A link comprises an intelligent interface and one or more segment couplers, whereby the couplers may exhibit different types of protection. Normally, a range of transmission rates are supported on the PROFIBUS DP side. The transmission rate for PROFIBUS PA is fixed at 31.25 kbit/s.
Links differ from pure segment couplers by the fact that they are PROFIBUS DP slaves on one side and PROFIBUS PA masters on the other. There is no direct communication between the
PROFIBUS DP master and the PROFIBUS PA slaves, i.e. the link is not transparent. Futher
Segment coupler Type A Type B Type C
Type of protection EEx [ia/ib] IIC EEx [ib] IIB None-Ex
Supply voltage 13.5 V 13.5 V 24 V
Max. power 1.8 W 3.9 W 9.1 W
Max. supply current ≤ 110 mA ≤ 280 mA ≤ 400 mA
No. of devices ca. 10 ca. 20 max. 32
Manufacturer / Model Description Type of protection
Supply current
Voltage DP-baudrate
Pepperl+Fuchs SK1 KFD2-BR-1.PA.2 Non-Ex 380 mA 22.0 V DC 93.75 kBit/s
Pepperl+Fuchs SK1 KFD2-BR-1.PA.93 Non-Ex 400 mA 24.0-26,0 V DC 93.75 kBit/s
Pepperl+Fuchs SK1 KFD2-BR-EX1.PA EEx [ia] IIC 100 mA 12.6-13.4 V DC 93.75 kBit/s
Pepperl+Fuchs SK1 KFD2-BR-EX1.2PA.93 EEx [ia] IIC 100 mA 12.6-13.4 V DC 93.75 kBit/s
Pepperl+Fuchs SK1 KFD2-BR-EX1.3PA.93 EEx [ia] IIC 100 mA 12.6-13.4 V DC 93.75 kBit/s
Pepperl+Fuchs SK2 KLD2-PL(2)-1.PA Non-Ex 400 mA 24,0-26,0 V DC 45.45 kBit/s - 12 MBit/s*
Pepperl+Fuchs SK2 KLD2-PL(2)-EX1.PA EEx [ia] IIC 100 mA 12.8 - 13.4 V DC 45.45 kBit/s - 12 MBit/s*
Siemens DP/PA-coupler 6ES7157-0AC80-0XA0 Nicht-Ex 400 mA 19.0 V DC 45.45 kBit/s**
Siemens DP/PA-coupler 6ES7157-0AD00-0XA0 EEx [ia] IIC 90 mA 12.5 V DC 45.45 kBit/s**
Siemens DP/PA-coupler 6ES7157-0AD80-0XA0 EEx [ib] IIC 110 mA 12.5 V DC 45.45 kBit/s**
Siemens DP/PA-coupler 6ES7157-0AD81-0XA0 EEx [ia] IIC 110 mA 13.5 V DC 45.45 kBit/s**
Siemens DP/PA-coupler 6ES7157-0AD82-0XA0 EEx [ia] IIC 110 mA 13.5 V DC 45.45 kBit/s**
4.3 Topology
The field devices on the PROFIBUS PA segment communicate with a master on the PROFIBUS DP system. The bus is designed according to the rules for PROFIBUS DP up to the segment coupler or link, see Chapter 3.2. Within the PROFIBUS PA segment, practically all topologies are permissible, see Fig. 4-3, below.
Fig. 4-3: Bus topologies (A:Tree, B:Bus, C:Bus + Tree, D:Bus + Tree + extension), PNK: process near component, SiK: Signal coupler, SG: Power supply, T: Terminator,
JB: Junction box, R: Repeater, 1...n: Field devices, Sk: Segment coupler Termination at JB possible
if spurs do not exceed 30 m
Cable type The fieldbus comprises two-core cable. According to IEC 61158-2 (MBP), four different cable types (A, B, C, D) can be used, only two of which (cable types A and B) are shielded.
• Cable types A or B are recommended for new installations. Their cable shielding guarantee adequate protection from electromagnetic interference and thus the most reliable data transfer.
On multi-pair cables (Type B), it is permissible to operate multiple fieldbuses (with the same degree of protection) on one cable. No other circuits are permissible in the same cable.
• Practical experience has shown that cable types C and D should not be avoided where possible since the lack of shielding means that electromagnetic interference characteristics generally do not meet the requirements described in the standard.
Table 4-4 lists the technical data of the four cable types as taken from the informative annex to the standard (i.e. this has not actually been specified.) The electrical data determine important characteristics of the design of the fieldbus, such as distances bridged, number of participants, electromagnetic compatibility, etc.
Table 4-4: Cable types according IEC 61158-2, Annex C
Intrinisic safety Cable for intrinsically safe applications as per the FISCO model must also satisfy the following additional requirements:
Table 4-5: Safety limits for the bus cable according FISCO
Cable type A Cable type B Cable type C Cable type D
Cable construction twisted pair,
shielded
Attenuation constant at 39 kHz 3 dB/km 5 dB/km 8 dB/km 8 dB/km
Capacitive unsymmetry 2 nF/km 2 nF/km — —
Envelope delay distortion (7.9...39 kHz) 1.7 µs/km — — —
Degree of coverage of shielding 90 % — — —
Max. recommended bus length (including spurs)
1900 m 1200 m 400 m 200 m
EEx ia/ib IIC EEx ib IIB
Loop resistance (DC) 15...150 Ω/km 15...150 Ω/km
Specific inductance 0.4...1 mH/km 0.4...1 mH/km
Specific capacitance 80...200 nF/km 80...200 nF/km
Max. spur length ≤ 30 m ≤ 30 m
Max. bus length ≤ 1000 m ≤ 1900 m
Cable manufacturers Table 4-6 list examples for Type A PROFIBUS-PA cable available from various manufacturers.
Table 4-6: PROFIBUS PA cable
Maximum overall cable length
The maximum network length depends on the type of ignition protection and the cable
specifications. The overall cable length is made up of the length of the main cable and the length of all spurs that are longer than 1 m.
Table 4-7: Maximum permissible overall cable length depending upon the cable type used
!
Note!• In FISCO systems with type of protection EEx ia, the maximum line length 1000 m
• If repeaters are used, the maximum permissible cable length is [length in table x (N+1)], where N is the number of repeaters.
• A maximum of four repeaters are permitted between station and master.
Maximum spur length The line between distribution box and field device is described as a spur. In the case of non ex-rated applications the max. length of a spur depends on the number of spurslonger than 1 m:
Table 4-8: Maximum length of a spur depending on the number of spurs
!
Note!• In FISCO systems with type of protection EEx ia, the max. length per spur is 30 m.
Number of field devices As far as the specification is concerned, a maximum of:
• 32 stations per segment in safe areas
• 10 stations in an explosive hazardous arrea (EEx ia IIC)
are possible. The actual number of stations is dependent on several factors, however, and must be determined during project planning.
Bus termination The start and end of each fieldbus segment must be terminated with a bus terminator. The terminator may be a separate component or be integrated into a bus component.
!
Note!• If the segment is in an explosion hazardous area, the terminators must be certified to the FISCO standard.
• In the case of a branched bus segment, the device furthest from the segment coupler represents the end of the bus.
• If the fieldbus is extended with a repeater then the extension must also be terminated at both ends.
Manufacturer Order No. Application Specific resistance
Turck Cable 483-*M Standard ≤ 44 Ω/km
Turck Cable 483B-*M EEx ia/ib IIC ≤ 44 Ω/km
Siemens 6XV1830-5FH10 Standard ≤ 44 Ω/km
Siemens 6XV1830-5EH10 EEx ia/ib IIC ≤ 44 Ω/km
Lapp 2170235 Standard ≤ 44 Ω/km
Lapp 2170234 EEx ia/ib IIC ≤ 44 Ω/km
Cable type A Cable type B Cable type C Cable type D
1900 m 1200 m 400 m 200 m
Number of spurs 1...12 13...14 15...18 19...24 25...32
Max. length per spur 120 m 90 m 60 m 30 m 1 m
4.4 Bus access method
PROFIBUS PA uses the central master/slave method to regulate bus access. The process near component, e.g. a PLC, is a Class 1 master that is installed in the PROFIBUS DP system. The field devices are configured from a PROFIBUS PA Class 2 master, e.g. FieldCare. The field devices on the PROFIBUS PA segment are the slaves.
4.4.1 Segment coupler
Segment couplers are transparent as far as the PROFIBUS DP master is concerned, so that they are not mapped in the PLC. They simply convert the signals and power the PROFIBUS PA segment. The do not need to be configured nor are they assigned an address.
The field devices in the PROFIBUS PA segment are each assigned a PROFIBUS DP address and behave as PROFIBUS DP slaves. A slave may be assigned to only one Class 1 master. A master communicates directly with its slaves:
• A Class 1 master, e.g. the PLC, uses the cyclic polling services to fetch the data provided by the field devices.
• A Class 2 master, e.g. FieldCare, transmits and receives field device data by using the acyclic services.
Fig. 0.1 Data exchange via segment coupler
Segment coupler SK1 from Pepperl+Fuchs
If the SK1 segment coupler is used, the transfer rate for the PROFIBUS DP is fixed at 93.75 kBd. In this case, if type A cable is used for the PROFIBUS DP the PROFIBUS DP segment can be up to a length of 1200 m. The length of the PROFIBUS PA segment depends on:
• whether the PROFIBUS PA segment in question is intrinsically safe or not.
• how many PROFIBUS PA stations are connected to the segment.
• how high is the current consumption of the individual PA slaves.
• how the PA slaves are distributed on the PROFIBUS PA segment.
The SK1 segment coupler works transparently. This means that PROFIBUS DP masters have direct access to every PROFIBUS PA slave. Addresses that have been assigned on a PROFIBUS PA segment are also occupied on the PROFIBUS DP.
PLC
Class 1 master Class 2 master
e.g. FieldCare
Segment
PROFIBUS DP
PROFIBUS PA-slaves PROFIBUS PA
cyclic data exchange
acyclic data exchange coupler