type and total number of devices on thesegment

StackRefers to the communications software suite that implements theISO/OSIlayers used in a device.

Total Number ofCommunicating Elements

Maximum Spur Length (M)

25-32 0

19-24 30

15-18 60

13-14 90

1-12 120


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Committee for EuropeanNormalisation

(CEN/CENELEC)

BSI DIN AFNOR MilitaryStandards

ISO/IECNATOStandards

American National StandardsInstitute(ANSI)

MilitaryStandards

DoD(USA)

ASMEISASAE

IEEETradeAssociations(NEMAEIA)

Standards organisationsWorldwide standards are organised by the International Standards Organisation (ISO) and the InternationalElectrotechnical Commission (IEC). A large number of national bodies support their work (see below). ISO coversstandardisation in all fields except electrical and electronic standards, which are the responsibility of the IEC.CENELEC is charted with developing voluntary electrotechnical standards, as a basis for supporting the creation ofa single European market without internal frontiers. Its work will override that of any European national group and isprimarily based on that of the IEC.

The main bodies involved in the developmentof fieldbus standards are the ISA and IEC forFoundation fieldbus and CENELEC forPROFIBUS, WorldFIP and Pnet (EN50170).

AFNOR Association Francaise de

Normalisation ANSI American National Standards

Institute ASME American Society of Mechanical

Engineers BSI British Standards Institute CENELEC European Committee for

Standardisation DIN Deutsche Institut fur Normun DoD Department of Defense EIA Electronic Industries Association ISO International Standards

Organisation

IEC International ElectrotechnicalCommission IEEE Institute of Electrical and

Electronic Engineers ISA Instrument Society of America NEMA National Electrical

Manufacturers Association SAE Society of Automotive Engineers

StarSeeHUBChicken Foot

Start DelimiterA bit sequence used to signal the start of the data portion of a frame. See alsoFrame,End delimiter


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Surge Protection Device (SPD)This is a device that routes electrical surges, such as from from lightning or welding, safely to earth. There is afieldbus-specific SPD available with a high bandwidth to pass the fieldbus signal, but line resistance (typically,about 10__ must be taken into account.

SwitchesAn Ethernet device which can be considered as a multi-port bridge. It can be used to replace ahubfor the designof STAR topologies. The advantage a switch has over a hub is that it can filter messages between networks suchthat devices connected to one switch share a local bandwidth and not that of the total network as with the hub.

TTCPTransmission Control Protocol: providesfull duplexstreaming service on which other protocols depend. See UDPFTPIPFDA

Token Passing

Deterministic method for controlling data communication on a network. Tokens special data messages areused to enable access to the network. In the simplest case, a device will receive a token which allows it to transmiton the network; all other devices listen. When the transmitting device has finished, it passes the token to the nextdevice and so on around the network. Rules vary, but usually a device can hold the token only for a limited timebefore passing it on.

In FOUNDATION fieldbus there is more than one type of token, these are:

- Compel Token initiates broadcast (Publisher/Subscriber) communication.- Pass Token initiates peer to peer messaging.- Probe Node Token detects new devices on the network.

Topology

Topology denotes the physical structure and layout of a network. In general, there are four types: Bus, Ring, Starand Tree.

BUS A shared data transmission path connecting all devices, with two defined ends. Each transmissiontravels the length of the medium (usually cable) and is seen by all other devices.

RING Network devices form a closed ring, with each connected to its neighbour. Data is transmitted inone direction only; as it circulates through each device, it is read. After circulating all around thering, it is removed by the source device. Failure of any device means failure of the entire network.

STAR One central device performs all signal switching and routing functions. The network is reliant onthe performance of this one device.

TREE A number of junction boxes are connected along the network, with devices connected in clustersat each junction box, via spurs. Also known as Chicken Foot topology.


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Transducer BlockThe Transducer Block contains parameters specific to the technology of the device. Some parameters are standardfor similar devices

TerminatorsTerminators are required at the end of each fieldbus Segment and match the cable impedance to reducereflections of the signal.

Tree TopologySeeChicken Foot

TrunkA Trunkis the main communication highway acting as a source for other line (spurs) from the control room. Seehomerun-backbone

UUDPUser Datagram Protocol: splits data into small parts and transmits the data as random packets. The receiverrebuilds the data. There is no confirmation of reception. SeeFDATCP

User LayerUnique layer found in FOUNDATION fieldbus to enhance interoperability and allow vendor innovation, with thefacility to use function blocks to build process applications (such as PID Controller. SeeISO


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VVirtual Field Device (VFD)Part of theApplication Layerfunctionality.

A Virtual Field Device is a model of thedevice for communications purposes. Itgroups variables into objects and thenallows access to the data via an individualobject dictionary (OD) for each VFD. Thisallows for simple access to complexfunctions built up in a VFD.

A physical device can contain one ormore VFDs.

Object Dictionary

Object1

Object2

Object3

Objects could be: process variables, range limits,C++ applications etc.

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