Sending of messages is basically random (broadcast principle), i.e. messages are not directed at a particular addressee. ECUs requiring information select relevant messages based on the message’s ID (content based addressing).
All ECUs sample the signal many times during one bit time to avoid short interferences. The sample time (bit timing) must be configured individually for each CAN controller.
CAN has the following message (or frame) types:
•Normal messages or data frames transferring ECU information on the CAN bus.
•Remote messages or remote frames enabling ECUs to request necessary information from other ECUs.
•Error frames are dispatched to signal an error that has occurred at the time of transfer.
Bus communication is executed as follows:
•Very important information, which is permanently required by numerous ECUs, is generally sent cyclically.
•As soon as an ECU requires specific information, it sends a message with a specific identifier, indicating that this information is required (remote frame).
•The ECU which can deliver the required information sends the information on the CAN bus and the requested ECU accepts the information independently from the CAN bus.
Structure of CAN Messages
SOF (start of frame): Identifies start of frame.
Arbitration field: Contains the identifier and the RTR bit (Remote Transmission Request). The RTR bit identifies whether the frame is a data frame or a request frame (remote frame).
Control field: To distinguish standard and extended frame formats, the IDE-bit (identifier extension bit) is used. A reserved bit intended for future extensions follows. The last 4 bits (DLC) identify the number of bytes included in the data field.
Data field: Contains the actual message data of message. Its length can range from 0 to 8 byte.
CRC field: Contains a check sum that is used to identify bit errors.
ACK field: Contains the ACK slot. This bit is recessive and overwritten by dominant receivers which have received the data correctly up to this time.
End of frame: Identifies the end of message.
Intermission: Minimum bit time separating consecutive messages. If no station accesses the bus after this time, the bus remains idle (bus idle)