Endress+Hauser PROline promag 53 Mode D'emploi page 34

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Stubs (PROFIBUS-DP)
Note the following points:
• Total combined length of all stubs < 6.6 m (at a max. of 1.5 MBit/s)
• At transmission rates > 1.5 MBit/s, stubs should not be used.
The line between the cable connector and the bus driver in the field device is called
a stub. Our experience with the systems, indicates that you should be quite careful
with the length of the stubs when planning your project. Therefore, we recommend
that you do not attempt to utilise the full theoretical maximum total combined length of
6.6 m for all stubs at 1.5 MBit/s. The order of the respective field devices makes more
of a difference in this case. We recommend that at transmission rates > 1.5 MBit/s you
avoid using stubs.
• If you must use stubs, do not install terminating resistors on them.
Shielding and grounding (PROFIBUS-DP/-PA)
When planning the shielding and grounding for a field bus system, there are three
important points to consider:
• Electromagnetic compatibility (EMC)
• Explosion protection
• Safety of the personnel
To ensure the optimum electromagnetic compatibility of systems, it is important that the
system components and above all the cables, which connect the components, are
shielded and that no portion of the system is unshielded.
Ideally, the cable shields will be connected to the field devices' housings, which are
usually metal. Since these housings are generally connected to the protective ground
conductor, the shield of the bus cable will thus be grounded many times.
This approach, which provides the best electromagnetic compatibility, can be used
without restriction in plants with good potential equalisation.
In the case of plants without potential equalisation, a mains frequency (50 Hz) equalis-
ing current can flow between two grounding points, which in unfavorable cases, e.g.
when it exceeds the permissible shield current, may destroy the cable.
To suppress the low frequency equalising currents on systems lacking potential equal-
isation, it is therefore advisable to connect the cable shield directly to the building (or
protective ground conductor) at only one end and to use capacitive coupling to connect
it to all other grounding points.
PROline Promag 53 PROFIBUS-DP/-PA
Endress+Hauser