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For reasons related to transmission technology, the bus is also equipped on the far end with
a terminating resistance T. When using the recommended bus cable, the theoretical
maximum cable length is 1900 meters. The theoretical maximum cable length is the sum of
all cable segments. During planning, also take into account the voltage drop over the wires
powering the field devices.
However, the power requirements of the individual nodes and the voltage drop on the cable
must also be calculated during projection. The individual field devices (FD) can be connected
at nearly any point in the bus system.
DP/PA couplers or DP/PA links are supplied using a safety extra-low voltage (SELV) power
supply. This power supply must have sufficient reserves to bridge over temporary power
failures.
The maximum number of devices which can be connected to one bus strand depends on
their power consumption and the conditions of use. When operated in the safe zone, the
couplers or links supply the bus with up to 400 mA.
When operated in explosive atmospheres, intrinsic safety is only guaranteed if all devices,
components, etc. connected to the bus (e.g. bus terminator) fulfill the following requirements:
● They are certified as intrinsically safe equipment.
● They fulfill the requirements of the FISCO model (Fieldbus Intrinsic Safety Concept).
Power supply devices in particular (bus couplers) must be certified as so-called FISCO
power supplies. Observe the safety-relevant maximum values and other specifications of the
EG type test certificate.
Connect power supplies (bus couplers) which are not explosion protected and certified to
intermediate EX-certified zener barriers. Observe the specifications of the EG type test
certificate.
For power supply to intrinsically safe PROFIBUS, use only power supplies, DP/PA
couplers, or DP/PA links certified as compliant with the FISCO model.
Switch through zener barriers if using non-EX-protected power supplies. See the
requirements of the EG type test certificate.
3.6.7
Number of connectable devices
The number of devices which can be connected to a bus strand can be calculated from the
sum of the maximum power consumption of the devices and the power available. By default,
assume 10 mA per device. For safety reasons, plan for a power reserve. Otherwise you run
the risk of a defective device overloading the bus with an increased power consumption. This
can interrupt the power supply and communication with the functioning nodes. The amount
of power reserved is based on the nominal power increase given by the manufacturer in
case of failure.
SIPART PS2 with PROFIBUS communication
Operating Instructions, 09/2007, A5E00127926-07
WARNING
Description
3.6 PROFIBUS PA
35
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