Siemens SIMATIC NET PROFIBUS Owner's Manual page 151

Bus terminal 12 M
The 12M bus terminal is used to connect data terminal equipment (DTEs) with an
RS-485 interface to the LAN cable.
It includes the following:
S 1 modular terminal block with 6 terminals for conductors with a cross-sectional
area <= 1.5 mm
necessary, the protective earth (PE)
S Screw down clamps for field contact
S Two switches:
Right switch ('Termination'), to terminate the end of an incoming, electrical
segment (A1, B1) with the characteristic impedance (switch on). At the same
time, the outgoing, electrical segment (A2, B2) is interrupted.
Left switch, used to set the range of the transmission rate 9.6 Kbps to 1.5 Mbps
and 3 Mbps to 12 Mbps.
S A 1.5 m long tap line with a 9-pin sub-D male connector for direct attachment to
a DTE.
The sub-D connector is plugged into the sub-D female connector of the DTE and
secured by screws. The 12M bus terminal requires a current of 90 mA at a
power supply of 5 V from the DTE between pins 5 (M5) and 6 (P5) of the sub-D
male connector.
A maximum of 32 BT12M modules can be connected to one bus segment. If
other components, such as repeaters are connected to a bus segment, this
reduces the maximum number of 12M bus terminals.
9.6k to 1.5M
3M to 12M
PE A1 B1 A2 B2 PE
Figure 4-30
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
2
for connection of the incoming and outgoing LAN cable and, if
off
R
on
on
Termination
off
Operator Controls
Passive Components for RS-485 Networks
4-53