Siemens SIMATIC S7-400 Hardware And Installation Manual page 132

Networking
Removing the Bus Connector
With a looped-through bus cable you can remove the bus connector from the
PROFIBUS-DP interface at any time, without interrupting data traffic on the bus.
Warning
!
Interference to the data traffic on the bus is possible.
A bus segment must always be terminated with the terminating resistor at both
ends. This is not the case, for example, when the last slave with bus connector is
not under power. Since the bus connector is powered by the station, the
terminating resistor has no effect.
Ensure that the stations at which the terminating resistor is switched on are
always under power.
5-22
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S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01