Image Processing Reference
In-Depth Information
no new messages are accepted until earlier entries have been consumed and deleted. This ensures
that no messages are lost before they are processed by the node.
Stimulated by the different ways of looking at and handling data exchange in fieldbus systems (and
embedded systems in general), two opposing paradigms have been established in the past and have
ignited a long, partly ierce debate: the “time-triggered” and the “event-triggered” paradigm []. [].The
time-triggered approach is specifically suited for periodic real-time data, and many fieldbus systems
actually use it in one form or another. he event-triggered approach was designed following the idea
that only changes in process variables are relevant for transmission. An additional aspect behind it
was that such events should be broadcast in the network, so that every node potentially interested in
the data can receive them. his makes extension of the network fairly easy by just adding new nodes
if the message identifiers they need for their application are known.
20.5.2 Fieldbus Systems and the OSI Model
Like all modern communication systems, fieldbus protocols are essentially modeled according to the
ISO/OSI model. However, in most cases only the layers , , and are actually used []. [].This is in fact
a tribute to the lessons learnt from the MAP failure, where it was found that a full seven-layer stack
requires far too many resources and does not permit an efficient implementation. For this reason,
the MiniMAP approach and based on it the IEC fieldbus standard explicitly prescribes a three-layer
structure consisting of physical, data link, and application layer. But what about the other layers and
the functions defined therein?
The reduced and simplified protocol stack reflects the actual situation found in many automation
applications pretty well, anyway. Many fieldbusses are single-segment networks with limited size, and
extensions are realized via repeaters or, at most, bridges. Therefore, network and transport layer—
which contain routing functionality and end-to-end control—are simply not necessary. The same
applies to the upper layers. Originally, fieldbus systems were not meant to be very sophisticated. Fully
implemented session and presentation layers are therefore not needed, either. Still, certain functions
from the layers - might be needed in reduced form. Rudimentary networking aspects could be
required or specific coding rules for messages that are better suited for the limited resources available
in typical fieldbus nodes. In such cases, these functions are frequently included in the layer or . For
the IEC fieldbus standard, the rule is that layer and functions can be placed either in layer
or layer , whereas layer and functionalities are always covered in layer (Figure .) [].
It would nevertheless be deceptive to think that all fieldbus systems just consist of a physical, data
link, and application layer. There are several examples where other layers were explicitly defined.
Particularly in the building automation domain, the situation is different. Owing to the possibly
Full OSI stack
Reduced fieldbus stack
IEC 61158 coverage
Application
Application
Application
Presentation
Session
Transport
Network
Data link
Data link
Data link
Physical
Physical
Physical
FIGURE .
Layer structure of a typical fieldbus protocol stack as defined by the IEC . (From Sauter, T., in
The Industrial Communication Technology Handbook
, CRC Press, Boca Raton, FL, , .-.. With permission.)
