Image Processing Reference
In-Depth Information
Interoperable.
On this level, the semantics of the data play an important role. Variables take on
physical relevance; they do not just have a data type, but also a physical unit that needs to be known
to the partners. Furthermore, accuracy ranges, meaning, and purpose of variables are defined. If a
device produces a larger data set, it is known how the individual data can be distinguished. Interop-
erable nodes can be used immediately without additional description, their network interfaces are
unique, and also the functional behaviors match in that is precisely deined what happens to the data
thataresentacrossthenetworkinthesenseofwhichactionsorsequenceofactionstheytrigger.
Interchangeable.
Even the temporal behavior of two nodes is the same with respect to the needs of
the application. Typically, such requirements are maximum reaction times to events or commands. If
such constraints are not equal, the overall behavior of the entire network can fundamentally change
even if a device replacing another is functionally equivalent.
The problem of interoperability has been disregarded in many cases until recently. In fact, it is
notaproblemoftheieldbusitself,butoftheapplication.Consequently,itmustbetackledbeyond
the ISO/OSI model. he definition of appropriate “profiles” addresses this problem. A profile defines
which variables carry which data, how they are coded, what physical units they have, etc. By virtue of
thisproile,itispossibletosubjectdevices,whichareclaimedtosatisfythisproile,toacorresponding
conformity test. he compatibility of a device is generally also tested when used within a multi-vendor
system.
The creation of profiles originated from the recognition that the definition of the protocol lay-
ers alone is not sufficient to allow for the implementation of interoperable products, because there
are simply too many degrees of freedom. Therefore, profiles limit the top-level functionality and
define specialized subsets for particular application areas []. Likewise, they specify communica-
tion objects, data types, and their encoding. So they can be seen as an additional layer on top of the
ISO/OSImodel,whichiswhytheyhavealsobeencalled“Layer”or“UserLayer.”Onethingto
be kept in mind is that nodes using them literally form islands on a fieldbus, which contradicts the
philosophy of an integrated, decentralized system. Different profiles may coexist on one fieldbus, but
a communication between the device groups is normally very limited or impossible at all.
The concept of profiles has many names. In MMS, they are termed Companion Standards. In
P-NET, they are equivalent to the Channel concept. Function blocks in Foundation Fieldbus essen-
tially pursue the same idea. In LonWorks, there are the Standard Network Variable Types, and in EIB,
interoperability is achieved by means of the EIB Interworking Standards.
From a systematic viewpoint, profiles can be distinguished into communication, device, and
branch profiles. A bus-specific “communication profile” defines the mapping of communication
objects onto the services offered by the fieldbus. A “branch profile” specifies common definitions
within an application area concerning terms, data types, their coding, and physical meaning. “Device
profiles” finally build on communication and branch profiles and describe functionality, interfaces,
and in general the behavior of entire device classes such as electric drives, hydraulic valves, or simple
sensors and actuators.
The work of defining profiles is scattered among different groups. Communication profiles are
usually in the hands of fieldbus user groups. They can provide the in-depth know-how of the
manufacturers, which is indispensable for bus-specific definitions. Device and branch profiles are
increasingly a topic for independent user groups. For them, the fieldbus is just a means to an
end—the efficient communication between devices. What counts more in this respect is the find-
ing and modeling of uniform device structures and parameters for a specific application. his forms
the basis for a mapping to a communication system that is generic within a given application
context.
The ultimate goal is the definition of fieldbus-independent device profiles []. [].This is an attempt
to overcome on a high level the still overwhelming variety of systems. Finally, such profiles are also
