Principle of Bus Access
18/32

In a FlexRay cluster, the FlexRay nodes are granted access to the communication medium in two different ways: first by the TDMA method (Time Division Multiple Access), and second by the FTDMA method (Flexible Time Division Multiple Access), whose core consists of the TDMA method.

The TDMA method is based on a communication schedule, which is organized into a number of time slots (static slots) of equal length, each assigned to a FlexRay node. During communicative operation a FlexRay node is granted access to the communication medium (bus) according to this schedule. From the first to the last static slot, the FlexRay nodes assigned to the static slots obtain exclusive access to the bus to transmit the messages assigned to the static slots.

The communication schedule is executed periodically by all FlexRay nodes during communicative operation; as a result, all static messages are transmitted with a specified period, i.e. deterministically. The communication schedule defines nothing other than a communication cycle, or stated more precisely, the FlexRay communication cycle.

For asynchronous processes,or for sporadic transmission of messages the TDMA method is not an ideal solution. Therefore, FlexRay technology offers the option of extending the cycle by a dynamic segment for messages in a FlexRay cluster that should not only be transmitted on a fixed time schedule, but also on an event driven basis.

The communication cycle is then composed of the combination of a static segment and a dynamic segment. The dynamic segment exhibits a fixed time length to assure deterministic data communication in the static segment despite the addition of dynamic message transmission.

The dynamic segment is based on the FTDMA method. The difference between the FTDMA and the TDMA method is that the dynamic messages defined in the communication schedule can be transmitted by the relevant FlexRay nodes as needed. This means that the time point of message transmission is not predictable. Because the dynamic segment has a finite length, there may be FlexRay nodes wishing to send that will not be able to transmit their dynamic messages in the current cycle.