FlexRay

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FlexRay is a serial , deterministic and fault-tolerant fieldbus system for use in automobiles , comparable to TTP . The FlexRay consortium was founded in 2000 by the companies BMW , Daimler AG , Motorola and Philips . From 2001 to 2004 the companies Bosch , General Motors and Volkswagen joined as core partners . In 2004 Freescale took over the rights and duties as a core member of the Motorola consortium. In 2006, NXP Semiconductors took over the rights and obligations as a core member in the Philips consortium. In 2010 the FlexRay consortium disbanded. The FlexRay standard was then converted into an ISO standard (ISO 17458-1 to 17458-5).

FlexRay logo

general description

Development and expectations

FlexRay should meet the increased requirements of future networking in the vehicle, which can not be satisfied by the CAN bus , in particular a higher data transmission rate , real-time capability and reliability (for X-by-Wire systems). However, the current focus is primarily on the higher data rate, which is necessary today due to the continuous increase in driver assistance systems in the drive and chassis area in premium vehicles. FlexRay defines layers 1 (physical layer) and 2 (data security layer) in the ISO / OSI reference model . It was first used in series production in 2006 in the BMW X5 . The FlexRay cluster in this vehicle is based on protocol version v2.0, the physical layer specification v2.1 revision A.

In order to also meet the requirements of active safety systems, FlexRay has been further developed , especially with regard to temporal determinism and fault tolerance . A bus guardian should enable centralized or decentralized monitoring of bus access based on the statically defined TDMA scheme , but is not used in practice. In addition to ByteFlight, FlexRay offers message communication with a defined TDMA scheme. FlexRay uses similar mechanisms, such as the time-triggered protocol TTP developed at the Technical University of Vienna . In addition to the TDMA scheme, the minislotting protocol adopted by ByteFlight offers a collision-free, prioritized, dynamic communication channel.

construction

To make a node, e.g. For example , to operate a control device on a FlexRay bus, you need two components: the bus transceiver and the communication controller . The bus transceiver establishes the direct connection to the data line: On the one hand, it writes the logical information that is to be sent in the form of voltage pulses on the bus; on the other hand it reads out the signals that are sent by other participants on the bus. This level is defined as physical or PHY Physical Layer called. FlexRay also includes the bus protocol. The bus protocol regulates how a network starts, how a global clock is established and which control devices are allowed to send at what time. The communication controller implements the bus protocol in every control device, for example it packs the information to be transmitted into a data packet and transfers this data packet to the bus transceiver at the right time for transmission.

Performance characteristics

FlexRay supports:

  • Bit rates up to 10 Mbit / s per channel
  • decentralized clock synchronization
  • guaranteed latency times
  • Two channels in the protocol
  • central & decentralized access control
  • Star and bus topologies as well as topologies with buses on the star arms

Communication protocol

TDMA (Time Division Multiple Access) according to FlexRay version 2.1

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

  • In the static segment, each control unit has its specific slot (time window) in which it can send messages. It must not exceed the length of its slot. If the message is too long, the next cycle or the dynamic segment must be used to continue the message.
This is the deterministic part of the protocol which can be used to ensure that important messages (e.g. steering, brakes) are transmitted within a known time.
  • 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 slot is insufficient or is required for more important messages.
    • If a control device does not want to send a message, its time window (mini slot) simply expires (mini 1 to mini 3).
    • If the control unit wants to send a longer message in minislot 4, for example, the point in time at which the next control unit can send is shifted back. In the worst case, the dynamic minislot 4 is so long that no further control units can transmit in this cycle.
    • Because control units that are at the rear end of the sequence in the dynamic slot are most likely to have to wait (or even drop out) with a message in this slot, the number of dynamic slots should be k> n (n is the number of control units that communicate on the bus and have a slot for this purpose).
In terms of its transmission structure, this part corresponds more closely to the CAN bus .
  • The symbol window (symbol) was intended for testing the bus access and will probably no longer be used.
  • NIT (Network Idle Time) should enable the control devices attached to the bus to synchronize again exactly with the bus.

The synchronization ensures that all control units on the bus send messages at the same rate and not send through time shifts in the minislot (time window) of an external control unit. The time cycle is negotiated by the control units according to certain rules when waking up. There is therefore no need for a master that sets the clock and could paralyze the bus if it fails.

The frame

The FlexRay frame

The FlexRay frame is structured as shown in the figure on the right.

hardware

To prevent line reflections, the line is terminated with its line 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 stub lines. BMW and other OEMs use special TP cables with PE insulation, since PE as an insulation material has considerable advantages over PVC in terms of temperature-related tolerance, and consequently the requirements for impedance can be met. Ethernet cables, both standard versions (CAT5 etc.) and Profinet cables, are suitable for laboratory setups , the latter are available in robust designs and are also suitable for cabling in vehicles.

The signals are transmitted by voltage levels of 1.5 V and 3.5 V, a 0 or 1 is transmitted depending on the position of these voltage levels on the lines. If both lines have a level of 2.5 V, the bus is idle. To save energy, the 0 V level can also be used for both lines.

See also

literature

  • FlexRay is described in detail in: Mathias Rausch: FlexRay - Basics, Functionality, Application . Hanser-Verlag, 2007, ISBN 3-446-41249-2 or ISBN 978-3-446-41249-1 .
  • A comparison with other automotive field buses takes place in: Werner Zimmermann and Ralf Schmidgall: Bus systems in vehicle technology - protocols, standards and software architecture. 5th edition, Springer Vieweg, 2014, ISBN 978-3-658-02418-5 .

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