The youngest of the bus systems in use today is FlexRay. It was developed because CAN is always more than a "bottleneck" was found. With 500 kbps bit rate, there was a practically usable CAN for demanding applications in the drive train and in particular in the area of ​​the chassis, was insufficient.

Flex­Ray

A second driving factor for the development of FlexRay were future X-by-wire application, such as break-By-Wire and Stear-By-Wire. Not only a high bit rate, but also new approaches for safety-critical applications are required. FlexRay offers its basic concept, with its two-channel approach to greater possibilities. FlexRay is technically from the proprietary bus system Byteflight BMW and TTP. One solution, developed by the University of Vienna and is now marketed by the company TTTech.

Specifications - FlexRay Communications System

The first use of FlexRay series took place in 2006. The first vehicle was the then BMW X5. The specification has been stable since 2005 in the version 2.1. There will be little further development, but they will be upwardly compatible.

Physical Layer and bus topology

How CAN use FlexRay, a bit stream based transmission. It is used a bidirectional two-wire bus system. The bus lines must be terminated at both ends with terminators. The second channel is optional. He can either redundant or to double the bandwidth used.

FlexRay - Physical Layer and bus topology

The bus access takes place with a scheduled time slicing, see the next section. It can be a maximum of 64 controllers in different topologies connected to the bus system. The known structure of CAN line with FlexRay also feasible, but here is the exception rather dar. The preferred topology is a star topology with an active neutral (Active-Star). The active star coupler point is a point at which the signals are regenerated. Thus distances between the controller and neutral point of up to 24 meters are possible. The standard bit rate of FlexRay is 10 Mbit / s. In contrast to CAN FlexRay has no inherent limitation on the bit rate. This means that in future, higher bit rates are used here.

Data Link Layer

FlexRay organized the timing of the transfers in so-called cycles, as illustrated. Within a cycle there is a static and a dynamic segment. The cycles are repeated periodically, the length of a cycle is fixed.

FlexRay Data Link Layer - Cycle

Within the static segment, we have time slots with a fixed length. The length of a time slot is such that exactly fits into a message. During the development phase is determined, which may send the control unit in which time slot and has to send. It is possible to be assigned in a particular time slot for different cycles have different ECUs. This is the so-called cycle-multiplexing on the function and meaning, we briefly discuss later. Each control device counts, starting with the beginning of the cycle, the time slots with, so that all control devices are based on in which time slot is the transmission system straight.

In the dynamic segment, we also have time slots, the so-called mini-slots. Here too, each mini-slot is assigned to a control device. That is, only one control unit has the broadcasting rights, so anything can re-run collision.

In contrast to the static segment the messages in the dynamic segment, but may now be of variable length. So you can take more than a mini-slot long. A control device may very well here to give up his broadcasting. Then there is the associated free mini slot, the slot counter continues to count and the next mini-slot may then include another control surface. If a control device, however uses his broadcast and his message takes longer than a mini slot, then the slot counter is simply stand until the message is complete. A message in the dynamic segment must be completely within the current cycle can be sent. If this is not the case, do not send the message to the communications controller. The message has to wait at least until the next cycle. The choice of mini-slots for a message is thus a kind of priority this message dar.

At the end of a communication cycle are the Network Idle Time and the Symbol Window. The symbol window is reserved for future extensions. The network idle time is used for synchronization between transmitters and receivers.

Cycle Multiplexing

The communication process is repeated periodically in the communication cycles. Typically, a message is sent once per cycle. be selected for time-critical messages, the cycle time accordingly small. In the BMW X5, for example, they took 5 milliseconds. To send the messages again with an even shorter cycle time, without the cycle to make it more succinctly, it has come up with a trick: the cycle multiplexing, as illustrated.

FlexRay - Cycle Multiplexing

One chooses to 3 milliseconds, the static segment a little larger than half the total cycle time. This makes it possible to send a message twice, at the beginning and in the middle of the cycle. The effective cycle time is 2.5 milliseconds. For those messages that range in which a larger period would, for that is a cycle time of 5 milliseconds is not necessary, it has created the cycle multiplexing. This must be no reservation slots, which are not required. When you use a multiplexing cycle and the same time slot for different messages from different control units. In the example shown here in Orange a message that is sent every 10 milliseconds, and in yellow a message to be sent only every 20 milliseconds to be. Overall, one can build on this, a scheme with a maximum of 64 communication cycles in which the time slots are used differently.

Frame Format

The message format of FlexRay is shown in the figure below. The transmission begins with a series of control bits. After that the frame ID that is identical to the number of slots or the mini-slots. This 2048 different values. Then follows the number of data words. In contrast to CAN or LIN, FlexRay will be in the amount of transferred data count in words (16 bit). A FlexRay message may be between zero and a maximum of 127 words, ie a maximum of 254 bytes of data. The header is secured by a checksum, a CRC. Together with the header is transmitted to the realization of the cycle multiplexing, a cycle counter. Completed, the data transmission through a 24-bit wide checksum that covers the entire message.

FlexRay - Frame Format

In the figure, the format of the messages at the level of the data link layer is shown, as they get to see the software. At the physical layer scatters the communications controller, very similar to CAN, a whole series of bits one: In the beginning, for example, the trailing-start sequence that is used for synchronization. Similarly, the frame start sequence, which is also used for synchronization purposes. Furthermore, the byte start sequence: These are 2 bits that are sent before any bytes, so that will be sent for every byte transferred effectively 10 bits. The protocol overhead of the overall protocol is thus somewhat larger than for CAN. The data rate is at 10 Mbit / s at best, about 500 kB / s. In practice, it will be somewhat less. Reason: In the beginning and end of a time slot is always left some space, because a message must fit in all circumstances in the time slot.

Another important aspect in the design of FlexRay systems is the fact that the time slots to the individual control units are clearly assigned. This must be in the development phase of the system happen. Also have a placeholder for future extensions to be provided. All these aspects are reduced significantly once the effective data rate for FlexRay systems. The arbitration will find, if one may not say so, in the vehicle, but in the development departments instead! Whoever "wins", is probably related not only with technical requirements.

Network boot and clock synchronization

For the proper functioning of the FlexRay system, it is essential that time-synchronous control units are all working together. During operation, this will be ensured by the clock synchronization. When network boot but special measures are necessary.

FlexRay - network startup and clock synchronization

To network boot there must be at least two, better three so-called cold start nodes. Only this can begin to communicate. Before they do, they check whether the bus system is at rest, or if there is already a communication. If a communication already takes place, they just latched without additional measures in this communication. However, if the bus system is at rest or when the bus earlier in the sleep state, was then first of all a wake-up pattern sent. Then it is checked whether the bus system is still at rest. If so, then sends a cold start node is a collision avoidance symbol. This signals to all other connected controllers any cold start node attempts to start the network. If the bus remains at rest, through this cold, cold start nodes start by sending its normal messages. In these messages are set in the header of the startup frame indicator bit and the sync frame indicator bit. So that these messages can be clearly recognized as messages from a cold start node.

The other cold-starting node to handle the communication only if they have received at least four such messages from the first cold start nodes. Then the way for the "normal" control devices are free to buy into the communication. They must, however, before they can receive the communication, have received at least two messages from two different cold start nodes.

Overall, this means that the launch of a network requires at least 8 full communication cycles. In the current operation will be adjusted, particularly in the Network Idle Time, the local clock signals of all communications controller, leaving the nodes in sync. At least two of the cold start nodes send messages continuously during operation with the corresponding sync frame indicator bit.

All messages for the cold start of the network and the synchronization of your phone only in the static segment. That is, to be configured in the configuration of a network must in any case, a static segment with at least two time slots.