Section 2 Transmission Technology RS485 cable type and speed
3BDS009029R5001 B 27
Project-specific requirements and limitations such as underground wiring, UV
resistance, being free of halogen, etc. should be considered when selecting the
cable. Cables may be used with deviate mechanically or electrically from the type A
cable, but the surge impedance must be retained.
From the line parameters specified above result the following bus segment lengths
:
If other transmission rate shall be used which is not listed in the table above, an
approximation can determine the permissible length through linear interpolation of
two adjacent transmission rates from the table.
Wire diameter > 0.64 mm
Wire cross-section > 0.34 mm
2
Note that cable for laying underground is not suitable for connecting to
PROFIBUS connectors because of the typically 10 mm diameter of the coating.
They can therefore only be connected to screw terminals of PROFIBUS devices.
Depending on the ambient EMC conditions, EN 50170 also allows for laying of
unshielded cable. However, the following reasons argue in favour of always using
shielded cable:
The only interference-free space is inside the switch cubicle, at best. As soon as a
relay is located within the switch cubicle, this is no longer the case.
Unshielded cable requires additional protective measures at the bus inputs to
protect against voltage overload.
Table 3. Dependencies of bus length and transmission rate
Transmission rate in kbit/s 9.6 19.2 93.75 187.5 500 1500 12000
Range in m/segment 1200 1200 1200 1000 400 200 100
In a PROFIBUS DP installation, it must be chosen a data transfer rate which is
supported by all devices connected to the bus. The chosen data transfer rate then
determines the maximum segment lengths as shown above.
Table 2. Basic data for a PROFIBUS DP reference cable
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