On-board troubleshooting
17
If the other half of the differential signal is missing, the cause can be a short circuit in
the bus, or a broken conductor in the bus cables, or faulty termination. (For
troubleshooting the problem with termination, please see the next section, Missing or
inappropriate termination on page 17). Locate the short circuit or broken conductor
and solve the problem.
If the entire signal is missing, the cause can be a short circuit in the bus or in bus
connectors. Locate the short circuit and solve the problem.
One possible cause for a low signal level can be bus overload. The bus is designed for
connecting Multifunction Vehicle Bus devices, not any other kinds of devices. If
inappropriate devices are connected to the bus, it becomes overloaded and cannot
function.
To find out whether the problem is caused by bus overload, check all connections to
make sure that there are no inappropriate devices connected to the Multifunction
Vehicle Bus. If you find any inappropriate devices, disconnect them.
If the problem was not caused by a low signal level or a missing signal, continue with
the troubleshooting tips presented in the following sections.
Missing or inappropriate termination
A transmission error, like cross-talk, can be caused by missing or incorrect termination
in one end of the bus.
To find out whether the transmission error is caused by any of the above, do the
following:
1 Set the vertical amplification of the oscilloscope to 2v/div and the time scale to 1µs/
div or 500ns/div.
2 If you need to measure line A, connect oscilloscope channel 1 to pin 1 and channel
2 to pin 2 of a bus connector. If you need to measure line B, connect oscilloscope
channel 1 to pin 4 and channel 2 to pin 5.
3 Connect the ground of the oscilloscope channels to PE (module front panel).
4 Set the oscilloscope for a differential measurement (channel 1 - channel 2) and
determine the cause of the problem by the resulting waveform.
The following pictures present waveforms indicating missing, defective or inappropriate
termination. If your transmission error results from any of the above, you can measure
both the terminator resistors and the terminator module with a multimeter. (See Table
A.2: Pin order, connector M1 and Table A.3: Pin order, connector M2.)
Remove and replace the defective component.
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