fieldbus jargon buster 23/4/04 9:47 am page 1 fieldbus...

FIELDBUS Jargon Buster

©A

BB

Lim

ited

200

4

Fieldbus Jargon Buster 23/4/04 9:47 am

Fieldbus Jargon Buster


The Fieldbus Jargon Buster, best viewed in Word 97, uses Bookmarks and Hyperlinks to aid navigation. Fieldbus terms are listed alphabetically with hyperlinks (indicated by underlining) to related terms.


Quick Menu: -

A

Fieldbus Access Sublayer

Fieldbus Message Specification Object Dictionary – Virtual Field Devices

Application Layer

B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Application Layer This corresponds to Layer 7 in the ISO/OSI communications model. This is NOT the process application — see ISO/OSI Model. It is the responsibility of this layer to provide protocols/services both internally within and between devices on a fieldbus. For Foundation fieldbus the Application layer is described in two parts, the Fieldbus Message Specification Layer (FMS) and the Fieldbus Access Sublayer (FAS). The Fieldbus Message Specification Layer (FMS) allows easy access to data via object dictionaries and also the structuring of Virtual Field Devices (VFDs) allowing access to data via TAGs. The Fieldbus Access Sublayer (FAS) is where relationships such as Publisher-Subscriber and Client-Server are set up.

Asset Management An application at the system level to monitor automation equipment and alert as to predictive or current maintenance conditions. Often confused with AMS which simply allows for the calibration and set up of field devices. ABB’s 800xA control system has built in Asset Monitors with fully integrated CMMS such as MAXIMO to support field device – ABB field devices have specifically written Asset Monitors to fully support predicative maintenance alerts.

Asynchronous communication Describes 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 stop character. The start and stop characters add extra overhead and reduce the effective transmission speed.

ASI BUS Status level bus typically used for communicating to on/off devices such as Actuators.

B

Backbone Often described as the home run cable or main trunk. It is the main communication highway acting as a source for other lines (spurs) from the control room.

Home Run or Backbone

Chicken foot

Repeater

Spurs Segment 2

↓

The diagram opposite shows how the home run is connected to the control room via a fieldbus controller. The home run is in fact just another H1 segment which happens to be at the top of the tree and connected to the control room. It may have other segments connected to it via devices such as Repeaters, Gateways, Bridges etc. As a consequence the home run carries all the communications traffic passed to it.

Balanced Transmission Line

9-32 Volts

H1 Fieldbus Signal

A pair of wires carrying an AC Analogue or DC Digital signal with both sides isolated from ground. Both ends must be terminated. This describes the transmission for H1 fieldbus.


Bandwidth The difference between the maximum and minimum frequency which can be transmitted along a network. This relates to the maximum data/information carrying capacity of the network.

Blocks Blocks are used to collect together related data in a single structure. Blocks can be interconnected or ‘soft-wired’ to perform more complex functions. See Function Blocks Resource Blocks Transducer Block

Broadcast A communications method where one device transmits onto the network and many devices read the data. The transmitting device does not get acknowledgement that all the devices who wished to receive the data actually received it. See Publisher/Subscriber & Token Passing

Bridges

Physical Layer Datalink Layer Application Layer


Datalink Layer Datalink Layer Physical Layer Physical Layer

H1 H1

Message Filtering

A Bridge will allow the connection between different segments that use the same media access method i.e. the segments may operate at different baud rates.

Bridge Device

Unlike repeaters, which simply repeat the data on the input port onto the output port, a bridge will allow you to filter only the data that needs to be transferred to the other segment, so reducing communications traffic. A Gateway provides a similar function but extends it to transfer data between protocols (Between FOUNDATION and PROFIBUS PA).

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


Chicken foot

1900m – Sum of Chicken foot spurs

Junction Box

C Segment

Chicken Foot Describes a network consisting of a number of devices connected to one Junction Box. See also Spur , Segment, Topology. Also referred to as a STAR configuration in Ethernet.

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

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

Coupler

Device

Trunk Trunk

Spur

Coupler

A power supply conditioner can be used to stop the fieldbus signal entering the power supply therefore making it compatible. The conditioner is usually an electronic equivalent of Inductor/Resistor. See Power Supply.

Conditioner

(Electronic

equivalent of Inductor/Resistor)

Traditional

PSU

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


An Active Coupler may contain components for signal amplification and retransmission. See Splice also Segment Coupler PBus

D

DDL Device Descriptions are written in a specialised computer programming language called Device Description Language (DDL).

Deterministic A deterministic communications protocol is predictable in the maximum time a device must wait in order to gain access to the network to make data transmissions. For real-time control a deterministic protocol is desirable so that data 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 error occurs in a message. So, in the real world one can only speak about a probability of successfully transmitting a message when access to the network is gained. The possibility of errors tends to equalise the differences in the Ethernet and Central control token approaches.

Device A physical entity connected to a fieldbus that is composed of at least one communication element. E.g. Pressure transmitter. See also Host.

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

DP Profibus Decentralised Periphery. Describes a fieldbus protocol for connection of complex externally powered devices over RS485 or fibre optic media. See PA and Master


DTM Device Type Manager. This is an engineering methodology used to enhance the support of intelligent devices such as those using HART – Profibus and other communications protocols. A DTM is an executable program provided by the device vendor – it provides full access for device configuration/parameterisation and asset optimisation. A DTM requires the engineering tool to support the open FDT (Field Device Tool standard) and could be integrated in the system or as a laptop standalone tool.

Duplex Simultaneous, two-way independent transmission of data

E

Ethernet Network protocol commonly used to link PC’s. 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 Delimiter A bit sequence used to signal the end of a frame. See also Frame, start delimiter

F

Fast Ethernet A term to describe Ethernet at 100/1000 Mbit/sec as opposed to 10 Mbit/sec. See H2 & Ethernet

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 the FDA is to provide the Fieldbus Messaging Service (FMS) and system management (SM) services over UDP/TCP to allow field devices to be directly connected to HSE and H1 devices.

FDT Field Device Tool. This is a standard set of interfaces to all the execution of DTM


(Device Type Managers) to engineer field devices such as transmitters – Positioners etc.

Fieldbuses A 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 process control, including devices such as pressure transmitters, then the most suitable protocols would be PROFIBUS PA/DP and FOUNDATION, as they both supply power on the network. LON and Devicenet are examples of sensor buses that cater for simple devices such as status switches. PROFIBUS is common in Europe and exists in three different versions:

PROFIBUS DP Mainly used to communicate between PLC, systems or Bulk I/O

evices. There is no power on the network - in Euronorm d P ROFIBUS FMS For PLC- PLC comms - in Euronorm.

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

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

FISCO Fieldbus Intrinsically Safe Concept. A set of simple to follow guidelines to design a fieldbus network for use in hazardous areas using intrinsically safe methods. The guidelines reduce segment length to 1km and spurs to 30m but then allow 110mA (depending upon gas group). Possibly best used in conjunction with multibarrier technology. Foundation fieldbus or Profibus PA

Flexible Function Blocks Flexible Function Blocks (FFB) resides in the application Layer along with the other function blocks (AI-AO-PID…). They are designed for complex batch, discrete and hybrid control applications and as such there are two types, Fixed and Configurable. The Fixed FFB has a predefined algorithm with set inputs and outputs, Configurable can be designed to suit the application with tools based upon standards such as IEC 61131-3.


FNICO Fieldbus NonIncedive Concept. Restriction of FISCO to ZONE2 hazardous areas. 265mA IIC gas group. Foundation fieldbus or Profibus PA

Frame A single set of data transmission from a device. See also End delimiter, Start delimiter.

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

Function Block The Function Block describes data and operations for self-contained control functions. There are many standard Function Blocks. Some parameters within them are the same across all Function Blocks (Universal), some are the sand some are the same forBlocks Transducer Block

G

Gateway A Gateway provides communications capability between fieldbuses of different protocols e.g. between FOUNDATION and PROFIBUS PA.

PhDaApLay

H1

It is different to a Bridge, in that

Control ValvePressureTransmitter

AI AOPID

ame for similar Function Blocks (Function parameters) similar devices (Profile parameters). See Resource

ysical Layer talink Layer plication er


Application Layer

Application Layer

Physical Layer Physical Layer Profibus PA

Message Filtering

Gateway Device


protocol conversion occurs above the application layer rather than in the datalink layer. There is bound to be some loss of functionality as a result of the conversion between protocols (e.g. PROFIBUS PA does not timestamp alarms at the field device (Dec98)).

GSD file Gerätestammdaten — Device database file, also called ‘device datasheet’ for PROFIBUS devices. The GSD file contains a description of the device and provides a way for an open configuration tool to read the data sheet (GSD File) and therefore take account of the device characteristics. The vendors of the devices describe the functionality of their devices by means of a GSD file. The format of the GSD file is clearly defined by the PROFIBUS standard. The configuration tool reads the GSD files into its internal data base and takes all characteristics of the individual devices into account.

PROFIBUSconfiguration toolSystem configuration

Electronic Device Data Sheets (GSD-files)

PLC

PROFIBUS

GSD GSD GSD GSD GSD GSD

PLC

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. See DTM H

HART Highway Addressable Remote Transducer. A protocol where digital communications is overlain on top of an analogue 4-20mA signal. Typically used to commission transmitters or Positioners in the field, now ith the advent of HART pass through these commands are available at the system allowing central device management and asset optimisation. See DTM


H1 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)

H1 segments are connected to HOST systems via FF HSE linking devices

H2 Refers to High Speed Ethernet (HSE) fieldbus at 100 Mbit/sec. A Gateway or Link Device such as ABB LD800HSE is used to enable inter H1/HSE communications. HSE uses the Collision Detection/Collision Avoidance communications control method. See FDA

High speed fieldbus See H2

HIST Refers to a series of test for Foundation H1 device integration into Host systems

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

Host A computer belonging to a user that hosts communication hardware and software that allows connection to the network. In Foundation fieldbus the term is also sometimes applied to a device containing the Link Active Scheduler. 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 and can be considered to be a Multi-port repeater. It is used to create STAR network topologies for Ethernet installations.

HUB


A Hub allows all connected devices to share the same network resources (bandwidth). Switches are now replacing hubs as a switch will filter messages between networks allowing improved network designs for optimum network performance.

I

IEC International Electrotechnical Commission; the standardisation body responsible for the agreement of an international fieldbus standard. See also Standardisation Organisations.

Industrial IT Enabled Certification to simplify and enhance automation equipment integration into the ABB 800xA control. System. This would include documentation and tested Object types. At a higher level this would also include asset management features to reduce OpEx spending.

Inrush Current Maximum allowed rate of change of quiescent current after a device powers up (non-transmitting). It should not exceed 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. See Maximum Current, Lift Off Voltage.

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

IP Internet Protocol. Includes error checking as integral part. IP Address: 32-bit address assigned to hosts that want to participate in a TCP/IP internet.


ISO/OSI model 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

12

34

5

6

7

USER LAYER

PHYSICAL LAYER

COMMUNICATION“STACK”

MANAGEMENT

MANAGEMENT

International Standards Organisation Open Systems Interconnection (ISO/OSI). This is a model containing seven ‘layers’ each are describing a function(s) which may be used to produce an open communications protocol. Foundation fieldbus uses three of these layers plus an extra layer (User Layer). User Layer Here you can use function blocks to build process applications e.g. PID Controller or add extra functions. Application Layer - This is not the process application, its function is to provide a standard order for data contained in 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 Safety - Entity method Method for limiting the amount of electrical power (600 mW) available to field devices. An IS power supply for a segment is likely to provide a voltage of less than 23V (typically, will be 19V) and provide power for about four devices (assumes each device is loop-powered and takes about 10 mA) in the hazardous area.

NOTE —
The PNO (PROFIBUS FISCO Model (from PTB)) and FOUNDATION (doc. AG-163) have different guidelines for IS. FFhave now adopted the FISCO guidelines too.


Protection can be via a Barrier or Isolator with the pros and cons of each being the same as for a traditional installation. FISCO and FNICO are other methods to design a safe network into a hazardous area.

ISP Interoperable 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 develop their own specifications based on PROFIBUS DP. A competing organisation, called WorldFIP was immediately formed by a different set of vendors basing their protocol on the French FIP standard.However, users then found their systems suppliers on one-side and instruments suppliers on the other and pressure from them led to the dissolution of ISP and IFC and the formation of Fieldbus Foundation.

J

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

K

KERMIT A popular file transfer protocol.

L

LAS Link Active Scheduler for Foundation fieldbus. Typically a function of the H1 card at the HOST system or the more modern Linking Devices (FF HSE). This function distributes Tokens which control access to the FF H1 Network. The LAS therefore contains a list and sequence of communication and other events which are executed at the H1 level for example Execute AI block in PI101 Publish AI from PI101 over H1 Subscribe to AI from PI101 at a postioner PC101 Execute PID in PC101 Execute AO in PC101 Failure of the LAS will result in a backup LAS taking over if configured.


Latency The delay incurred by a switching or a bridging device between receiving and transmitting data.

Layers Refers to the layered ISO/OSI model for specifying open communication protocols.

Layout Tool Software to help design Foundation fieldbus H1/HSE and Profibus DP/PA topologies taking into account power requirements – topology rules and network performance. ABB has layout tools based upon XL for Profibus and Foundation fieldbus.

Lift Off Current See Inrush Current

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

Linking Device Allows foundation H1 devices (Pressure transmitters etc.) to link to foundation H2 (High Speed Ethernet devices). Also to connect Profibus PA devices to Profibus DP without restricting the DP baud rate. See Segment Coupler

M

Manchester Bi-phase L

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 1ENDDELIMITER

BITBOUNDARIES

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

Manchester Bi-phase L is a signaling method for synchronizing receivers and transmitters. It produces a pattern from which the information to synchronize the receiver clock can be extracted. It avoids the need for separate data and clock circuits. Used by Foundation fieldbus H1 and Profibus PA See Physical Layer


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

Master Class I and II Refers to a Profibus network. All profibus masters reside at the Profibus DP level and can be either Class I (e.g. a PLC) or Class II (e.g. a configuration tool). Class I masters deal with Cyclic transfer of data ClassII masters deal with acyclic data. See PA and DP

MAU Media Access Unit. Interface circuitry between the bus and the rest of the communications circuitry in a device. The MAU regulates the current and shapes the trapezoidal waveform when transmitting.

Primary

Sensor

ADC

Microprocessor

FieldbusController

MediaAccess

Unit(MAU)

v+

Vo

+

-Fieldbus WireMedium

Power

Network powered device

When receiving, it filters out the signal and extracts the bits. It also extracts the current from the network and supplies it to the device components.

Maximum Current A limit on the maximum device current applies during the interval 500 �s to 20 ms after connection of a device to the network or after the application of power to the network. Limit is rated quiescent current + 20 mA. See also Inrush Current, Lift Off Voltage.

Multi-variable device See Sensor Fusion


N

Node Is a communications device on the network e.g. a pressure transmitter, valve Workstaton.

O

Object 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 then use the indices to access the data.

Object Type within ABB’s 800xA system Structure within ABB’s 800xA system which collects all aspects of a pressure transmitter for example. Aspects would include data sheets, manuals, certificates, engineering data ….. ABB instrumentation has pre built and tested object types to simplify engineering and reduce testing requirements.

P

PA Profibus field device network. This is a bus based upon the IEC 1158-2 physical layer as foundation H1 but comprises slave devices only. The Profbus master controls all communications on the bus. See DP

Physical Layer This layer defines everything required to make a physical connection to the fieldbus network. Both FOUNDATION and PROFIBUS PA 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 also ISO model


Power Supply Fieldbus power supplies are different to traditional power supplies as they have extra components to stop the fieldbus signal from entering the power supply which would then corrupt it by regulating out the voltage variation. Traditional PSU can be upgraded so that they can be used in fieldbus installations by the use of PSU Conditioners.

PSU Multiplexer

PSU-1

PSU-2

To Fieldbus Cable

To Fieldbus Cable

Segment 2Segment 1

Redundant Configuration

Redundant PSUs are possible and facilitated with devices such as PSU multiplexer from RELCOM. Maximum non-IS output is 32 V DC max. For IS Output Depends on the barrier used (typically about 19V). See Surge Protectors Segment Inrush Current Maximum Current

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

Profiles A profile defines what must be present (e.g. parameters or services) to provide a useful level of interoperability between 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 should support Publisher/Subscriber and time distribution services. See Token Passing Publisher Subscriber Collision 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.

Profibus PA DP See PA and DP

Protocol This is the overall set of rules and formats for data transfer between the same ISO/OSI layers of different devices.

Publisher Subscriber Describes a particular transmission/reception method for sharing data on a network. A device can publish data to the whole network, all devices which wish to subscribe to that data will then read it. Typically used when distributed control functions are used with foundation fieldbus, Publisher Subscriber communications are determined by the LAS and are deterministic. See Token Passing Application Layer.

R

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

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

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

R R R R

1900 1900 1900 1900

Trunk

Segment

H1

R R

R

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 a delay such that only one of these repeaters can be used on one segment. Could be used to connect differing media. 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 distance repeaters allow more devices to be connected. Each additional device on the network will use up some of the bandwidth and as more devices are added performance will deteriorate.

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

S

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

Non-IS application. 1Km segment length (22�/km). 20V PSU. Devices consume 10mA. Current = Voltage/Resistance =20V-9V/44� =250mA (9v min Voltage) =25 devices Note: double the segment length halves the number of devices in this case.

IS application. 1Km segment length (22�/km). 20V PSU. Devices consume 10mA. Current = Voltage/Resistance =20V-9V/(44+105)� =74mA (9v min Voltage) =7 devices

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


Segment Coupler Gateway device to allow connection of Profibus PA devices to a Profibus DP segment. Profibus DP is based upon RS485 (in this case) Profibus PA has power on the bus and is based upon the international standard IEC1158-2. This coupler reduces the DP baud rate to 93.75Kbit/sec recent Profibus Linking devices remove this limitation.

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

Service Services are operations on communications objects of a defined class. A measured value, for example, is such an object that belongs to the class of Variables. Read and Write are the services allowed for this class of objects.

SIL Safety Integrity Level. Defines the failure conditions and modes of single devices or whole control systems. There are SIL2 and SIL3 field devices available which cold be used in a safety system, these devices are certified (TUV for example) based upon either historical data or redundancy built into the devices/system.

Slow Speed fieldbus See H1

SMTP Simple Mail Transfer Protocol SMTP specifies how two mail systems interact and the format of control messages they exchange to transfer mail.

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


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

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

TCE

SRu

SW(InfiIEbfrb

N

ote: 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 =1,900

Segment length + All Spur Lengths < 1,900m

Maximum Spur Length is reduced

by 30m for each device on the spur

tack efers to the communications software suite that imsed in a device.

tandards organisations orldwide standards are organised by the Internatio

SO) and the International Electrotechnical Commisational bodies support their work (see below). ISOelds except electrical and electronic standards, whC. CENELEC is charted with developing voluntary

asis for supporting the creation of a single Europeaontiers. Its work will override that of any Europeanased on that of the IEC.

otal Number of ommunicating lements

Max. Spur Length (M)

25-32 0

19-24 30

15-18 60

13-14 90

1-12 120

plements the ISO/OSI layers

nal Standards Organisation sion (IEC). A large number of

covers standardisation in all ich are the responsibility of the electrotechnical standards, as a n market without internal

national group and is primarily


The main bodies involved in the development of fieldbus standards are the ISA and IEC for Foundation fieldbus and CENELEC for PROFIBUS, WorldFIP and Pnet (EN50170).

Committee for European

Normalisation (CEN/CENELEC)

BSI DIN AFNOR

Military Standard

ISO/IEC NATO Standard

American National Standards Institute (ANSI)

Military Standard

DoD (USA)

ASMEISA SAE

IEEE

• AFNOR —

Association Francaise de Normalisation

• ANSI — American National Standards Institute

Trade Associations(NEMA…EI

• 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 Electrotechnical Commission • IEEE — Institute of Electrical and Electronic Engineers • ISA — Instrument Society of America • NEMA — National Electrical Manufacturers Association • SAE — Society of Automotive Engineers

Star See HUB Chicken Foot

Start Delimiter A bit sequence used to signal the start of the data portion of a frame. See also Frame, End delimiter


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

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

T

TCP Transmission Control Protocol: provides full duplex streaming service on which other protocols depend. See UDP FTP IP FDA

Token Passing Deterministic method for controlling data communication on a network. Tokens — special data messages — are used to enable access to the network. In the simplest case, a device will receive a token which allows it to transmit on the network; all other devices listen. When the transmitting device has finished, it passes the token to the next device and so on around the network. Rules vary, but usually a device can hold the token only for a limited time before 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, Star and Tree. BUS A shared data transmission path connecting all devices, with two defined

ends. Each transmission travels 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 in one direction only; as it circulates through each device, it is read. After circulating all around the ring, 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 on the performance of this one device. TREE A number of junction boxes are connected along the network, with devices

connected in clusters at each junction box, via spurs. Also known as ‘Chicken Foot’ topology.

Transducer Block The Transducer Block contains parameters specific to the technology of the device. Some parameters are standard for similar devices. You would find detail relating to the transducer e.g. 2 or 3 wire RTD input.

Terminators Terminators are required at the end of each fieldbus Segment and match the cable impedance to reduce reflections of the signal.

Tree Topology See Chicken Foot

Trunk A Trunk is the main communication highway acting as a source for other line (spurs) from the control room. See Home Run - backbone

U

UDP User Datagram Protocol: splits data into small parts and transmits the data as random packets. The receiver rebuilds the data. There is no confirmation of reception. See FDA TCP

User Layer Unique layer found in FOUNDATION fieldbus to enhance interoperability and allow vendor innovation, with the facility to use function blocks to build process applications (such as PID Controller. See ISO


V

Virtual Field Device (VFD) Part of the Application Layer functionality.

A Virtual Field Device is a model of the device for communications purposes. It groups variables into objects and then allows access to the data via an individual object dictionary (OD) for each VFD. This allows for simple access to complex functions built up in a VFD.

Object Dictionary

Object 1

Object 2

Object 3

VViirrttuuaall FFiieelldd DDeevviiccee

A physical device can contain one or more VFD’s.

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

fieldbus jargon buster 23/4/04 9:47 am page 1 fieldbus...

Documents