Proline Promag 55 PROFIBUS DP/PA Wiring
Endress+Hauser 37
Spurs
Note the following points:
• Length of spurs < 6.6 m (21.7 ft) (at max. 1.5 MBit/s)
• No spurs should be used for transmission rates > 1.5 MBit/s. The line between the connector and
the bus driver is described as a spur. Experience has shown that you should proceed with caution
when configuring spurs. For this reason, you cannot assume that the sum of all spurs at
1.5 MBit/s will be 6.6 m (21.7 ft). This is affected greatly by the arrangement of the field devices.
Therefore, we recommend that if possible, you do not use any spurs at transmission rates
>1.5MBit/s.
• If you cannot avoid using spurs, then they may not include any bus terminators.
Bus termination
It is important to terminate the RS485 line correctly at the start and end of the bus segment since
impedance mismatch results in reflections on the line which can cause faulty communication
transmission ( ä 68).
Further information
General information and further notes regarding the wiring are contained in BA034S/04:
"Guidelines for planning and commissioning, PROFIBUS DP/PA, field communication."
4.1.2 PROFIBUS PA cable specifications
Cable type
Twin-core cables are recommended for connecting the device to the fieldbus. Following
IEC 61158-2 (MBP), four different cable types (A, B, C, D) can be used with the fieldbus, only two
of which (cable types A and B) are shielded.
• Cable types A or B are particularly preferable for new installations. Only these types have cable
shielding that guarantees adequate protection from electromagnetic interference and thus the
most reliable data transfer. In the case of type B multi-pair cables, 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 used due to the lack of
shielding, since the freedom from interference generally does not meet the requirements
described in the standard.
The electrical data of the fieldbus cable have not been specified but determine important
characteristics of the design of the fieldbus, such as distances bridged, number of users,
electromagnetic compatibility etc.
Type A Type B
Cable structure Twisted pair,
shielded
One or more twisted pairs, fully
shielded
Wire cross-section 0.8 mm
(AWG 18) 0.32 mm
(AWG 22)
Loop-resistance (DC) 44 /km 112 /km
Characteristic impedance at 31.25 kHz 100 ± 20% 100 ± 30%
Attenuation constant at 39 kHz 3 dB/km 5 dB/km
Capacitive asymmetry 2 nF/km 2 nF/km
Envelope delay distortion (7.9 to 39 kHz) 1.7 μs/km *
Shield coverage 90% *
Max. cable length (incl. spurs >1 m (> 3 ft)) 1900 m (6200 ft) 1200 m (4000 ft)
* Not specified
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