Siemens SIMATIC NET PROFIBUS Owner's Manual page 145

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PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Warning
Possible disturbance of data traffic on the bus
A bus segment must always be terminated at both ends with the
terminating resistor. This is, for example, not the case when the power
supply to the last node with a bus connector is turned off. Since the
bus connector is supplied with power from the station, the terminating
resistor has no effect.
Make sure that the power supply for the stations on which a
terminating resistor is activated is always turned on.
As an alternative, you can also use the PROFIBUS terminator as an
active terminating resistor (see Section 5.7).
Passive Components for RS-485 Networks
4-47