Image Processing Reference
In-Depth Information
to many developers, and still in the mid-s, one could read statements like []: “The idea of the
fieldbus concept seems promising. However, with reasonable effort it is not realizable at present.”
The alternative and somewhat more conservative approach was the development of so-called “field
multiplexers,” devices that collect process signals in the field, serialize them, and transfer them via
one single cable to a remote location where a corresponding device demultiplexes them again []. For
quite some time, the two concepts competed and coexisted [], but ultimately the ield multiplexers
mostly disappeared, except for niches in process automation, where many users still prefer such
“Remote I/O” systems despite the advantages of fieldbus solutions []. [].The central field multiplexer
concept of sampling I/O points and transferring their values in simple data frames also survived in
some fieldbus protocols especially designed for low-level applications.
The desire to cope with the wiring problem getting out of hand in large installations was certainly
the main impetus for the development of fieldbus systems. Other obvious and appealing advantages
of the concept are modularity, the possibility to easily extend installations, and the possibility to have
muchmoreintelligentielddevicesthatcancommunicatenotjustforthesakeofprocessdatatransfer,
but also for maintenance and configuration purposes [,]. A somewhat different viewpoint that led
to different design approaches was to regard bus systems in process control as the spine of distributed
real-time systems []. While the wiring optimization concepts were in many cases rather simple
bottom-up approaches, these distributed real-time ideas resulted in sophisticated and usually well
investigated top-down designs.
An important role in the fieldbus evolution has been played by the so-called automation pyra-
mid. his hierarchical model was defined to structure the information flow required for factory and
process automation. he idea was to create a transparent, multilevel network—the basis for computer-
integrated manufacturing (CIM). The numbers vary, but typically this model comprised up to five
levels [,]. While the networks for the upper levels already existed by the time the pyramid was
defined, the field level was still governed by point-to-point connections. Fieldbus systems were there-
foredevelopedalsowiththeaimofinallybridgingthisgap.heactualintegrationofield-level
networks into the rest of the hierarchy was in fact considered in early standardization [], for most
of the proprietary developments, however, it was never the primary intention.
In the automation pyramid, fieldbusses actually populate two levels: the field level and the
cell/process level. For this reason, they are sometimes further differentiated into two classes:
•
Sensor-actuator busses or device busses have very limited capabilities and serve to con-
nect very simple devices with, e.g., programmable logic controllers (PLCs). hey can be
found exclusively on the field level.
•
Fieldbusses connect control equipment like PLCs and PCs as well as more intelligent
devices. hey are found on the cell level and are closer to computer networks.
Depending on the point of view, there may even be a third sublevel []. This distinction may
seem reasonable but is in fact problematic. here are only few fieldbus systems that can immediately
be allocated to one of the groups, most of them are being used in both levels. herefore, it should be
preferable to abandon this arbitrary differentiation.
How do fieldbus systems compare to computer networks? he classical distinction of the different
network types used in the automation pyramid hinges on the distances the networks span. From top
down, the hierarchy starts with global area networks (GANs), which cover long, preferably interconti-
nental distances and nowadays mostly use satellite links. On the second level are wide area networks
(WANs). They are commonly associated with telephone networks (no matter if analog or digital).
Next come the well-known LANs, with Ethernet as the today most widely used specimen. hey are
the classical networks for office automation and cover only short distances. he highest level of the
model shown in Figure . is beyond the scope of the original definition, but is gaining importance
