CANBusInterface

CAN MOST Flexray
and other bus systems
Electronic control units
in the car
everal years ago the benefit of the digital computer world allready
Spro-voked a revolution in the communication technique. Today every
third household in Germany has access to the internet, every second one
owns a PC. Of course, people want to profit from these means of communication
and entertainment as well as from the available information in their
cars, taking into consideration that the average German is spending more
and more time on the road. The car industry reacted upon the
customers’ demand and thought about how to realize it practically. At first
sight, it seemed to be difficult to develop a central communication unit
resistant even to the extreme external influences inside the car, which
offers a high functionality but at the same time does not distract the driver’s
attention from the traffic. To comply with all these requirements, this
optional equipment is supposed to have the following qualities:
A maximum functionality with a minimum number of operation elements,
the best display quality to read all information concerning communication,
navigation and advice. The conqueror of this new technique was BMW
who applied it in the BMW 7 series E65 and E66. It was the first car
produced in series with a data bus-controlled information system which is
able to put communication, navigation, car operation and multi media
components on a screen and which can be operated by a single button.
The data supply of this so-called i-Drive is realized by the data bus
software installed in the car and the systems belonging to it. The various
data bus systems in the BMW 7 distinguish from each other in their
purpose of use: the development of the multi media oriented data system
„MOST“ for example mainly aimed at a high transmission rate for picture
and sound so that a DVD can be enjoyed as well as on the home DVD
player.
The abbreviation MOST means Media Oriented System Transport. The
system consists of a glass fibre light guide which combines the MOSTrelevant
elements of a car by means of an annular structure. Due to the
use of an optical fibre as transmission medium the system is absolutely
insensitive to electromagnetic interferences. The immense advantage of
this transmission technique is its high transmission rate: this bus working
with real-time is able to transmit up to 24,8 Mbit/s of synchronous und
asynchronous data. The result is a picture and sound transmission in DVD
quality. Just in time with the appearance of the digital video broadcast
DVB-T the second MOST generation is carrying double speed to the data
bus.
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Furthermore, the „K-CAN P“ data bus controls the function of the door
contacts, the window lifts and so on. In this case it is not the high
transmission rate which counts but the simple transport of status information.
In this system conventional copper cables serve as data transmitters.
Several other data bus systems are installed in the BMW 7:
K-CAN S, byteflight, PT-CAN and others. All these systems are necessary
to maintain the communication between the up to 90 control units. The
successor of the CAN bus systems is yet designated: Flexray. By means
of Flexray it will be possible to send datapackets no longer serially
(as customary with CAN bus), but also parallelly.
Another kind of data bus system also working with glass fibre cables is
used in the Mercedes C-class W203. This so-called D2B bus is responsible
for the data communication between radio, CD-changer, intercom
station, voice command system and sound system. In the future D2B will
be replaced by MOST because of its higher picture transmission rate.
With the change from E-class W210 to W211 the replacement was
already practised. Also the C-class year of construction 2004 on will be
delivered exclusively with MOST components.
A data bus nearly used in all cars from 1995 on, is the CAN bus. It mainly
transmits information with relevance to engine or gear but it also connected
with the safety elements, e. g. the airbag. The twisted two-wire
technique ensures that the system can even work if one conductor is
interrupted or short-circuited with the chassis. If a connected control unit is
defective the other units will recognize it due to the system’s construction
and exclude it from the communication circle. This concept creates a
maximum safety in the case of an accident. Furthermore, it helps the
technician to localize and to eliminate a defect in the car electrical installation.
Without these complex data bus systems it is impossible to satisfy the
customers’ requirements concerning more comfort and functionality as well
as the legislator’s claims for more safety and lower fuel consumption.
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