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FIGURE .
ANSI/ISA-.- hierarchical model. (From Pitzek, S. and Elmenreich, W., Configuration and
management of fieldbus systems. In R. Zurawski, ed.,
he Industrial Communication Technology Handbook
.CRCPress,
Boca Raton, . With permission.)
•
Separation of application logic from physical dependencies allows hardware-independent
design that enables application development before the hardware is available, as well as
easing migration and possibly allowing the reuse (of parts) of applications.
For configuring a physical fieldbus system from such an application model, we must examine () how
thisapplicationmodelmapstothephysicalnodesinthenetworkand()howinformationlowis
maintained in the network.
In order to map the application model to actual devices, fieldbuses often provide a model for spec-
ifying physical devices as well. For example, in the PROFIBUS DP, the physical mapping between
function blocks and the physical device is implemented as follows (see Figure .). A physical device
can be subdivided in several modules that take the role as virtual devices. Each device can have one
(in case of simple functionality) up to many slots, which provide the mapping from the physical
devices to the function blocks. A function block is mapped to a slot, whereas slots may have associ-
ated physical and transducer blocks. Physical and transducer blocks represent physical properties of
a fieldbus device. Parameters of a function block are indexed, and the slot number and the parameter
index cooperatively define the mapping to actual data in the device memory.
In contrast, the FF follows an object-oriented design philosophy. Thus, all information items
related to configuring a device and the application (control strategy) are represented with objects.
This includes function blocks, parameters, as well as subelements of parameters. These objects are
collected in an object dictionary (OD), whereas each object is assigned an index. his OD defines the
actual mapping to the physical memory on the respective device. In order to understand the methods
for controlling the communication flow between the application components, we first examine some
recurring important communication properties in fieldbus applications:
