Frame Payload

A maximum of 254 user bytes (payload) can be transported by one message. The payload length parameter shows the size of the payload in words. The payload length exhibits the same value for all messages transmitted in the static segment. The system designer must define this value during the configuration phase. Since dynamic messages are not restricted to a fixed payload size, the payload length may assume different values for such messages.

In a message transmitted in the static segment, it is possible to use the first twelve bytes to transmit the network management vector. This requires setting the payload preamble indicator in the header. The network management vector is available for implementing network management in a FlexRay cluster.

If the payload preamble indicator is set for a dynamic FlexRay message, this indicates that the payload begins with a network management vector. In a dynamic message, this indicates that the first two bytes are the message identifier. The message identifier gives the system designer a way to specify the payload more precisely; it may be used as a basis for making finer distinctions in acceptance filtering.

In special cases, the sender may send the payload of a message exclusively with zeros. Such a case exists if a FlexRay controller needs to send a static message according to the communication schedule, but the buffer corresponding to the message is blocked by the host. This might occur, for example, if the host is accessing this buffer itself at the given moment. Because the FlexRay controller cannot access the data in the buffer, it automatically transmits the static message as a null frame. In this case, the null frame indicator assumes the value zero in the message header.

To protect the payload, the CRC method (CRC: Cyclic Redundancy Check) is used. This is a very powerful error detection method. In the framework of the CRC method, a CRC sequence is computed based on the header and payload and a generator polynomial defined by the FlexRay specification. This CRC sequence is appended to the header and payload as the trailer.

The CRC sequence for a message corresponds to a multiple of the header and the payload. The receiver of the message can detect any transmission error with very high reliability. An error is detected when division by the generator polynomial yields a remainder. With a payload of up to 248 bytes, the CRC method guarantees a hamming distance of six. For a larger payload the hamming distance is four, resulting in a lower error detection capability.


Last modified: Friday, 27 April 2018, 9:23 AM