June 2021

BY Damien Coleman, Product Manager/ EBI Specialist at Snap-on I n recent years to meet the demands of Drive by 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 26 AFTERMARKET JUNE 2021 TECHNICAL/SNAP-ON www.aftermarketonline.net Part 3: FlexRay Networks AUTOMOTIVE COMMUNICATION NETWORKS 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 Damien concludes his three-part series with an look at FlexRay Fig. 2 Fig. 1 Fig. 3