June 2021

JUNE 2021 AFTERMARKET 27 FlexRay is similar to CAN Bus as the data is transmitted over two twisted wires ” Waveforms Due to the incredibly high data transfer speed of FlexRay, in-depth waveform analysis is difficult. However, the oscilloscope can be used to validate that a signal is present on the network and to test for an open or short with in the circuit. The waveform as seen in Fig.5 shows a trace captured from a FlexRay system. The zero lines are set up to show the transition between a Logic 1 and Logic 0 bit due to the voltage switch for Bus (+) and Bus (–). The waveform as seen in Fig.6 shows the differential voltage between Bus (+) and Bus (–). This is observed using a single channel test with channel 1 connected to Bus (+) and the oscilloscope scope ground connected to Bus (–). The waveform as seen in Fig.7 shows Bus (+) and Bus (–) separated to show the mirror image of both traces. increases to 3.1 volts and the Bus (–) voltage reduces to 1.9V. This creates a differential voltage of +1.2 volts. When a logic bit of 0 is required the Bus (+) voltage reduces to 1.9 volts and the Bus (–) voltage increases to 3.1 volts. This creates a differential voltage of -1.2 volts. Two channel network voltage To see two channel network voltage, please refer to Fig.2. Differential voltage To see differential voltage, please refer to Fig.3. BMW X5 vertical dynamics control module The E70 BMW X5 was the first production vehicle to have a FlexRay network fitted. It is implemented for the vertical dynamics control module, which controls the suspension height for each individual suspension strut. The network is configured in a Star bus (see Fig.4). This ensures the system can still operate if a satellite unit fails. It is also an advantageous setup when long lengths of wiring are required, as normal electrical interference will only affect one leg of the network due to this configuration. A satellite unit is located in each corner of the vehicle. VDM – Vehicle dynamics control module. S1 – Satellite sensor left front. S2 – Satellite sensor right front. S3 – Satellite sensor left rear. S4 – Satellite sensor right rear. PT CAN – Powertrain CAN Bus. A terminating resistor of 90 – 110 Ohms is fitted in each of the satellite units. Next issue: Damien goes back to CAN Bus for our July/August double issue, with a deep dive case study looking at a Volvo XC60 and its medium speed CAN Bus failure Fig. 4 Fig. 5 Fig. 6 Fig. 7