FlexRay Frame

The signals in a FlexRay cluster are transmitted by FlexRay messages. A FlexRay message always consists of a header, payload and trailer. The header comprises five status bits, the ID, data length code (DLC) and cycle counter. The ID identifies the FlexRay message, while the DLC indicates the number of words transmitted in the FlexRay message. A total of 127 words may be transmitted. The cycle counter indicates the current communication cycle. The Header-CRC protects the DLC, ID and last two status bits. The individual bits of the status field specify the FlexRay message more precisely.



The "null frame indicator" status bit indicates whether or not the payload is valid. The "sync frame indicator" status bit indicates whether the message is a FlexRay message that can be utilized for synchronization. The "startup frame indicator" status bit indicates whether the message is a FlexRay message that is used during startup.

The final bit in the status field, the "payload preamble indicator" status bit, is then transmitted. The meaning of this bit depends on which segment within the communication cycle is being used to transmit the frame. If the frame is transmitted in the static segment, a set bit indicates that a network management vector will be transmitted at the beginning of the payload segment. If the frame is being transmitted in the dynamic segment, a set bit indicates that a Message ID will be transmitted at the beginning of the payload segment.

Transmission of a FlexRay messages ends with a trailer that includes the CRC. The CRC protects the header and payload. For a payload of up to 248 useful bytes, the CRC guarantees a Hamming distance of 6. For a larger payload the Hamming distance is 4.