Siemens SITRANS T3K PA Instruction Manual page 111

Instruction Manual 7NG3213
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Control takes place with the central process control system PLS or with a PC
when requirements are low.
As a rule the functions signal conversion DP-PA, bus feeeding and bus
termination are combined in a coupling module. Depending on the number of
PROFIBUS-PA field devices to be operated in the automation system and the
required timing, a DP/PA coupler or a more powerful DP/PA link in the case of
higher requirements is used.
For transmission-technical reasons, a terminating resistor T must be fitted to the
remote end of the bus. When using the recommended bus cable, the theoretically
possible line length (sum of all line sections) is a maximum 1900 mm. Additionally
the voltage drop over the lines supplying the field devices must also be taken into
account in the planning.
The current requirements of the individual users and voltage drop on the cable
must also be taken into account in the planning however. The individual field
devices FD can be connected at almost any point in the bus system. See the
"PNO Guide PROFIBUS-PA" for further information (see appendix 3, page 117,
"Literature and Catalogs" /1/).
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 adequate reserve to be
able to bridge temporary mains interruptions.
The maximum number of devices that can be connected to a bus channel de-
pends on their current consumption and the respective application conditions.
When operating in the safe area, the couplers/links can feed up to 400 mA into the
bus.
When operating in hazardous areas the intrinsic safety is only guaranteed if the
maximum power supply fed into the bus does not exceed certain voltage and
current values.
These are usually:
EEx ia IIC Current I < 110 mA, Voltage U < 15 V
S 0
See appendix 3, page 117 "Literature and Catalogs" /1/ for further information.
SITRANS T3K PA
111
C79000-B7174-C55-04