The format of a CAN message
Image 2 shows the format of a standard CAN message. The beginning of a message is signaled by a leading dominant bit, followed by the 11-bit message identifier and a further bit which distinguishes between a data telegram and a data request telegram (remote frame). With a remote frame, a network node can trigger the transmission of a certain message by another node in the system. The control field specifies the transmission format (standard/extended) of a message and the number of subsequent data bytes.
The data field of a CAN message can contain between zero and eight data bytes. The data field is followed by the 15-bit CRC segment. This field is used by the receiver to check the received message. In the acknowledge field the transmitter of a message expects the acknowledgement signaling the error-free reception of the transmitted message from at least one receiving network node. This acknowledgement is given by the transmission of a dominant bit in the acknowledge slot by all nodes in the network which received the message free of errors. This acknowledgement is used exclusively for troubleshooting on the transmission side.
Finally, the end-of-frame field displays the complete error-free transmission of a CAN message.