Automotive communication networks

Part 3: FlexRay Networks

Published:  02 September, 2021

Damien concludes his three-part series with an look at FlexRay

In recent years to meet the demands of Drive wire systems as well as the addition of advanced driver assistance systems (ADAS), vehicle manufacturers in association with electronic component manufacturers investigated possible solutions for high-speed data transfer. In the previous article, we looked at CAN (Controller Area Network) Bus communication. However, with safety-critical drive-by-wire systems, a higher transfer data rate and improved error detection is required. With this in mind, a number of vehicle manufacturers, namely Volkswagen, BMW, Daimler and General Motors, became core members of the FlexRay consortium.

Physical layer
FlexRay is similar to CAN Bus as the data is transmitted over two twisted wires, which act to reduce the effects of external interference on the differential voltage between both wires. Data transfer speeds can be up to 10 times faster than CAN bus, operating at speeds up to 1 Mbits per second. Although the normal data transfer speed for CAN is 500 kbits per second, it has the capability to transmit at speeds of 1 Mbits per second.
FlexRay can support network redundancy or dual channels (see Fig.1). This ensures greater fault tolerance as well as increased bandwidth for additional data transfer. However, this additional network is rarely implemented.

Signal conditioning
To match the impedance of the network cabling, a terminating resistance of 90 to 110 Ω within the modules at either end of the network is required. This ensures signal reflections are reduced, which is an issue with such high data transfer speeds.

Bus access
CAN Bus messages are subject to arbitration to ensure messages of higher priority have access to the network first. FlexRay supports both event-triggered messaging and deterministic messaging which allows for high data rates and guarantees message delivery when required. Each node or module has a predetermined time slot to transmit its data. This is referred to as time division multiple access or TDMA. For sporadic messaging (event-driven) TDMA is not optimal so flexible time division multiple access or FTDMA is used.

FlexRay voltages
FlexRay bus voltage is similar to CAN Bus. With the bus idle or in a recessive state, the voltage present on both wires, Bus (+) and Bus (–), is 2.5 volts.

Approximate voltage levels
When a logic bit of 1 is required the Bus (+) voltage 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 the 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.

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 next waveform displayed 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 final waveform shows Bus (+) and
Bus (–) separated to show the mirror image of both traces.

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