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          A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

CAN-Bus 3 - Data Security








Magnets and transitional resistors have no chance.

Digital data-lines have a clear advantage in the event of disturbances. If, e.g., the signal from the fuel tank transducer is transfered analog, every - be it only very small - disturbance, perhaps through an (electric) magnet or through transitional resistances, has an affect. Should only 4 volts instead of 5 volts be received, then, in theory, the fuel-gauge would show 20% less. In the case of a digital transfer there is no such loss of information, as long as the bits can be recognised. The undefined range when transferring 5 V signals should lie between 2 - 3,5 volts. All voltage levels above- and below this are unambibgious.

Even more data-security through CAN_H and CAN_L.

The designers of the CAN-Busses were not yet satisfied. Mostly one takes two data-lines, CAN_H(igh) and CAN_L(ow). Thereby, one line tranfers the exactly inverted signal of the other. If then, CAN_H increases the voltage, then CAN_L reduces it and vice versa. The receivers on the CAN-Bus, e.g., in the Highspeed-Bus, evaluate the difference between the two. Should the voltage difference exceed a certain value, the transfer of the bit is considered valid. Should a disturbance occur in both data-lines, then both are shifted by a certain voltage, in either the plus- or the minus direction. The difference however, remains constant.

Varying voltage ranges depending on the BUS-speed.

Theoretically, in a comfort data-bus, CAN_H oscillates between 0 and 5 volts, Can_L between 5 and 0 volts. Thus, if one measures the tension, instead of 5 V with CAN_H, there are only approx. 4,3 V and with CAN_L, instead of 0 V, 0,7 V. The respective difference of 0,7 V is due to the diode-path of the transistor circuit in the IO unit of the individual BUS-participants. You can find the level for the faster Busses (e.g., for the engine) in the table shown below. Recessive (1) is the voltage level, when no data-transfer is taking place. 05/07

ComfortDominantRecessive
CAN_Happrox. 4,3 Vapprox. 0 V
CAN_Lapprox. 0,7 Vapprox. 5 V
High-SpeedDominantRecessive
CAN_Happrox. 3,5 Vapprox. 2,5 V
CAN_Lapprox. 1,5 Vapprox. 2,5 V




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