Siemens SIMATIC NET PROFIBUS, Owner'S Manual, Page: 443 / 490

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PROFIBUS Optical Bus Terminal (OBT)
C79000-G8976-C122-02
Functional Description
The OBT is a repeater with 3 channels.
3.1 Interfaces
The OBT has the following interfaces for attachment to PROFIBUS DP segments:
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Channel 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.
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Channel 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.
3.2 Optoelectric 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
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output on channel 1 as an electrical signal
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changed 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.
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