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CAN-Bus 2 (data communication)



To perform the error analysis it is not necessary to be familiar with the data structure.

If you want to find errors, it most often suffices to watch the signals in total, it is not necessary to drill down to the single bits. It is possible to trace disturbances on CAN_L and CAN_H successfully by separating out single control devices of the Controller Area Network Bus without knowing its data structure. Separating out single components is made easier because some companies use an easy accessible plug bar to which many control devices are connected centrally. Before you dismantle, e.g., four door linings check the wiring diagram.

Every switch able wire is, in principle, a data line.

Let us venture forward to the bit level. With all care we may describe the simple circuit of a rear light or even an indicator lamp as transmission of data. The plus wire would be the data line. Indeed, the Controller Area Network Bus mostly features two data lines; however, it also works with just one. Indeed, the voltage would have to be maximized to 5 volts and the ground would have to feature an additional wire. In addition, the current would have to be substantially lower, demanding a suitable cable cross section. Unfortunately, the rear lights cannot communicate back, and by this another important feature of the Controller Area Network Bus is absent.

'Sending information through the Bus'-> putting on resistors.

We will remain one more moment with the example. Turning the light switch brings a signal onto the wire (Bus). Activating the direction indicator switch it is even a signal with varying bits. We can assume therefore that 'sending information through the bus' indicates to manipulate the bus that it has a voltage of 0 or 5 Volt on the wire at a certain time. If two control devices put at the same time different bits on the Bus, the voltage always asserts itself against the high voltage. 'Read something from the Bus' indicates to take the voltage of the data line(s) no matter whether one sends information or not. However, the temporal arrangement must be kept exactly to transfer a bit properly. Thus, e.g., the voltage is switched on by the transmitter at a certain time and also held for a certain time.

Keeping the same scanning rate on the Controller Area Network Bus is very important.

The maximum transmission rate depends among other things on the length of the wire. An idea of the speed of the processes provides the stroke rate for cars, 500 kBit/s, which is 500,000 bits per second or 500 bits per millisecond. Sending or receiving a single bit must not take longer than 2 microseconds (s). Thus, it is essential that the transmitter(s) and receiver(s) are chronologically exactly coordinated. Therefore, the data transmission always starts with 1 bit, in whose flanks (voltage increase and voltage drop) all recipients can adjust to one common stroke rate (Synchronization). If for some time there is no traffic on the Bus, data packets are send through, in order to synchronize the timers (time unit transducer) of the single control units. 05/07






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