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FIELDBUS
Jargon Buster
Fieldbus Jargon Buster
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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.
Fieldbus Message
Specification
Object Dictionary – Virtual
Field Devices
Application
Layer
Fieldbus Access
Sublayer
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
Fieldbus Jargon Buster
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.
Chicken foot
Home Run or Backbone
↓
Repeater
The diagram opposite
Segment 2
Spurs
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
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.
H1
Fieldbus
Signal
9-32
Volts
Fieldbus Jargon Buster
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
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.
Datalink Layer
Physical Layer
Datalink Layer
Physical Layer
H1
H1
Physical Layer
Datalink Layer
Application
Layer
Bridge
Device
Message
Filtering
Physical Layer
Datalink Layer
Application
Layer
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.
Fieldbus Jargon Buster
C
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.
Segment
Chicken foot
Junction Box
1900m – Sum of Chicken foot spurs
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.
Traditional
PSU
Conditioner
(Electronic
equivalent of
Inductor/Resistor)
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.
Coupler
Coupler
Coupler
The Coupler connects
Trunk
Trunk
devices to the
network. It can be
Spur
simply terminals in a
small box, plugs and
sockets or specially
designed DIN rail
Device
mounted connection
units. It has at least
three connections: one for the spur and one for each side of the trunk.
Fieldbus Jargon Buster
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 nondeterministic 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
Fieldbus Jargon Buster
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
Fieldbus Jargon Buster
(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
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
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.
Fieldbus Jargon Buster
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 selfAI
AO
PID
contained control
functions. There are
many standard
Function Blocks.
Pressure
Control Valve
Some parameters
Transmitter
within them are the
same across all
Function Blocks
(Universal), some are the same for similar Function Blocks (Function parameters)
and some are the same for similar devices (Profile parameters). See Resource
Blocks Transducer Block
G
Gateway
A Gateway
provides
communications
capability
between
fieldbuses of
different
protocols e.g.
between
FOUNDATION
and PROFIBUS
PA.
It is different to a
Bridge, in that
Gateway
Device
H1
Application
Layer
Physical Layer
Physical Layer
Datalink Layer
Application
Layer
Application
Layer
Physical Layer
Message
Filtering
Profibus PA
Physical Layer
Datalink Layer
Application
Layer
Fieldbus Jargon Buster
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
io n
u rat
g
i
f
of the
PROFIBUS
co n
t em
configuration tool
devices
S ys
describe the
functionality
PLC
of their
devices by
GSD
GSD
GSD
GSD
GSD
GSD
means of a
GSD file.
Electronic Device Data Sheets (GSD-files)
PROFIBUS
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.
The advantage is that the configuration is very simple and can be done by a vendorindependent 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
Fieldbus Jargon Buster
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
Fieldbus Jargon Buster
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 (nontransmitting). 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.
Fieldbus Jargon Buster
ISO/OSI model
Fieldbus Model
ISO-OSI Model
M
APPLICATION LAYER
A
N PRESENTATION LAYER
A
SESSION LAYER
G
E
TRANSPORT LAYER
M
NETWORK LAYER
E
DATA LINK LAYER
N
T
PHYSICAL LAYER
7
6
5
4
3
2
1
M
A
N
A
G
E
M
E
N
T
USER LAYER
USER LAYER
FIELDBUS MESSAGE
SPECIFICATION
FIELDBUS ACCESS
SUBLAYER
COMMUNICATION
“STACK”
DATA LINK LAYER
PHYSICAL LAYER
PHYSICAL LAYER
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. FF
have now adopted the FISCO guidelines too.
Fieldbus Jargon Buster
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.
Fieldbus Jargon Buster
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
Manchester Bi-phase L is a signaling method for synchronizing receivers and
transmitters.
BIT
BOUNDARIES
It produces
a pattern
1
1
0
1
1
0
0
0
from which
PREAMBLE
the
N+
1
N+
0
1
NNinformation
0
START
to
DELIMITER
synchronize
1
1
N+
N+
1
0
NNEND
the receiver
DELIMITER
clock can be
extracted. It
N+ Non data symbol - positive.
NNon data symbol - negative.
avoids the
need for
separate data and clock circuits. Used by Foundation fieldbus H1 and Profibus PA
See Physical Layer
Fieldbus Jargon Buster
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
v+
Media
Access
Unit
(MAU)
Power
Fieldbus
Controller
Micro
processor
A
D
C
Primary
Sensor
Media
Access Unit.
Interface
circuitry
between the
+
bus and the
rest of the
communicati
Fieldbus
ons circuitry
Wire
in a device.
Medium
The MAU
regulates
the current
and shapes
the
trapezoidal
waveform when transmitting.
Vo
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
Fieldbus Jargon Buster
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
Fieldbus Jargon Buster
Power Supply
Fieldbus power
supplies are
different to
Redundant Configuration
traditional power
supplies as they
PSU-1
PSU
have extra
Multiplexer
components to
PSU-2
stop the fieldbus
signal from
entering the
Segment 1
Segment 2
power supply
which would
then corrupt it
by regulating out
To Fieldbus
To Fieldbus
the voltage
Cable
Cable
variation.
Traditional PSU
can be
upgraded so
that they can be used in fieldbus installations by the use of PSU Conditioners.
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
Fieldbus Jargon Buster
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).
Segment
H1
Trunk
R
R
1900
1900
1900
1900
R
R
R
R
Terminators and Power Supplies not shown
(Each end of each segment must be terminated)
R
Repeaters for Ethernet have the same function as for fieldbus however there are two
types of repeater available.
Fieldbus Jargon Buster
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)
IS application. 1Km segment
length (22 /km). 20V PSU.
Devices consume 10mA.
Current = Voltage/Resistance
=20V-9V/(44+105)
=74mA (9v min Voltage)
=25 devices
Note: double the segment length
halves the number of devices in this
case.
=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
Fieldbus Jargon Buster
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.
Fieldbus Jargon Buster
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
Note:
¾ 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
¾ Maximum Spur Length is reduced
by 30m for each device on the spur
Total Number of
Communicating
Elements
Max.
Spur
Length
(M)
25-32
0
19-24
30
15-18
60
13-14
90
1-12
120
Stack
Refers to the communications software suite that implements the ISO/OSI layers
used in a device.
Standards organisations
Worldwide standards are organised by the International Standards Organisation
(ISO) and the International Electrotechnical Commission (IEC). A large number of
national bodies support their work (see below). ISO covers standardisation 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 of a single European market without internal
frontiers. Its work will override that of any European national group and is primarily
based on that of the IEC.
Fieldbus Jargon Buster
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
AFNO
R
ISO/IEC
AFNOR —
Association
Francaise de
Normalisation
American National Standards
ANSI —
Institute
American
(ANSI)
National
Standards
Institute
IEEE
Trade
ASME
ASME —
Associations
ISA
American
SAE
(NEMA…EI
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
Military
Standard
NATO
Standard
Military
Standard
DoD
(USA)
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
Fieldbus Jargon Buster
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
communication.
Pass Token
Probe Node Token
initiates broadcast (Publisher/Subscriber)
initiates peer to peer messaging.
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
RING
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.
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.
Fieldbus Jargon Buster
STAR
TREE
One central device performs all signal switching and routing functions. The
network is reliant on the performance of this one device.
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
Fieldbus Jargon Buster
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.
A physical device can
contain one or more VFD’s.
Virtual Field Device
Object Dictionary
Object
1
Object
2
Object
3
Objects could be: process variables, range
limits, C++ applications etc.