FD67 Profibus-DP Manual
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3.1.2 General information for successful planning in a Profibus network
Up to 32 units (including repeaters) are allowed in a bus segment.
According to the selected baud rate, the extension of a Profibus segments must not exceed the max.
recommended values ; see Table 3-1.
Repeaters can be used to divide a network into several segments. So it is possible to operate more than
32 units in an extended Profibus network. However signal delays resulting from the installation of
repeaters must be considered during configuration of the system. Up to 9 repeaters can be installed
between two units.
3.1.3 Topology
♦
Fig. 3-1 : Example of a Profibus network
3.1.4 Termination of bus segments
Both sides of a segment must be fitted with a terminal resistor. The terminal resistors must be supplied to
guarantee a correct physical signal level.
The terminal resistor is either located in the connector or directly integrated into the device. In both cases
it must be observed that in case of failure or application-related voltage cut-off of the unit at the end of a
segment, the terminal resistor may also be switched off to the detriment of the operatability of the whole
segment.
So it might be preferable to install be bus master at the beginning or the end of the bus segment. In case
of breakdown of the bus master, the switching off of the terminal resistor is of no importance. Additional
repeaters or externally supplied terminal resistors may be installed for reliable termination of the other
end of the segment.
The need of additional measures must be determined for every specific case.
Fig. 3-1 shows an example of correct termination in a typical Profibus network.
MASTER
Slave
Slave
Slave
Slave
Repeater
Repeater
Slave
Slave
Slave
Slave
Terminal resistor