VEGA VEGACOM 557 Profibus FMS - Manual (Page 48)

48 VEGACOM 557 Profibus FMS
Supplement
The image of the functions of VFD to the real
device is provided in the PROFIBUS commu-
nication model by the Application Layer Inter-
face (ALI). Image 4 shows the correlation
between the real field device and the VFD. In
this example, only the variables pressure,
level and temperature are part of the VFD
and can be read and written via the two com-
munication relationships.
All communication objects of a PROFIBUS
participant are entered in its own local object
directory. In simple devices, the object direc-
tory (OV) can be pre-defined. In complex
instruments, the object directory is projected
and loaded into the device locally or remotely.
The OV contains description, structure and
data type as well as the assignment between
the instrument-internal address of the com-
munication objects and the name on the bus
(Index/Name).
The OV consists of:
- Header (contains information on the struc-
ture of the OV)
- List of the statistic data types (list of the
supported statistic data types)
- Statistic object directory (contains all sta-
tistical communication objects)
- Dynamic list of the variable list (list of the
currently known variable lists)
- Dynamic program list (list of the currently
known programs)
The individual sections of the OV must be
only available if the instrument also supports
the respective functions.
In PROFIBUS the logical addressing is deter-
mined as preferred method for the address-
ing of the communication objects. There, the
access to the communication objects is done
via a short address, the so-called index. The
index is a number of the type Unsigned 16.
By this, efficient telegrams are enabled and
the protocol overhead is reduced. An index
is determined in the OV for each communica-
tion object. The logical addressing must be
supported by all PROFIBUS participants.
In addition, PROFIBUS-FMS enables the
following optional addressing procedure for
special applications:
- The
addressing with names : Here, the
symbolic name of the communication ob-
jects is transferred via the bus.
- The physical addressing : Here, an indi-
vidual physical storage address with the
services Physical-Read and Physical-Write
can be accessed in the field instruments.
PROFIBUS-FMS services
From the point of view of the application proc-
ess, the communication system provides a
service, the so-called FMS-services. The
handling of these services is described by
service primitives. They represent an interac-
tion between requester and responder.
Figure 5 gives an overview of the available
PROFIBUS services.
The large number of available PROFIBUS
application services is used to fulfil the mani-
fold communication requirements of widely
differing field instruments. Only a few appli-
cation services must be implemented. The
selection of further services depends on the
respective application. This selection is deter-
mined in so-called profiles specifically for the
respective application.