Physical signal transmission in a FlexRay cluster is based on transmission of voltage differences (differential signal transmission). Therefore, the transmission medium (FlexRay bus) consists of the two lines Bus Plus (BP) and Bus Minus (BM).

The Electrical Physical Layer Specification defines four bus levels, which are assigned either to the recessive or the dominant bus state. The recessive bus state is characterized by a differential voltage of 0 Volt. The dominant bus state, on the other hand, has a differential voltage not equal to zero Volt.

Both, the idle bus level and the idle low power bus level, are recessive. The idle bus level is characterized by the two lines exhibiting a 2.5 Volt potential, resulting in a differential voltage of 0 Volt. The valid range for the idle bus level lies between 1.8 Volt and 3.2 Volt.

The idle low power bus level appears on the FlexRay bus when all FlexRay transceivers are in their low-power mode. This bus level is also characterized by a differential voltage of 0 Volt, but in this case the lines exhibit a potential of 0 Volt. The valid range here is between -0.2 Volt and 0.2 Volt.

The two bus levels Data_1 and Data_0 are dominant bus levels. With the Data_1 bus level the BP bus line exhibits a potential of 3.5 Volt, and the BM bus line a potential of 1.5 Volt. The resulting differential voltage is 2 Volt. The Data_1 bus level represents logical one.

With the Data_0 bus level the BP bus line has a potential of 1.5 Volt, and the BM bus line a potential of 3.5 Volt. The resulting differential voltage is -2 Volt. The Data_0 bus level represents logical zero.

Bus states and bus levels can be seen in the figure “FlexRay Bus Level”. Also shown are the relevant voltage thresholds for sender and receiver.