Siemens SIMATIC RS 485 Manual page 24

Connecting
5.4 Configuration options with the RS 485 repeater
Bus segments 1 and 2 looped through
The following figures show how to couple two bus segments using an RS 485 repeater. Each
bus cable is looped through on the RS 485 repeater.
Bus segment 1
Bus segment 2
Figure 5-5
Note
When you turn off the power supply of a complete bus segment, the terminating resistors of
the connected nodes are also without power supply. This can lead to disruptions or
undefined signal states in this bus segment that are not recognized by the RS 485 repeater
and can then lead to problems in the other bus segment.
Whenever possible, we recommend the following procedure:
 Alternative 1:
 Alternative 2:
 Alternative 3:
See also
Design of the RS 485 repeater (Page 11)
24
Connecting two bus segments to the RS 485 repeater
Disconnect the two bus segments before turning off the power supply using switch 6
(refer to chapter Design of the RS 485 Repeater) on the repeater (set to "OFF").
Connect the RS 485 repeater to the power supply of the bus segment to be turned off so
that the repeater is also turned off. In this case, make sure that the RS 485 repeater is
not at the end of the previous bus segment, since the RS 485 repeater then acts as the
terminator which has no effect if there is no power supply. If this solution is required, use
a PROFIBUS terminator with a permanent power supply downstream from the RS 485
repeater.
If you want the RS 485 repeater to retain its power supply, use PROFIBUS terminators to
terminate the bus segment you want to turn off since these also require a permanent
power supply. You require 1 terminator if the bus segment to be turned off ends at the
RS 485 repeater, otherwise you require 2 terminators.
Bus segment 1
R
Bus segment 2
Terminating resistor
Bus segment 1
Do not enable!
Terminating resistor
Bus segment 2
Do not enable!
RS 485 Repeater
Manual, 02/2011, A5E03382072-01