CAN FD is an advanced version of CAN. The significant difference lies in the data rate it can offer.
Original CAN offers 11-bit (standard) and 29-bit (extended) data rates, whereas CAN FD offers flexible data rates, ranging from 0-64 bytes per frame. BOSCH developed both to support the ever-growing need for data and technology in the automotive industry.
BOSCH originally made the Controlled Area Network Bus (CAN Bus), a communication protocol for automotive applications. With vehicle innovation and multiple ECU's coming into the picture, there was a need for a communication method that could cope with ever-growing complexity, speed, and lots of data. CAN bus played an essential role, preventing the need for miles of wire for communication between different modules.
On the other hand, CAN FD was released as an advancement to the original CAN-Bus protocol when the features offered by the original CAN started falling short of the technology it had to support. CAN Flexible Data rate (specified in ISO 11898-1:2015) is practically a more capable CAN. This flexible signal transmission provides automotive electronics communication with increased bandwidth, and all the required functionality cost-effectively. Moreover, it offers upgradation to almost every feature CAN holds and is a better alternative for more advanced data and bandwidth requirements.
The original CAN 2.0 bus has many features that make it an ideal choice for applications where the number of ECU’s is more and the bandwidth utilization is less. CAN bus supports a maximum message payload of 8 bytes per frame at a maximum data rate of 1Mbps. CAN FD supports a flexible message payload, ranging from 0, 8, 12, 16, 20, 24, 32, 48, 64 bytes per frame at 2, 5 and 8 Mbps. The CAN FD Protocol also has two independent bit rates for the arbitration and data phases. The arbitration phase use the same bit timing as the original CAN, but the data bit rate is either similar or higher than the arbitration bit rate.
The below table consists of a few basic differences between a original CAN and CAN FD:
There are many more differences that can be understood by knowing both the technologies in detail. The knowledge of frame formats, error handling techniques, and other features give a clearer picture of how one differs from the other.
Frame Formats:
Frame Format for a standard CAN bus (11 bit)
SOF: Start of Frame. Marks the beginning of the data frame. (A dominant 0)
ADDRESS FIELD: This field decides the priority of the data. The lower the address higher the priority. An address with a maximum number of dominant "0" will hold the highest priority. No two nodes can transmit the same message address at the same time.
RTR: Remote Transmission Request is used to request data from the nodes that are not needed to send the data continuously. This is used if any node wishes to communicate with another specific node. This directly reduces congestion on the bus as direct communication takes place between the nodes.
ARBITRATION FIELD: The arbitration process is the one that helps in deciding which node gets to play master for a given point of time. Any node that transmits a logical "1" when another node transmits a logical 0 "drops out" or loses the arbitration. This means that the node that transmits the first "1" loses arbitration; a dominant "0" always wins. The node that fails arbitration re-queues its message for re-transmission, and the CAN frame bit-stream continues without error until only one node is left transmitting.
IDE: Identifier Extension Bit is for future use/extension. A recessive IDE will result in another 18 bits of the address.
RSRV: Reserve Bit. As the name suggests, it is kept for future up-gradations. • DLC: Data Length Code (0-8 bytes).
DATA: User-defined data (0-64 bits).
CRC: Cyclic Redundancy Check for error/data corruption detection.
ACK: Acknowledgement by the receiving end.
EOF: End of Frame (7s bit recessive "1" s)
Frame Format for CAN FD
The Frame Formats for original CAN and CAN FD may not seem very different. But a few added fields in a CAN FD frame format are not present in the CAN bus.
RRS: Remote Request Substitution (always a dominant 0). The remote frames are not at all supported in CAN FD. (In original CAN, there is RTR (Remote Transmission Request) for identifying the data frames and remote frames)
EDL: Extended Data Length (always a recessive 1) for managing larger payloads and faster bit-rates in CAN FD.
FDF: Flexible Data Rate Format (always a recessive 1) used to indicate Flexible data frame format usage.
BRS: Bit Rate Switch helps determine the bit rate of a data frame. 1. Dominant 0 signifies that the arbitration rate for the CAN FD data frame is up to 1Mbit/sec.
2. Recessive 1 signifies a higher/faster arbitration rate for the CAN FD data frame ranging up to 5Mbit/sec.
ESI: Error State Indicator
1. A dominant 0 indicates the error-active mode.
2. A recessive 1 indicates the error-passive mode.
DLC: Data Length Code is a 4-bit code in CAN FD which denote the number of data bytes in the frame. (DLC values ranging from 1001 to 1111 are used to specify the data lengths of 12, 16, 20, 24, 32, 48, and 64 bytes).
CRC: The Cyclic Redundancy Check is 17 bits long for up to 16 bytes of data or 21 bits for 20-64 bytes. Its length depends upon the length of EDL and DLC bits. CAN FD always uses 4-fixed stuff bits that improve communication reliability.
Features of CAN and CAN FD:
Original CAN
CAN uses pair of twisted wire cables.
It offers easy and simplified handling.
Multiple ECU’s can be connected on the same CAN-BUS.
Developed to support and enable high speed and efficient communication in automobiles.
It has reduced weight and wire costs.
Error reduction.
A quick exchange of data. Uses arbitration process; hence top priority data get access of the bus.
Scope for upgradation.
Standard CAN 2.0A allows 11Bit data transmission (meaning, a total of 2048 different unique messages can be introduced).
Extended CAN 2.0B allows 29Bit data transmission (that sums up to 536+ million messages).
In addition to all the features that the original CAN offer, CAN FD has:
Flexibility to switch between faster and slower data rates.
Increased Protocol efficiency.
Reduced protocol overhead.
Allows more data to fit into a single message.
Better reliability. • Improved network bandwidth.
Up to 30 times more efficient and faster communication between multiple ECU’s.
Decreased number of undetected errors with advanced CRC.
Influx provides you with a wide variety of products that support CAN and CAN FD to cater to the automotive industry's ever-changing needs.
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