CAN-Bus 6 (Blocks of Data)
|Control units keep lists of all possible messages|
In the Controller Area Network Bus messages are not only dispatched, but may also be requested. A participant of the Bus requires, e.g., the controller of the temperature sensor's values. Also these requirements are
programmed, like all other possible messages, into the control unit's of the Controller Area Network Bus. This takes place already in the factory. Thus, upon leaving the factory, a chip is aware, provided that it is not
altered, of all the data information that it is able to provide or request. Only the contents of the data blocks are subject to change.
|A correct reception of the message is acknowledged by the participants.|
Another important feature of the Controller Area Network Bus is the data security. In this case data security stands for the verification of a correct reception of the data field. One possibility would be to send the sum of
the digits as extra information. In this case all participants would add the bits in the data block and compare the number with the provided number. However, because it is possible that two mistakes cancel each other
out, the test methods applied to the Controller Area Network Bus are substantially more complex.
If you watch more closely single messages produced on the Controller Area Network Bus, a slight protruding
point at the end of every message might catch your attention. This is the bit that all participants transmit to provide feedback for the correct reception of the message. It originates from all participants, thus, it is slightly
higher in voltage and stands out. The last bit is not part of the information, but comes from the other participants. This becomes especially important, when analyzing error messages.
By the way, the positive feedback of one single control unit might be sufficient for the transmitter. However, without any feedback, it must repeat its message. 05/07