According to the FlexRay™ definition, a compliant physical layer communication channel consists of four elements: bus driver (BD), twisted pair (TP) bus line, bus termination, and an active star (AS). The BD (transceiver) is an interface between the microcontroller (including the FlexRay™ communication controller) and the FlexRay™ bus. The bus driver converts the digital data stream to differential analog bus voltage and vice versa. The twisted pair bus line, with defined surge impedance, connects two or more bus nodes. The bus line is terminated by a resistor. Usually an active star is placed in the center point of a FlexRay™ network. The active star can be used as a stand-alone device which redistributes the incoming bus signals to all other bus lines (branches). Optionally it can be used simultaneously as a signal router and a transceiver. In this case the locally transmitted data are sent to all FlexRay™ branches. All incoming data is mirrored at a single digital transceiver pin RxD. This functionality is needed e.g. in network gateway applications.
Because automotive network topologies are restricted by the car's body geometry and its available mounting space, linear FlexRay™ network topologies unquestionable. In addition, crash-prone car regions, for instance the front part of the body, have been identified. In case of a branch failure, the remaining intact parts of the network can be separated from the defect ones.
Networks including an active star show improved EMC/EMI behavior compared to mere passive star networks and are preferable due to the fact of better parameter reproducibility and predictability. Automotive networks often require topology variants depending on the count of used electronic control units. The network partitioning always considers the minimum number of needed branches. On the other hand reduction of overall system costs can be achieved by utilization of the same hardware, connectors, wiring, etc. Therefore unused branches can be disabled by the active star.