Siemens SIMATIC NET PROFIBUS User Manual

4-3

PROFIBUS Optical Bus Terminal (OBT)
C79000-G8976-C122-02

The OBT allows the attachment of a single PROFIBUS DP node. To attach
RS-485 segments with more than one node, a further network component is
available, the OLM 12M. The requirement for using the OLM 12M is that  the
PROFIBUS DP network is operated at a transmission rate of 187.5 Kbps, 500
Kbps, 1.5 Mbps or 12 Mbps.

The optical channels of the OLM 12M can be connected as follows with the OBT or
with other SIMATIC devices on the optical PROFIBUS DP: 
The connection is established with a two-fiber plastic FO cable (maximum length
50 m) or PCF FO cable (maximum length 300 m). The fiber-optic cable has two
simplex connectors at each end. The optical channel of the OLM 12M connected
to the OBT must be operated in the “Line“ mode with the monitoring deactivated.

1) PROFIBUS cable
    (terminated at both ends)
2) PROFIBUS cable
3)  PROFIBUS connector (not terminated)
4)  Plastic FO or PCF FO with two fibers

ET 200M without
FO interface

DP node without 
FO interface

SIMATIC S7-400
with IM 467

ET 200M without
FO interface

À

Terminating resistor activted

2)

2)

2)

3)

4)

4)

1)

1)

OBT

OBT

OBT

À

À

À

À

À

À

3)

Figure 4-2

Example of Attaching RS-485 Segments