Siemens SIMATIC NET PROFIBUS User Manual

3-2

PROFIBUS Networks SIMATIC NET

6GK1970-5CA20-0AA1 Release 2 05/2000

PROFIBUS networks were specially designed for use in an industrial environment
and one of their main features is their degree of immunity to electromagnetic
interference resulting in high data integrity. To achieve this degree of immunity,
certain guidelines must be adhered to when configuring electrical networks.

The following parameters must be taken into account when planning an electrical
network:

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The transmission rate required for the task 
(within a network, only one uniform transmission rate can be used)

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The required number of nodes

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The type of network components required (bus terminals, bus connectors,
connecting cables)

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The LAN cables to be used

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The required segment lengths

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The electromagnetic and mechanical environment of the cabling (for example
surge voltage protection, cable route)

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The number of RS-485 repeaters between any two DTEs is limited to a
maximum of 9

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Increasing the overall span of a network by using repeaters can lead to longer
transmission times that may need to be taken into account when configuring the
network (see Section 3.3).

Regardless of the transmission rate, the ends of all segments must be terminated
by activating the terminating resistor in the connector. After the terminating resistor
has been activated, no further cable sections are permitted.

The terminating resistor is only effective when it is supplied with voltage. This
means that the corresponding DTE or the RS-485 repeater must be supplied with
power.  As an alternative, the PROFIBUS terminator can be used as a permanent
terminating resistor.