Fieldbus is the name of a family of industrial computer network protocols used for real-time
distributed control, standardized as IEC 61158. A complex automated industrial system — such
as manufacturing assembly line — usually needs a distributed control system—an organized
hierarchy of controller systems—to function. In this hierarchy, there is usually a Human
Machine Interface at the top, where an operator can monitor or operate the system. This is
typically linked to a middle layer of programmable logic controllers via a non-time-critical
communications system. At the bottom of the control chain is the fieldbus that links the
PLCs to the components that actually do the work, such as sensors, actuators, electric
motors, console lights, switches, valves and contactors.
Description Fieldbus is an industrial network system for
real-time distributed control. It is a way to connect instruments in a manufacturing
plant. Fieldbus works on a network structure which typically allows daisy-chain, star,
ring, branch, and tree network topologies. Previously, computers were connected using
RS-232 by which only two devices could communicate. This would be the equivalent of the currently
used 4-20 mA communication scheme which requires that each device has its own communication
point at the controller level, while the fieldbus is the equivalent of the current LAN-type
connections, which require only one communication point at the controller level and allow multiple
of analog and digital points to be connected at the same time. This reduces both the length
of the cable required and the number of cables required. Furthermore, since devices that
communicate through fieldbus require a microprocessor, multiple points are typically provided by
the same device. Some fieldbus devices now support control schemes such as PID control
on the device side instead of forcing the controller to do the processing.
History Bitbus
The oldest commonly used field bus technology is Bitbus. Bitbus was created by Intel Corporation
to enhance use of Multibus systems in industrial systems by separating slow i/o functions from
faster memory access. In 1983, Intel created the 8044 Bitbus microcontroller by adding
field bus firmware to its existing 8051 microcontroller. Bitbus uses EIA-485 at the physical layer,
with two twisted pairs - one for data and the other for clocking and signals. Use of
SDLC at the data link layer permits 250 nodes on one segment with a total distance of 13.2 km.
Bitbus has one master node and multiple slaves, with slaves only responding to requests from
the master. Bitbus does not define routing at the network layer. The 8044 permits only
a relatively small data packet, but embeds an efficient set of RAC tasks and the ability
to develop custom RAC tasks. In 1990, the IEEE adopted Bitbus as the Microcontroller
System Serial Control Bus. Today BITBUS is maintained by the BEUG - BITBUS
European Users Group. Standardization
Although fieldbus technology has been around since 1988, with the completion of the ISA
S50.02 standard, the development of the international standard took many years. In 1999, the IEC
SC65C/WG6 standards committee met to resolve difference in the draft IEC fieldbus standard.
The result of this meeting was the initial form of the IEC 61158 standard with eight
different protocol sets called "Types" as follows:
Type 1 Foundation Fieldbus H1 Type 2 ControlNet
Type 3 PROFIBUS Type 4 P-Net
Type 5 FOUNDATION fieldbus HSE Type 6 SwiftNet
Type 7 WorldFIP Type 8 Interbus
This form of standard was first developed for the European Common Market, concentrates
less on commonality, and achieves its primary purpose—elimination of restraint of trade
between nations. Issues of commonality are now left to the international consortia that
support each of the fieldbus standard types. Almost as soon as it was approved, the IEC
standards development work ceased and the committee was dissolved. A new IEC committee
SC65C/MT-9 was formed to resolve the conflicts in form and substance within the more than
4000 pages of IEC 61158. The work on the above protocol types is substantially complete.
New protocols, such as for safety fieldbuses or real-time ethernet- fieldbuses are being
accepted into the definition of the international fieldbus standard during a typical 5-year
maintenance cycle. Both Foundation Fieldbus and Profibus technologies
are now commonly implemented within the process control field, both for new developments and
major refits. In 2006, China saw the largest FF systems installations at NanHai and SECCO,
each with around 15000 fieldbus devices connected. IEC 61158 specification
There were many competing technologies for fieldbus and the original hope for one single
unified communications mechanism has not been realized. This should not be unexpected since
fieldbus technology needs to be implemented differently in different applications; automotive
fieldbus is functionally different from process plant control. The final edition of IEC standard
IEC 61158 allows 8 technologies.This are the some hierarchic layer of the automation protocols.
IEC 61158 consists of the following parts, under the general title Digital data communications
for measurement and control – Fieldbus for use in industrial control systems:
Part 1: Overview and guidance for the IEC 61158 series
Part 2: Physical Layer specification and service definition
Part 3: Data Link Service definition Part 4: Data Link Protocol specification
Part 5: Application Layer Service definition Part 6: Application Layer Protocol specification
Standards There are a wide variety of competing fieldbus
standards. Some of the most widely used ones include:
AS-Interface CAN
EtherCAT FOUNDATION fieldbus
Interbus LonWorks
Modbus Profibus
BITBUS CompoNet
SafetyBUS p RAPIEnet
See List of automation protocols for more examples.
Cost advantage The amount of cabling required is much lower
in Fieldbus than in 4-20 mA installations. This is because many devices share the same
set of cables in a multi-dropped fashion rather than requiring a dedicated set of cables per
device as in the case of 4-20 mA devices. Moreover, several parameters can be communicated
per device in a Fieldbus network whereas only one parameter can be transmitted on a 4-20 mA
connection. Fieldbus also provides a good foundation for the creation of a predictive
and proactive maintenance strategy. The diagnostics available from fieldbus devices can be used
to address issues with devices before they become critical problems.
Networking With the exception of ARCNET, which was conceived
as early as 1975 for office connectivity and later found uses in industry, the majority
of fieldbus standards were developed in the 1980s and became fully established in the
marketplace during the mid-1990s. In the United States, Allen-Bradley developed standards
that eventually grew into DeviceNet and ControlNet; in Europe, Siemens and other manufacturers
developed a protocol which evolved into PROFIBUS. During the 1980s, to solve communication problems
between different control systems in cars, the German company Robert Bosch GmbH first
developed the Controller Area Network. The concept of CAN was that every device can be
connected by a single set of wires, and every device that is connected can freely exchange
data with any other device. CAN soon migrated into the factory automation marketplace.
Despite each technology sharing the generic name of fieldbus the various fieldbus are
not readily interchangeable. The differences between them are so profound that they cannot
be easily connected to each other. To understand the differences among fieldbus standards,
it is necessary to understand how fieldbus networks are designed. With reference to the
OSI model, fieldbus standards are determined by the physical media of the cabling, and
layers one, two and seven of the reference model.
For each technology the physical medium and the physical layer standards fully describe,
in detail, the implementation of bit timing, synchronization, encoding/decoding, band rate,
bus length and the physical connection of the transceiver to the communication wires.
The data link layer standard is responsible for fully specifying how messages are assembled
ready for transmission by the physical layer, error handling, message-filtering and bus
arbitration and how these standards are to be implemented in hardware. The application
layer standard, in general defines how the data communication layers are interfaced to
the application that wishes to communicate. It describes message specifications, network
management implementations and response to the request from the application of services.
Layers three to six are not described in fieldbus standards.
Technical committees, with representatives of many different companies, have been responsible
for turning the original specifications into international ISO standards. Bury, among others,
reports that work is underway to implement a common fieldbus protocol. This will entail
a common set of application-layer services that can be provided regardless of the lower-layer
implementation details. Although very much in its infancy, it is expected that this protocol
may become reality by 2010. Whether designed for low-level sensor communications or high-level
machine connectivity, a fieldbus is an important enabling technology for an open architecture
controller. Features
Different fieldbuses offer different sets of features and performance. It is difficult
to make a general comparison of fieldbus performance because of fundamental differences in data
transfer methodology. In the comparison table below it is simply noted if the fieldbus in
question typically supports data update cycles of 1 millisecond or faster.
Process Fieldbus vs. Device Networks It should be noted that requirements of fieldbus
networks for process automation applications are different from the requirements of fieldbus
networks found in discrete manufacturing applications such as automotive manufacturing, where large
numbers of discrete sensors are used including motion sensors, position sensors, and so on.
Discrete fieldbus networks are often referred to as "device networks".
Ethernet and Fieldbus Recently a number of Ethernet-based industrial
communication systems have been established, most of them with extensions for real-time
communication. These have the potential to replace the traditional fieldbuses in the
long term. Here is a partial list of the new Ethernet-based
industrial communication systems: AFDX
EtherCAT EtherNet/IP
Ethernet Powerlink FOUNDATION HSE
BACnet PROFINET IO
PROFINET IRT SafetyNET p
SERCOS III TTEthernet
VARAN RAPIEnet
For details, see the article on Industrial Ethernet
Safety Fieldbus can be used for systems which must
meet safety-relevant standards like IEC 61508 or EN 954-1. Depending on the actual protocol,
fieldbus can provide measures like counters, CRC's, echo, timeout, unique sender and receiver
ID's or cross check. Ethernet/IP and SERCOS III both use the CIP Safety protocol, Ethernet
Powerlink uses openSAFETY, while FOUNDATION Fieldbus and Profibus can address SIL 2 and
SIL 3 process safety applications. In January 2006, the Fieldbus Foundation announced
that TÜV Rheinland Industrie Service GmbH, Automation, Software and Information Technology,
a global, independent and accredited testing agency, had granted Protocol Type Approval
for its Safety Specifications. The Foundation Technical Specifications - Safety Instrumented
Functions are in compliance with International Electrotechnical Commission 61508 standard
requirements up to, and including, Safety Integrity Level 3.
Market In process control systems, the market is
dominated by FOUNDATION fieldbus and PROFIBUS PA. Both technologies use the same physical
layer but are not interchangeable. As a general guide, applications which are controlled and
monitored by PLCs tend towards PROFIBUS, and applications which are controlled and monitored
by a DCS tend towards FOUNDATION Fieldbus. PROFIBUS technology is made available through
Profibus International with headquarters in Karlsruhe, Germany. FOUNDATION Fieldbus technology
is owned and distributed by the Fieldbus Foundation of Austin, Texas.
See also Parallel Redundancy Protocol
Media Redundancy Protocol Notes
References Chatha, Andrew.. Fieldbus: The Foundation
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External links USA: Fieldbus Foundation
Foundation Fieldbus End User Councils: Middle East: Foundation Fieldbus End User
Council - Middle East Australia: Foundation Fieldbus End User Council
Australia Inc Independent organizations:
Iran: Iranian Fieldbus Foundation Manufacturing Systems Integration Research
Institute at Loughborough, UK The P-NET Fieldbus