FlexRay is a serial, deterministic and fault-tolerant fieldbus system for use in automobiles, comparable with TTP. The FlexRay Consortium was founded in 2000 by the company BMW, Daimler AG, Motorola and Philips. From 2001 to 2004 occurred as a core partner in the company Bosch, General Motors and Volkswagen. In 2004, Freescale 's rights and obligations as a core member of the consortium of Motorola. In 2006, NXP Semiconductors ' rights and obligations as a core member of the consortium of Philips. 2010, the FlexRay consortium broke up, but the specification is still on the FlexRay website available. FlexRay standard is currently being transferred to an ISO standard.

General Description

Development and expectations

FlexRay should meet the increased requirements of future automotive networks, in particular higher data rates, real- time capability and reliability ( for X-by -wire systems ). The current focus is, however, primarily the higher Datenrate, which by the continuous increase of Fahrerassistenzsystemen in the field of drive and chassis in Premiumfahrzeugen today is necessary. FlexRay defines the Layer 1 ( physical layer ) and 2 ( data link layer ) in the ISO / OSI reference model.

In the area of ​​safety bus guardian concepts have been developed. These provide centralized or decentralized monitoring of bus accesses based on the static fixed TDMA scheme. The series automotive use was the first time in the BMW E70 in 2006. FlexRay cluster in this car is based on the protocol version v2.0, the Physical Layer Specification v2.1 Revision A.

In order to meet the requirements of active safety systems, FlexRay was developed primarily in relation to temporal determinism and fault tolerance further. FlexRay has ByteFlight addition to a message communication with a fixed TDMA scheme. FlexRay uses similar mechanisms, as developed at the Technical University of Vienna Time-Triggered Protocol TTP. In addition to the TDMA scheme adopted by the ByteFlight minislotting protocol provides collision free, prioritized, dynamic communication channel.


In order to operate a node, such as a control unit to a FlexRay bus, requires two components: the bus transceiver and the communication controller. The bus transceiver connects directly to the data line here: On the one hand he writes the logical information that is to be sent in the form of voltage pulses on the bus; on the other hand reading of the signals sent from other nodes on the bus. This level is referred to as a physical or physical layer PHY. In addition, FlexRay still includes the bus protocol. The bus protocol determines how a network begins as a global clock is established and which control units may transmit data at what time. The communication controller sets the bus protocol in each control device to, for example, it packages the information to be transmitted into a data packet and transfers this data packet at the appropriate time for transfer to the bus transceiver.


FlexRay supports:

  • Bit rates up to 10 Mbit / s per channel
  • Distributed clock synchronization
  • Guaranteed latency
  • Two channels in the protocol
  • Central and decentralized access control
  • Star, bus topology and topologies with buses to the star arms

Communication protocol

The communication on the bus takes place in cycles. Each of these cycles is divided into different segments:

  • In the static segment, each control device its specific slot ( time slots ) in which it can send messages. It may not exceed the temporal length of its slots. If the message is too long, the next cycle or the dynamic segment must be used to continue the message.
  • The dynamic segment can be used by a control unit if it wants to send longer or additional messages, and for example, the width of its static slots is not sufficient or is needed for more important messages. If a control device wants to send any messages to settle, its time slot (mini slot) just runs off (Mini 1 to 3 Mini ).
  • Wants to send a long message, for example in mini- slot 4, the control device, shifts the time when the next control device can send, to the rear. In the worst case, the dynamic mini- slot 4 is so long that, in this cycle can not send a further control device more.
  • Because for control units connected to the rear end of the sequence are in the dynamic slot which is most likely that they would have to wait a message in this slot (or would fall out ), the number of dynamic slots should k> n be ( n is the number of control devices that communicate on the bus and to have a slot ).
  • The symbol window ( symbol ) was provided for the test bus access and will probably no longer be used.
  • NIT (Network Idle Time ) to enable the hanging at the bus control units, re-synchronize exactly with the bus.

Synchronization causes all control devices on the bus to the same clock send messages and not by temporal shifts in the mini slot ( time slot ) of a foreign controller to send. The timing is negotiated by the control devices according to certain rules when you wake up. There is no master is therefore necessary that sets the pace and could paralyze case of its failure the bus.

The frame

The FlexRay frame ( engl. frame) is constructed as shown in the adjacent figure.


To prevent line reflections, the line is terminated with its characteristic impedance in the range of 80 Ω to 110 Ω. The lines are twisted. The maximum cable length depends on the data rate and the number, length and position of the spurs. BMW and other OEMs use special TP cables with PE insulation, because as insulation against PVC, PE has considerable advantages in the temperature-induced tolerance, therefore, the requirements can be met with the impedance. For laboratory set up Ethernet cables are both standard types ( CAT5, etc. ) as well as Profinet cable, the latter is available in robust design and are also suitable for wiring in the vehicle.

The signals are transmitted by voltage level of 1.5 V and 3.5 V, depending on the location of the voltage level on the lines is a 0 or 1 transfer. If both lines the level of 2.5 V, the bus is idle ( idle). For energy savings and the level can be 0 V is used for both lines.