Siemens CPU 313C Hardware And Installation Manual page 75

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Disturbance of data traffic might occur on the bus.
A bus segment must always be terminated at both ends with the terminating
resistor. This, for example, is not the case if the last slave with bus connector is off
power.
Since the bus connector draws its power from the station, the terminating resistor
has no effect.
Please make sure that power is always supplied to stations on which the
terminating resistor is active.
As an alternative, you can also use the PROFIBUS Terminator as active bus
termination.
S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03
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