Siemens PROFIBUS PA SIPART PS2 Manual page 22

Introduction
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20
An additional terminating resistor T must be fitted at the far end of the
bus for transmission-technical reasons. When using the recommended
bus cable, the theoretically possible line length (sum of all line sections)
is a maximum of 1900 mm. In addition, the voltage drop over the lines
supplying the field devices must be taken into account in the planning.
However, the current requirements of the individual users and voltage
drop on 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. Further information can be found in the "PROFIBUS
PPNO Guide A".
DP/PA-coupler or DP/PA-Link are supplied by a power supply unit with
SELV (Safety Extra Low Voltage). This power supply must have ade-
quate reserves for bridging brief power failures.
The maximum number of devices that can be connected to a bus chan-
nel depends on their current consumption and the respective conditions
of use. When operating in a safe area, the couplers/links can feed up to
400 mA into the bus.
In zones subject to explosion hazard, intrinsic safety is only ensured if
the maximum power fed into the bus does not exceed certain voltage
and current values. See chapter 7 "Technical Data" device in intrinsi-
cally safe version.
WARNING
To power the intrinsically safe PROFIBUS only certified power supply
units (DP/PA couplers or links) must be used. See the EC--Type Ex-
amination Certificate "for use in Potentially Explosive Atmospheres".
When explosion-proof versions of positioners are used in areas where
the atmosphere is potentially explosive, they may be supplied with
auxiliary electrical power only within the enclosed housing.
The openings for the electronic connections must be executed with
EEx-d-certified cable glands or sealed with EEx-d-certified plugs, or
the "Conduit Tube System" must be used to provide an explosion bar-
rier at a maximum distance of 46 cm (18 inches) from the housing.
The number of devices which can be connected to a bus channel can
be determined from the maximum current consumption of the devices
connected (according to standards ≥ 10 mA per device) and the avail-
able current. A current reserve should be planned for safety reasons,
otherwise there is a risk that a defective device could overload the bus
with its increased current consumption and the power supply and com-
munication with all unaffected users could collapse. The power reserve
quantity depends on the current increase in the event of an error speci-
fied by the manufacturer.
Every device has its own address to distinguish between the connected
process devices.
SIPART PS2 PA Manual
A5E00127926-03