CAN Basics

Standard CAN frame format | Extended CAN frame structure

This page compares standard CAN frame format vs extended CAN frame format and mentions various fields used in standard CAN frame format and extended CAN frame structures.

Standard CAN Frame Format fields

Figure-1 depicts standard CAN frame structure. Following table-1 describes fields used in standard CAN frame format. It uses 11 bit identifier.

Fields	Description
SOF	Start of Frame bit. It marks start of message. It is used to synchronize nodes on the CAN bus.
Identifier	It is 11 bit (binary) in size. It establishes priority of message. Lower the value, higher is the priority.
RTR	It stands for Remote Transmission Request bit. This field is dominant when node requires information from another remote node. All the nodes receive request and all the nodes receive reply. Specific node processes the request based on identifier and transmits the reply.
IDE	Stands for Identifier Extension bit. It indicates standard CAN frame is being transmitted with no extension.
r0	It is reserved for future use.
DLC	Stands for Data length code. It is 4 bits in size. It indicates number of bytes to be transmitted over the CAN bus.
Data	It contains upto 64 bits of application data.
CRC	It is used for error detection. It is 16 bits in size. It holds checksum for application data preceding to it.
ACK	It is 2 bits in size. It contains first bit as ack bit and second bit as delimiter. Each node uses this to show integrity of its data. Node receiving correct message overwrites this bit in original received message with dominate bit as mentioned above to indicate error free message has been transmitted. The node receiving erroneous message leaves this bit as recessive. Moreover it discards the message and hence prompts the sending node to re-transmit the message after re-arbitration process.
EOF	Stands for End of Frame. It is 7 bits in size. It marks end of CAN frame or message.
IFS	stands for Interframe space. It is 7 bits in size. It contains time required by controller to move correctly received frame to its proper position in message buffer area.

Extended CAN Frame format fields

Figure-2 depicts extended CAN frame structure. Following table-2 describes fields used in extended CAN frame format. It uses 29 bit identifier.

Fields	Description
SRR	It stands for Substitute Remote Request. This bit replaces RTR bit of standard CAN message location as placeholder in this extended CAN format.
IDE	It functions as recessive bit in identifier extension. It indicates that more identifier bits are followed. 18 bit extension follows IDE.
r1	It is additional reserved bit for future use.

Note: Illustrated Pictures and Tables are taken from Internet sources for better understanding.