FlexRay is a new communication system, which aims to achieve reliable, real-time and efficient data transmission between electrical and Mechatronic components, so as to connect today’s and future innovative functions in vehicles. The data transmission rate of FlexRay is 10mbit / s, which is significantly higher than the 500K high-speed can bus used in the car today.

What are the advantages of FlexRay?

High bandwidth (10mbit / s, compared with 0.5mbit/s of can)

Deterministic (= real time) data transmission

Reliable data communication

Support system integration

Automotive industry standards

Bus topology

FlexRay bus system can be installed in vehicles in different topologies and forms. Here you can use the following topology:

Linear bus topology

Topology of star bus

Hybrid bus topology.

Linear topology:

Similar to can bus, all control units adopt twisted pair, and are connected in parallel.

Star topology:

There are central control unit and satellite control unit, the main control unit is located in the center, and the satellite control unit is connected with the main control unit.

In the real FlexRay, the hybrid connection form of linear and star is often used. There are up to eight branches in FlexRay, each of which is connected with a linear control unit, and each branch is connected with the central gateway module j533. The figure below shows the FlexRay bus system with six branches.

Diagnosis and maintenance of FlexRay

Resistance measurement method:

Similar to high-speed can bus, the connection relationship of the system can be judged by measuring the resistance. Under normal circumstances, there are two terminal resistors in each branch, and each resistance value is about 90-110 Ω. If measured in parallel, the resistance value is about 45-55 Ω.

For example, if there is an open circuit at x, the open circuit point can be determined by measuring the line between 3 and n.

Different from high-speed can bus, if the resistance of one branch is within the normal range, it can’t explain the situation of other branches. It needs to measure the resistance of all branches to judge the circuit connection of FlexRay, and they need to be diagnosed separately.

Voltage measurement method;

Similar to high-speed can bus, FlexRay system can be judged by measuring voltage; under normal conditions, flexray-h voltage is about 2.6V, while flexray-l voltage is about 2.4V. If the voltage is 0V or 12V, the system is short circuited to the ground or power supply.

Different from high-speed can bus and similar to resistance measurement method, even if the voltage value of a branch is normal, it can only indicate the branch; it needs to measure the voltage value of all branches to judge whether the system is normal or not.

Measurement with oscilloscope:


Need special oscilloscope, and the connection is complex, need technicians for dual channel oscilloscope can be very skilled operation, at the same time can accurately analyze the difference of different waveforms.

Similar to high-speed can bus, its normal waveform is very similar, but its speed is 20 times of high-speed can bus.

In case of interference, the waveform is as follows:

Different from high-speed can bus, it is the same as resistance method and voltage method. Even if the waveform of one branch is normal, it does not represent the situation of other branches. It is necessary to measure the waveforms of all branches to judge whether the system is normal or not.

Synchronous signal diagnosis:

Because FlexRay has a star connected part, there are synchronization signals in FlexRay, that is, when the control unit is replaced, it needs to be synchronized with other control units; in some materials, it is called cold start control unit, generally 2-3 units; for example, central gateway control unit, engine control unit, ABS control unit, these units can not be replaced at the same time, because they need to be replaced at least A control unit is used for signal synchronization.

Summary notes:

FlexRay diagnosis and high-speed can bus have the same place, but also more complex place, only familiar with these characteristics, find the control unit connected to each branch, can diagnose the fault faster.

Editor in charge: CC

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