Media Orientated Systems
    Transport

Assignment

An individual program for each passenger which originates from a central computer, this is the target of the engineers who are working on the comfort-orientated-electronics in the coming years. The type of data makes no difference to the digital system. In this case, as long as it concerns entertaining audio- and video data, the data security is of no consequence. This changes however, when the information to be transferred becomes relevant for the driving, e.g., when roads, vehicles and/or traffic signs are 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 ring made up of radio, tuner, sound, telephone, navigation, voice input and of course, the operation through the already available display. Thereby, apart from the direct linking, the data could also be 'analogised'. Of course, there would also be some type of gateway as a carrier to the vehicles standard network.

An important yardstick for the quality of the signal transmission, is the absorption. If from 30 Watts of output, only 15 Watts comes over, this can be used to determine the absorption. That means that it 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 3dB. If the transmission power at 3dB is halved, the reception strength, with even higher absorbtion, is lower.

The subject of absorption is not as bad as it sounds, because of course, each control device lying on the 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 absorbtion by all the involved components, e.g., the glass-fibre cable and the plug-in connectors with the resistance of the power supply line, 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 all goes well, a short sign of life, then nothing more. In this case, a ring-interruption diagnosis, the initiation of which differs quite strongly depending on the manufacturer, helps. It is mostly triggered through the gateway as the only connection to the remaining vehicle-network. This means, if the ring-interruption diagnosis cannot be started, the gateway may (also) be defective.

The diagnosis can of course, not be made through the possibly defective optical fibre, but through a tacted single-core wire, e.g., one giving a 12V signal. The control devices are then requested, through light signals, to undertake certain actions, if nothing happens it is noted. By the way, this occurs, first at full-, then at half strength (3dB), to test whether the signals still come through when the absorption is stronger.

If a control device does not report at all, it could also mean that it was never installed. Therefore, the error-list must also be compared with the specified features. If one has localised a defect in this way, the problem could still lie with the cable to and from the control device, e.g., the plug-in connectors. If the control device is the problem, and the replacement is not worthwhile, it can be possibly be left out and can, at this point , be short-circuited. This has been done by using a simple cable-clamp. We would 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 diagnosis by error memory. It is no longer sufficient to describe the error as '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. All that's left now is the reset counter, which determines, after how many further occurences the error is deleted. 01/11