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1.1.2
Properties of the PROFIBUS PA
The PROFIBUS PA (PA = Process Automation) enables di-directional communication between a bus
master and the field devices over a two-wire cable. At the same time the power is supplied to the two-wire
field devices through the same cables (current I in Figure 1--1).
Supplementary to the EN standard 50170, the PNO (PROFIBUS user organization) has defined the scope
of functions of the individual field device types in a so-called profile description. These profiles define the
minimum functional requirements and optional extensions. The device-internal "Device Management"
supplies all the necessary basic information for finding the profile parameters to the control system
configuration tools. In this way a parametering tool can operate all devices which conform to profile no
matter what their type and manufacturer.
The system must be implemented with one or more PROFIBUS PA lines depending on the automation job
and thus the number of field devices. A PROFIBUS PA line consists of the components illustrated in
Figure 1--2.
Control station
PROFIBUS DP
PLS, PC
Figure 1--2 PROFIBUS PA architecture
The central process control system PLS or, in the case of low requirements, a PC, is responsible for the
control. As a rule the functions signal conversion DP-PA, bus feeding and bus termination are unified in the
"coupler" device. A DP/PA coupler or, in the case of higher requirements, a more powerful DP/PA link is
used depending on the number of PROFIBUS PA field devices to be operated in the automation system.
An additional terminating resistor must be fitted at the far end of the bus for transmission-technical
reasons. The theoretically possible line length (total of all line sections) is a maximum 1900 m (see
chapter 2.1) when using the recommended bus cable. However, the current requirements of the individual
users and voltage drop at the cable must also be taken into account in the planning. The individual field
devices FD can be connected almost anywhere in the bus system. See the "PNO Guide
PROFIBUS PA" /1/ for further information.
The DP/PA coupler or DP/PA Link is supplied by a power supply unit with safety extra-low voltage (SELV).
This power supply must have adequate reserves for bridging brief power failures.
The maximum number of devices that can be connected to a bus line depends on their current
consumption and the respective application conditions. When operating in an area where there is no risk of
explosion, the couplers/links can feed up to 400 mA into the bus.
When operating in explosion risk areas, the intrinsic safety is only guaranteed when the maximum power
fed into the bus does not exceed certain voltage and current values.
These are normally:
Current I
SITRANS F with PROFIBUS PA
C79000-B5676-C114-03
PROFIBUS PA
Coupler
T
FD
Power
supply
FD = Field device
< 128 mA, voltage U
S
Field
T
additional
line termination
FD
< 15 V
0
Instruction Manual
5
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