Siemens SIMATIC NET PROFIBUS Owner's Manual page 152

Passive Components for RS-485 Networks
Termination
The termination must be activated on the first and last node on the bus segment. If
termination is activated (termination on), the connection between the incoming (A1,
B1) and outgoing (A2, B20 segment is interrupted. The advantage of this is that if a
bus terminating resistor is activated incorrectly, the stations after the bus terminal
can no longer be accessed. When a segment is started up, you can then make
sure that no bus terminating resistors are activated that are not located at the
beginning or end of the network.
Note
Restriction when using the 12M bus terminal at 500 Kbps
This restriction only affects segments longer than 80 m.
If the 12M bus terminal is operated at a transmission rate of 500 Kbps along with
the RS-485 bus terminal with a 3.0 m tap line (6GK1 500-0AB00), a minimum
clearance of 5 m (= 5 m PROFIBUS cable) must be maintained between the
RS-485 bus terminal with the 3.0 m tap line. The 12M bus terminals can be
arranged at any point in the segment; in other words, no minimum clearance
needs to be maintained. The 12M bus terminal can also be included between two
RS-485 bus terminals with a 3.0 m tap line. The only important point in this respect
is that the PROFIBUS cable between the two RS-485 bus terminals with 3.0 m tap
lines must be a total of 5 m long.
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PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000