Siemens SIMATIC NET PROFIBUS Owner's Manual page 155

Note
The shield clamps are used solely to contact the shields and are not suitable as
strain-relief clamps. This means that the LAN cables must be secured as close as
possible to the bus terminals to provide mechanical strain relief.
Note
Bus terminals installed at the end of segments require the 5 V power supply from
the DTE interface to supply the activated, integrated terminating resistor.
The sub-D male connector must therefore always be plugged in and secured by
screws. The attached DTE must not be turned off.
Note
The same wires (green or red must always be connected to the same terminal A
or B in all bus terminals and with all bus connections) and be uniform throughout
the segment.
The following scheme is recommended for a PROFIBUS network:
Terminal A: green wire
Terminal B: red wire
Note
Notes on the 12M bus terminal
The 12M bus terminal must only be plugged in to an interface with the power
turned off.
At the ends of a segment, the PROFIBUS cable must only be connected to
terminal pair A1, B1. Terminals A2, B2 are disconnected from the bus when the
terminating resistor is activated.
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Passive Components for RS-485 Networks
4-57