Transmitting More than Eight Bytes of Data on the CAN Bus Network Protocol

The Controller Area Network (CAN) protocol is a widely used protocol for transmitting messages in distributed control systems. It is designed for real-time communication in harsh environments, such as automotive and industrial settings. However, the standard CAN frame has a maximum payload size of only 8 bytes. When more than 8 bytes of data need to be transmitted, specific techniques are employed to ensure data integrity and efficient communication. This article explores how to handle data transmission beyond the 8-byte limit on a CAN bus network protocol.

Segmentation of Data

One common method to transmit more than 8 bytes of data on a CAN bus is through the use of segmentation of data. This involves breaking the data into smaller segments that fit within the limitations of a standard CAN frame.

Multiple Frames:

When you need to send a message larger than 8 bytes, you can split the data into multiple CAN frames. Each frame can carry up to 8 bytes of data.

Frame Structure:

Each CAN frame consists of an identifier, control bits, data length code (DLC), data field, and CRC (Cyclic Redundancy Check). The sender will transmit multiple frames sequentially, with each frame containing a portion of the data.

Using CAN FD (-flexible data rate)

Another method for transmitting more than 8 bytes of data is to use CAN FD (flexible data rate). CAN FD allows for a larger data payload, up to 64 bytes in each data frame, making it more suitable for applications requiring higher data rates or greater amounts of data.

Protocol Layers

Higher Layer Protocols (HLP) are often implemented to handle larger messages in CAN networks. These protocols provide mechanisms for fragmentation and reassembly of messages, ensuring that the receiver can reconstruct the original message from the received frames.

Data Transfer Management

These higher-layer protocols manage message segmentation, ensuring that the receiver can reconstruct the original message from the received frames. This includes:

Handling fragmented data segments Ensuring data integrity Providing mechanisms for error handling and retransmission if necessary

Reassembly on the Receiving End

The receiving node must be able to recognize the sequence of frames and reassemble them into the original larger message. This typically involves:

Keeping track of the order of the frames and their identifiers Recognizing the beginning and end of each message type

For example, if you need to send a 20-byte message, you might send it in three CAN frames as follows:

Frame 1: Contains bytes 1-8 Frame 2: Contains bytes 9-16 Frame 3: Contains bytes 17-20

Handling Lost Messages

When dealing with lost messages, it is crucial to implement error handling mechanisms. This can include:

Retransmission of lost frames if the receiver detects missing data Timeouts and retries Checksums or other mechanisms to detect and correct errors

By using these techniques, you can efficiently transmit and handle data larger than 8 bytes on a CAN bus, ensuring reliable communication in your networked systems.