Frame Format

The following figure shows the FlexRay frame format. The FlexRay frame has three segments: header, payload, and trailer.

• Header—Includes the Frame ID, Payload Length, Header CRC, and Cycle Count. The Frame ID identifies a frame and is for prioritizing event-triggered frames. The Payload Length contains the number of words transferred in the frame. The Header CRC is for detecting errors during the transfer. The Cycle Count contains the value of a counter that advances incrementally each time a Communication Cycle starts. Additionally, the header includes some indicators to help identify the frame type. The Payload Preamble indicator indicates whether an optional vector is contained within the payload segment of the transmitted frame (for example, a network management vector). The Null Frame indicator indicates whether the frame is a normal or null frame (a frame that does not contain a valid payload). The Sync Frame indicator indicates whether the frame is a special sync frame used for clock synchronization. Finally, the Startup Frame indicator indicates whether the frame is a startup frame to help start the FlexRay cluster.
• Payload—Contains the data the frame transfers. The FlexRay payload or data frame length is up to 127 words (254 bytes), which is more than 30 times greater than CAN.
• Trailer—Contains three 8-bit CRCs to detect errors.