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MOST-Bus
Assignment
An individual program for each passenger which originates from a central system up front, this is the target of the engineers who are working on comfort-orientated-electronics in the coming years. The type of data
makes no difference to the digital system. As long as it concerns entertaining audio- and video data, the data security is of no consequence. This changes however, when the to be transferred becomes relevant for
driving, e.g., when roads, vehicles and/or traffic signs are being scanned.
Function
Since about 1985 we've been hearing about the development of the CAN-Bus. This however, is concerned especially with the the
transferring of control-data, and that, with a high degree of security. Parallel to the CAN-Bus, the, based on fibre-optic, DB2-optical-bus designed for a broad bandwidth and large data amount in the area of audio- and
video systems, was developed. From this Bus-system, the Transport (Multi-) Media Orientated System - MOST - with a further increased transfer rate of 22.5 Mbit/s came into being. In the near future a transfer rate of
150 Mbit/s is targeted.
A typical set-up would be a POF-circuit (Polymere Optical Fiber), made up of radio, tuner, sound, telephone, navigation, voice input and of course, the operation using the already available
display. POF is similar to plexi-glass, only much more flexible than we know it in everyday use. Thus bending radii in the region of a few millimeters are possible.
An important yardstick for the quality of the signal transmission is the supression. This means that if from 30 Watts of output, only 15 Watts come over in a certain section, this can be used to determine the
supression. Thus the absorption can be determined from the transmission- to reception strength ratio, not however, simply by division, but by logarithmic calculation. In this case it would amount to 3 dB. If the
transmission at 3 dB is halved, the reception strength, with even higher supression, is lower.
The subject of supression is not as bad as it sounds, because of course, each control device lying on the data-bus, whether it taps the data or not, converts this into electric signals. After a renewed conversion they
are sent again to the next junction at the full light transmitting power. Nevertheless, the supression by all the involved components, e.g., the glass-fibre cable and the plug-in connectors with the resistance of the power
supply cable, are added together.
However, due to the failure of one control device, the MOST-Bus is hit sensitively. Since the individual control devices are all attached to the power supply, they give, if anything at all, a short sign of life, then nothing
more. In this case, a circuit-interruption diagnosis helps, the initiation of which differs quite strongly, depending on the manufacturer. It is mostly triggered through the gateway, as the only connection to the remaining
vehicle-network. This means, if the circuit-interruption diagnosis cannot be started, the gateway may (also) be faulty.
Gateway as carrier for the remaining vehicle-network
The diagnosis can of course, not be made through the possibly faulty optical fibre, but through a clocked single-core wire, e.g., one giving a 12V signal. The control devices are then prompted, through light signals, to
undertake certain actions, if nothing happens it is noted. This by the way, occurs, first at full-, then at half strength (3dB), to test whether the signals still come through when the suppression is stronger.
If a control device does not report at all, it could also mean that this was never installed. Therefore, the error-list must also be compared with the specified features. If one has localised a fault in this manner, the
problem could still lie with the cable going to and coming from the control device, e.g., the plug-in connectors. If the control device is the problem, and replacing it is not worthwhile, it can be possibly be left out, and
can, at this point, be short-circuited. Apparently, this has already been done by using a simple screw-terminal. We would however, recommend a proper repair-set, which also does not cost the earth.
By the way, one now tends to concentrate more and more on the error memory after diagnosing. It is no longer sufficient to describe the fault as being 'sporadic' or 'permanent'. In the meantime an error priority, e.g.,
between 1 and 8, is also saved, the lower the number, the more urgently one is sent into the workshop. The frequency with which the error occurs can also be indicated by the numbers 1 to 254. 02/12
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