Image Processing Reference
In-Depth Information
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Distribution
. A network is the prerequisite of distributed systems, many data-processing
tasks can be removed from a central controller and placed directly in the field devices
if they are sufficiently smart and the interface can handle reasonably complex ways of
communication.
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Flexibility and modularity
. A fieldbus installation like any other network can be extended
much more easily than a centralized system, provided the limitations of addressing space,
cable length, etc. are not exceeded. For the special case of fieldbusses, simplification of
the parameterization and configuration of complex field devices is an additional benefit
making system setup and commissioning easier.
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Maintainability
. Monitoring of devices, applying updates, and other maintenance tasks
are easier, if at all possible, via a network.
All these aspects show that apart from the names, there is not so much difference in the general
ideas of fieldbusses and embedded networks. Still there are peculiarities in the history as well as in
the technical characteristics of fieldbus systems that are worth to be looked at and that might be
interesting also for contemporary networked embedded systems. The purpose of this chapter thus
is to give an overview of the nature of fieldbus systems. It briefly reviews the evolution from the
historical roots up to the tedious standardization efforts. It discusses in detail typical characteristics
that distinguish fieldbusses from other types of networks. Current activities aiming at using Ethernet
in automation are reviewed, and evolution prospects are given. It appears that fieldbus systems have
reached their climax, and that future years will bring a mix of Ethernet-/Internet-based solutions and
still-to-be-developed, new field-level networks.
20.2 What Is a Fieldbus?
As said in the introduction, fieldbus systems have to be seen as an integrative part of a comprehensive
automation concept and not as standalone solutions. he name is therefore programmatic and evoca-
tive. Interestingly enough, not even the etymology of the term itself is fully clear. he English word
“fieldbus” is definitely not the original one. It appeared around when the fieldbus standardiza-
tion project within IEC TC was launched [] and seems to be a straightforward literal translation of
the German term “Feldbus,” which can be traced back until about []. Indeed, the overwhelm-
ing majority of early publications in the area is available only in German. he word itself was coined
in process industry and primarily refers to the process field, designating the area in a plant where
lots of distributed field devices, mostly sensors and actuators are in direct contact with the process
to be controlled. Slightly after the German expression and sharing its etymological root, the French
word “réseau de terrain” (or “réseau d'instrumentation,” instrumentation network) emerged. This
term was not specifically targeted at the process industry, but refers also to large areas with scattered
devices. he connection of such devices to the central control room was traditionally made via point-
to-point links, which resulted in a significant and expensive cabling need. he logical idea, powered
by the advances of microelectronics in the late s, was to replace this starlike cabling in the field
by a party-line, bus-like installation connecting all devices via a shared medium—the fieldbus [,].
Given the large dimensions of process automation plants, the benefits of a bus are particularly evi-
dent. However, the concept was not undisputed when it was introduced. he fieldbus approach was
an ambitious concept: a step toward decentralization, including the preprocessing of data in the field
devices, which both increases the quality of process control and reduces the computing burden for the
centralized controllers []. Along with it came the possibility to configure and parameterize the field
devices remotely via the bus. his advanced concept, on the other hand, demanded increased com-
munication between the devices that goes far beyond a simple data exchange. his seemed infeasible
