Image Processing Reference
In-Depth Information
But, what exactly is a fieldbus? Even after a quarter of a century of fieldbus development, there
exists no clear-cut definition for the term. he “definition” given in the International Electrotechnical
Commission (IEC) fieldbus standard is more a programmatic declaration or a least common
multiplecompromisethanaconciseformulation[]:“Aieldbusisadigital,serial,multidrop,data
bus for communication with industrial control and instrumentation devices such as—but not limited
to—transducers, actuators, and local controllers.” It comprises some important characteristics, but is
far from being complete. On the other hand, it is a bit too restrictive.
A more elaborate explanation is given by the Fieldbus Foundation, the user organization support-
ing one of the major fieldbus systems []: “A Fieldbus is a digital, two-way, multidrop communication
link among intelligent measurement and control devices. It serves as a local area network (LAN) for
advanced process control, remote input/output (I/O), and high speed factory automation applica-
tions.” Again, this is a bit restrictive, for it limits the application to process and factory automation,
the primary areas where the Foundation Fieldbus is being used.
The lack of a clear definition is mostly due to the complex evolution history of fieldbusses. In most
cases, bus systems emerged primarily to break up the conventional star-type point-to-point wiring
schemes connecting simple digital and analog I/O devices to central controllers, thereby laying the
grounds for the implementation of really distributed systems with more intelligent devices. As it
wasdeclaredintheoriginalmissionstatementoftheIECwork,“theFieldBuswillbeaserialdigital
communication standard which can replace present signaling techniques such as - mA . . . so that
more information can flow in both directions between intelligent field devices and the higher-level
control systems over shared communication medium. . .” [,]. Still in more recent publications this
aspect is seen as the only raison d'être for fieldbus systems [], which is, however, short-sighted and
does not do the fieldbus justice.
Today, fieldbus systems comprising communication networks and devices would likely be called
embedded networks or networked embedded systems. In the s, when their era began, these
terms were still unknown. Yet, their main features and development stimuli are comparable to what
is behind the modern concepts, which are by the way similarly weakly defined:
•
Focused solutions
. Like embedded systems and their networks, fieldbus systems were no
general-purpose developments, even if they were said to be. They were always devel-
oped with a concrete application field in mind and designed to meet the respective
boundary conditions (like temporal behavior, efficiency, reliability, but also cost) in the
best possible way.
•
Smart devices
. An essential objective for embedded systems and fieldbusses alike is to
bring more intelligence to the field, i.e., to the end devices. Like in embedded systems,
fieldbus developers also used the technological building blocks available at the time
where possible, such as standard microcontrollers to keep costs low. However, if special
needs were to be met, also dedicated solutions were devised.
•
Limited resources
. Both embedded applications and fieldbus systems share the fundamen-
tal problem that resources are limited. No matter what the state of art in microelectronics
is, embedded devices (and ield devices) are less powerful than standard computers. Com-
munication (sub)systems usually have less available bandwidth than computer networks,
and power consumption is an issue.
•
Comprehensive concepts
. Fieldbus systems are not just networks. Communication is only
part of a distributed automation concept with comprehensive application software and
tool chains. In some advanced cases, fieldbusses were embedded into special frameworks
exhibiting many characteristics of distributed operating systems. his is a typical feature
also in modern networked embedded systems.
