Image Processing Reference
In-Depth Information
Depending on the capabilities of the field devices, the actual implementation of the MIB may be
distributed. As the MIB is structure-oriented, field devices can easily control their branch locally, and
the fieldbus server needs to hold only a mirror of the data. Alternatively, the fieldbus server can act as
a proxy and contain the full database. On the upper level, the data base repository summarizes all the
ieldbus MIBs in the system. However, the pure ieldbus-related data are not sufficient to fully describe
a plant, hence the repository also holds global data such as particular application information or
overall system configuration, which can be accessed by appropriate tools.
Access to the fieldbus servers and the MIB data is possible through several interfaces to allow for
easy integration in existing environments:
Two native programming interfaces permit direct access to the MIB objects. hese inter-
faces were defined in the predecessor project RACKS, which had the goal of specifying
fieldbus-independent ways of accessing fieldbus data.
SNMP suggests itself as a solution. Even though the NOAH MIB is not compliant with
standard SNMP MIBs, a mapping is possible. Unlike SNMP, however, NOAH supports
functions to manipulate the MIB at runtime. The proposed way to circumvent this
probleminSNMPistomaptheMIBmanipulationfunctionstodedicatedMIBvari-
ables, so that access to these variables triggers changes in the MIB. This idea was not
tested in practice, and in view of the severe problems with SNMP MIBs concerning
inflexibility and consistency [], it is doubtful whether such an approach could work
reliably.
Object linking and embedding (OLE) was also a natural choice. By the time the decisions
on the interfaces were taken, OPC had already begun to make its way. The adoption of
two different views on the MIB objects clearly shows this influence.
An HTTP-based interface permits easy access with Web browsers. The NOAH-specific
information is coded in special tags inside the HTML files, and the read/write access to
the actual objects is controlled via the URLs, which are specified in the communication
with the HTTP server.
The common difficulty that a gateway can translate only data and basic services, but not more
involved fieldbus-specific functions is solved in theusualway.Specialfunctionscanbeactivatedby
accessing dedicated MIB objects. In this case, the agent in the fieldbus server (which is fieldbus-
specific in any case) has to execute the respective mechanism on the fieldbus.
After the project end in , the results were submitted to CENELEC and IEC for standardization.
The expectations after the successful termination of the research project were high. Nevertheless,
the documents describing the research results first had to be transformed into a format useful for
standardization which took time, and the work became entangled in tactical debates. Thus far, the
device description has been the most successful outcome of NOAH: it was published as EN 
and IEC -, respectively. Work on the rest, especially the device profiles in part  of the IEC
standard is going rather slowly.
20.6 Networking Networks—Interconnection in
Heterogeneous Environments
Fieldbus systems were originally meant as part of a larger, comprehensive networking concept for
automation. As the protocols were tailored to the needs of process control with its stringent real-time
requirements and limited communication and computing resources, concepts from the higher layers
of the networking hierarchy, notably the office area could not be used. Network interconnections
were thus not straightforward. his situation is not restricted to fieldbus systems; it is common for all
 
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