Endress+Hauser PROline prowirl 72 PROFIBUS PA Documentation Technique page 27

PROline Prowirl 72/73 PROFIBUS PA
!
Endress+Hauser
Explanation of the FISCO model (PROFIBUS PA)
The German Federal Physical-Technical Institute (PTB) has developed the FISCO model
which was published in Report PTB-W-53 "Examination on Intrinsic Safety for Field Bus
Systems".
The FISCO model makes possible the interconnection of intrinsically safe apparatus
and one intrinsically safe associated apparatus, without having to have separate certi-
fication for respective connections.
The criteria for the intrinsic safety of an interconnection (bus segment) is given under
the following interrelationships:
1. To transmit power and data, the bus system uses the physical configuration defined
by IEC 61158-2 (MBP). This is the case for PROFIBUS PA and the H1 bus.
2. Only one active source is permitted on a bus segment (here the power repeater). All
other components work as passive current sinks.
3. The basic current consumption of a field device is at least 10 mA.
of the bus device ≥ U
4. U , I and P
i i i
supply).
5. Each instrument must fulfill the following requirement: C
6. The permissible line length for EEx ia IIC applications is 1000 m.
7. The permissible spur length for Ex applications is 30 m per spur.
8. The transmission line that is used must conform to the following cable parameters:
Resistor coating: 15 Ω /km < R' < 150 Ω /km
Inductance coating: 0.4 mH/km < L' < 1 mH/km
Capacitance coating: 80 nF/km < C' < 200 nF/km (including the shield)
9. The bus segment must be terminated on both ends of the line with a terminal bus
resistor. A terminal resistor is integrated into the power repeater so that an external
bus terminator is only required on the other end. According to the FISCO model the
fieldbus terminator must conform to the following limits:
– 90 Ω < R < 100 Ω
– 0 µF < C < 2.2 µF
Note!
Cable parameters and length restrictions are respected.
Cable specifications for PROFIBUS PA
Cable construction
Core cross-section (nominal)
Loop resistance (direct current)
Impedance at 31.25 kHz
Attenuation constant at 39 kHz
Capacitive unsymmetry
Envelope delay distortion (7.9...39 kHz)
Degree of voltage of shielding
Max. bus segment length
(incl. spur lines)
Specific inductance
Specific capacitance
Loop resistance
Max. spur length
, I and P of the associated equipment (bus power
o o o
Cable type A (reference
twisted pair, screened
2
0.8 mm / AWG 18
44 Ω /km
100 Ω ±
20%
3 dB/km
2 nF/km
1.7 µ s/km
90%
1000 m
0,4...1,0 mH/km
80...200 nF/km
15...150 Ω /km
≤ 30 m
<
≤ 5 nF, L ≤ 10 µH
i i
Cable type B
one or more twisted pairs,
common screening
2
0.32 mm / AWG 22
112 Ω /km
100 Ω ±
30%
5 dB/km
2 nF/km
–
–
1000 m
11