FlexRay Fundamentals. With its data rate of up to 10 MBit/s FlexRay is targeting applications such as X-by-wire and the powertrain, which require a deterministic . Nov 9, FlexRay Protocol HardWare. 6. 11/9/ Demo: There are 13 nodes in our network. 1 is TMSLS, and others are TMSLS Sep 27, FlexRay: Communication in distributed systems within automotive context. • developed by the FlexRay consortium (BMW, DaimlerChrysler.

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FlexRay Automotive Communication Bus Overview

For automobiles to continue to improve safety, increase performance, reduce environmental impact, and enhance comfort, the speed, quantity and reliability of data communicated between bbasics car’s electronic control units ECU must increase. Advanced control and safety systems–combining multiple sensors, actuators and electronic control units–are beginning to require synchronization and performance past what the existing standard, Controller Area Network CANcan provide.

After years of partnership with OEMs, tool suppliers, and end users, the FlexRay standard has emerged as the in-vehicle communications bus to meet these new challenges in the next generation of vehicles. Adoption of a new networking standard in complex embedded designs like automobiles takes time. While FlexRay will be solving current high-end and future mainstream in-vehicle network challenges, it will not displace the other two dominant in-vehicle standards, CAN, and Flexrag.

Understanding how FlexRay flextay is important to engineers across all aspects of the vehicle design and production process. This article will explain the bwsics concepts of FlexRay. Many aspects of FlexRay are designed to keep costs down while delivering top performance in a rugged environment. FlexRay uses unshielded twisted pair cabling to connect nodes together. FlexRay supports single- and dual-channel configurations which consist of one or two pairs of wires respectively.

FlexRay Automotive Communication Bus Overview – National Instruments

Differential signaling on each pair of wires reduces the effects of external noise on the network without expensive shielding. Most FlexRay nodes typically also have power and ground wires available to power transceivers and microprocessors.

Most first-generation FlexRay networks only utilize one channel to keep wiring costs down, but as applications increase in complexity and safety requirements, future networks will use both channels.

FlexRay buses require termination at the ends, in the form of a resistor connected between the pair of signal wires. Only the end nodes on a multi-drop bus need termination. Too much or too little termination can break a FlexRay network. While specific network implementations vary, typical FlexRay networks have a cabling impedance between 80 and ohms, and the end nodes are terminated to match this impedance.

Termination is one of the most frequent causes of frustration when connecting a FlexRay node to a test setup. Modern PC-based FlexRay interfaces may contain on-board termination resistors to simplify wiring.

FlexRay supports simple multi-drop passive connections as well as flextay star connections for more complex networks. Depending a vehicle’s layout and level of FlexRay usage, selecting the right topology helps baslcs optimize cost, performance, and reliability for a given design.

FlexRay is commonly used in a simple multi-drop bus topology that features a single network cable run that connects multiple ECUs together. The ends of the network have termination resistors installed that eliminate problems with signal reflections.

The multi-drop format also fits nicely with vehicle harnesses that commonly share a similar type of layout, simplifying installation and reducing wiring throughout the vehicle. The FlexRay standard supports “Star” configurations which consist of individual links that connect to a central active node.

If one of the branches of the star is cut or shorted, the other legs continuing functioning. Since long runs of wires tend to conduct more environmental noise such as electromagnetic emissions from large electric motors, using multiple legs reduces the amount of exposed wire for a segment and can help increase noise immunity.


The bus and star topologies can be combined to form a hybrid topology. Future FlexRay networks will likely consist of hybrid networks to take advantage of the ease-of-use and cost advantages of the bus basicss while applying the performance and reliability of star networks where needed in a vehicle.

FlexRay accomplishes this hybrid of core static frames and dynamic frames with a pre-set communication cycle that provides a pre-defined space for static and dynamic data.

This space is configured with the network by the network designer. As with any multi-drop bus, only one node can electrically write data to the bus at a time.

If two nodes were to write at the same time, you end up with contention on the bus and data becomes corrupt. There are a variety of schemes used to prevent contention on a bus. CAN, for example, used an arbitration scheme where nodes will yield to other nodes if they see a message with higher priority being sent on a bus. While flexible and easy to expand, this technique does not allow for very high data rates and cannot guarantee timely delivery of data.

Every FlexRay node is synchronized to the same clock, and each nodes waits for its turn to write on the bus. This provides many advantages for systems that depend on up-to-date data between nodes.

Embedded networks are different from PC-based networks in that they have a closed configuration and do not change once they are assembled in the production product. By designing network configurations ahead of time, network designers save significant cost and increase reliability of the network.

Every FlexRay network may be different, so each node must be programmed with correct network parameters before it can participate on the bus.

To facilitate maintaining network configurations between nodes, FlexRay committee standardized a format for the storage and transfer of these parameters in the engineering process. The Field Bus Exchange Format, or FIBEX file is an ASAM-defined standard that allows network designers, prototypers, validaters, and testers to easily share network parameters and quickly configure ECUs, test tools, hardware-in-the-loop simulation systems, and so on for easy access to the bus.

The FlexRay communication cycle is the fundamental element of the media-access scheme within FlexRay. The duration of a cycle is fixed when the network is designed, but is typically around ms. There are four main parts to a communication cycle: The smallest practical unit of time on a FlexRay network is a macrotick. FlexRay controllers actively synchronize themselves and adjust their local clocks so that the macrotick occurs at the same point in time on every node across the network.

While configurable for a particular network, macroticks are often 1 microsecond long. Because the macrotick is synchronized, data that relies on it is also synchronized.

Illustration of a static segment with 3 ECUs transmitting data to 4 reserved slots. The static segment, represented as the blue portion of the frame, is the space in the cycle dedicated to scheduling a number of time-triggered frames. The segment is broken up into slots, each slot containing a reserved frame of data.

When each slot occurs in time, the reserved ECU has the opportunity to transmit its data into that slot. Because the exact point in time is known in the cycle, the data is deterministic and programs know exactly how old the data is. This is extremely useful when calculating control loops that depend on consistently spaced data. Figure 3 illustrates a simple network with four static slots being used by three ECUs.

Actual FlexRay networks may contain up to several dozen static slots. Illustration of a static slot with ECU 2 missing. Most embedded networks have a small number of high-speed messages and a large number of lower-speed, less-critical networks.

To accommodate a wide variety of data without slowing down the FlexRay cycle with an excessive number of static slots, the dynamic segment allows occasionally transmitted data. The segment is a fixed length, so there is a limit of the fixed amount of data that can be placed in the dynamic segment per cycle. To prioritize the data, minislots are pre-assigned to each frame of data that is eligible for transmission in the dynamic segment.


Higher priority data receives a minislot closer to the beginning of the dynamic frame. Once a minislot occurs, an ECU has a brief opportunity to broadcast its frame.

If it doesn’t broadcast, it loses its spot in the dynamic frame and the next minislot occurs.

If the dynamic frame window ends, then the lower-priority minislots must wait flexgay the next cycle for another opportunity to broadcast. Dynamic slots illustration showing ECUs 2 and 3 broadcasting in their minislots and leaving no time for the lower-priority minislots. Figure 5 shows Basiics 1 broadcasting in its minislot since the first 7 minislots chose not to broadcast. ECU 1 must wait for the next cycle to broadcast.

The Symbol window is primarily used for maintenance and identification of special cycles such as cold-start cycles. Most high-level applications do not interact with the symbol window. The network idle time is of a pre-defined, known length by ECUs. The ECUs make use of this idle time to make adjustments for any drift that may have occurred during the previous cycle.

The FlexRay network provides scalable fault-tolerance by allowing single or dual-channel communication. For security-critical applications, the devices connected to the bus may use both channels for transferring data.

However, it is also possible to connect only one channel when redundancy is not needed, or to increase the bandwidth by using both channels for transferring non-redundant data. Within the physical layer, FlexRay provides fast error detection and signaling, as well as error containment through flexrray independent Bus Guardian.

The frame is divided into three segments: Header, Payload, and Trailer. The Frame ID defines the slot in which the frame should be transmitted and is used for prioritizing event-triggered frames.

The Payload Length contains the number of words which are transferred in the frame. The Header CRC is used to detect errors during the transfer. The Cycle Count contains the value of a counter that advances incrementally each time a Communication Cycle starts. The payload contains the actual data transferred by the frame.

The length of the FlexRay payload or data frame is up to words byteswhich is over 30 times greater compared to CAN. FlexRay data is represented in bytes. Flexraj applications require data to be represented in real decimal values with units, scaling, and limits. When you take one or absics bits or bytes from a FlexRay frame, apply a scaling and offset, you get a signal that is useful for communicating actual parameters between ECUs.

Bsaics ECUs programs work with FlexRay data as signals and leave the conversion of signals to raw frame data up to the driver or lower-level communication protocols.

A typical vehicle has hundreds to baskcs of signals. This makes writing programs for FlexRay networks easier as designers can simply refer to the signal name in the code.

Simplified Synchronization process of a FlexRay network. FlexRay has the unique ability to sync up nodes on a network without an external synchronization clock signal. To do so, it uses 2 special types of frames: Startup Frames and Sync Frames. To start a FlexRay cluster, at least 2 different nodes are required to send startup frames.

The action of starting up the FlexRay bus is known as a cold-start and the nodes sending the startup frames are usually known as cold-start nodes. The startup frames are analogous to a start trigger, which bascis all the nodes on the network to start.