Siemens SIMATIC NET PROFIBUS manual Functional Description, Interfaces

Functional Description 3

The OBT is a repeater with 3 channels.

3.1Interfaces

The OBT has the following interfaces for attachment to PROFIBUS DP segments:

SChannel 1 (CH1) is an electrical RS-485 interface. This is implemented as a

9-pin D SUB female connector. A single PROFIBUS DP node can be connected via this channel or a PC, PG or OP can be connected to the OBT. The maximum permitted segment length is 100 m. The copper segment should, however, be kept as short as possible since disturbances can be coupled into the optical PROFIBUS DP from this segment.

SChannel 2 (CH2) and channel 3 (CH3) are optical interfaces. They are designed as duplex sockets. The end of a two-fiber plastic or PCF fiber-optic cable with two simplex connectors is connected to each of these duplex sockets.

The OBT also has a block with three terminals for connecting the 24 V power supply and, if necessary, a grounding conductor.

3.2Optoelectric Signal Conversion and Signal Regeneration

The OBT converts the RS-485 level signal received at channel 1 into an optical signal level that is then output via channel 2 and channel 3.

Signals received in channel 2 or 3 are converted to electrical signals and

Soutput on channel 1 as an electrical signal

Schanged back to an optical signal and then output again on the other optical channel.

The receive channels have no echo, in other words received signals are not sent back on the same channel.

The OBT regenerates the signals in amplitude and time. This allows up to 126 modules to be cascaded in an optical bus. The cascading depth is limited solely by the monitoring times of the attached devices.

The propagation delay per OBT is 6 bit times.

PROFIBUS Optical Bus Terminal (OBT)	3-1
C79000-G8976-C122-02