Siemens SIMATIC NET PROFIBUS - Manual (Page 459)

6-1
PROFIBUS Optical Bus Terminal (OBT)
C79000-G8976-C122-02
Troubleshooting
Table 6-1
LED Display Possible Cause of Problem
L+ 24V LED not lit - Power outage
- OBT defective
L+ 24V LED flashing - The transmission rate could not be set
CH1 LED not lit - Break on one or more wires of the RS-485 LAN cable
- Wires A and B of the RS-485 LAN able connected to wrong terminals
- Attached PROFIBUS node is defective or not sending
- PROFIBUS node not attached or attached node is not turned on
CH1 LED lit
But PROFIBUS nodes signa-
ling bus problems
- Wires A and B of the RS-485 LAN able connected to wrong terminals
- Short-circuit on the RS-485 LAN cable
- Break on one of the wires of the RS-485 LAN cable and wires A and B con-
nected to wrong terminals
- No or wrong termination
CH2, CH3 LED not lit - Send and receive FO cords swapped over
- Break on FO cord receiving receiving from partner module
- No partner module connected or partner module is not turned on
- Attached partner module is defective (not sending)
CH2, CH3 LED lit
But PROFIBUS nodes signa-
ling bus problems
- FO connector is loose
- FO cable distance to next module too long
- Receive FO cord interrupted and disturbance caused by extraneous light
If none of the LEDs indicates a problem and you still encounter communication
problems (for example no confirmation, unexpected frames), check the monitoring
times set on the PROFIBUS nodes (such as the Slot Time).
For more detailed information on these parameter settings, refer to the description
of your PROFIBUS DTEs and the configuration software.
6