VEGA VEGACOM 557 Profibus FMS - Manual (Page 47)

VEGACOM 557 Profibus FMS 47
Supplement
5.2 Special information on
PROFIBUS-FMS
PROFIBUS-FMS enables the communication
among automation devices as well as the
communication of the automation devices with
the intelligent field devices. Here the possible
functionality is more important than a short
system reaction time. In many applications
the data exchange is mainly made acyclically
on request of the application process.
PROFIBUS layer 7 (Application Layer)
Layer 7 of the ISO/OSI multilayer system
provides the communication services which
are available to the user. These application
services enable efficient, open data traffic
between application processes. The
PROFIBUS application layer is specified in
DIN 19 245 part 2 and consists of:
- Fieldbus Message Specification (FMS) and
- Lower Layer Interface (LLI).
Virtual field device (VFD) with object directory (OV)
Field device 1
VFD
Field device 2
VFD
logical connec-
tion
logical connec-
tion
For
communi-
cation, VFD is the
visible part of
the real field device
Real field device
Set value Formulation
Limit value
Virtual field device (VFD)
Level
Pressure Temperature
Object directory (OV)
Data type directory
Index Object code Meaning
1 Data type Integer8
2 Data type Integer16
……
6
Data type Floating Point
Statistic object directory
Index Object Data Internal Symbol
code type Address
20 VAR 1 4711 H Pressure
21 BAR 6 5000 H Temperature
22 VAR 2 100 H Level
!
FMS describes the communication objects,
the application services and the resulting
models from the communication partner’s
view.
LLI is used for adaption of the application
functions to the various characteristics of
PROFIBUS layer 2.
PROFIBUS communication model
An application process comprises all pro-
grams, resources and tasks not assigned to
a communication layer. The PROFIBUS com-
munication model enables the unification of
distributed application processes to one
complete process through communication
relationships.
The part of an application process in a field
device which can be reached via the commu-
nication is called virtual field device (VFD).