Siemens SIMATIC S7-300 Hardware And Installation Manual page 106

Wiring
Removing bus connectors
With a looped-through bus cable, you can unplug the bus connector from the PROFIBUS-DP interface
at any time, without interrupting data communication on the network.
Possible data traffic errors
Warning
Data traffic error 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.
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S7-300 Programmable Controller Hardware and Installation
A5E00105492-02