Siemens SIMATIC NET PROFIBUS Owner's Manual page 174

Active Components for RS-485 Networks
Segments 1 and 2 Connected Through
Figure 5-5 shows the connection between two segments via an RS-485 repeater
with each LAN cable connected through:
Segment 1
Segment 2
Figure 5-5 Connecting Two Bus Segments to the RS-485 Repeater
Note
If you turn off the power supply of a complete segment, the terminating resistors of
the connected nodes are also without power supply. This can lead to disruptions or
undefined signal states in this segment that are not recognized by the repeater
and can then lead to problems in the other segment.
Whenever possible, we recommend the following procedure:
S Alternative 1:
Disconnect the two segments before turning off the power supply using switch
5 (Table "Description and Functions of the RS-485 Repeater") on the repeater
(set to "OFF").
S Alternative 2:
Connect the repeater to the power supply of the segment to be turned off so
that the repeater is also turned off. In this case, make sure that the repeater is
not at the end of the previous segment, since the repeater is then the
terminating resistor which has no effect if there is no power supply. If this
solution is required, use a PROFIBUS terminator with a permanent power
supply after the repeater.
S Alternative 3:
If you want the repeater to retain its power supply, use PROFIBUS terminators
to terminate the segment you want to turn off since these also require a
permanent power supply. You require one terminator if the bus segment to be
turned off ends at the repeater, otherwise you require two terminators.
5-8
Segment 1
R
Segment 2
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Terminating resistor Terminating resistor
Bus segment 1
off! off!
Terminating resistor Terminating resistor
Bus segment 2 Bus segment 2
off! off!