Siemens SIPART PS2 Operating Instructions Manual page 36

Description
3.5 PROFIBUS PA
In general, the following functions are combined into one coupling assembly:
● DP/PA signal transfer
● Bus power
● Bus termination
Depending on the number of the PROFIBUS PA field devices to be operated in the automation
system and the time behavior required, a DP/PA coupler or a DP/PA link is used. For standard
requirements, you use a DP/PA coupler; for higher requirements, a more powerful DP/PA link.
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.
36
WARNING
SIPART PS2 with PROFIBUS PA
Operating Instructions, 05/2019, A5E00127926-AC