A single CAN Bus message can transmit up to 8 bytes of data.
Understanding the CAN Bus Message Structure
The Controller Area Network (CAN) bus is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other in applications without a host computer. A fundamental aspect of CAN communication is the message or "frame" structure, which includes various fields.
While the overall frame has many parts (like arbitration ID, control field, CRC, etc.), the core payload, where the actual application data is carried, has a specific size limit.
The Data Field: The Payload of Your Message
The most direct answer to the question "CAN bus message size?" usually refers to the size of the data field within a CAN frame. This field is where the information you want to send (e.g., engine speed, sensor readings, button status) is placed.
Based on the standard specification:
- Up to 8 bytes of data can be transmitted through a single CAN Bus message.
This means a single message can carry anywhere from 0 bytes (a data-less frame, often used for remote requests) up to a maximum of 8 bytes of payload data.
Practical Implications of the 8-Byte Limit
The 8-byte data limit per message has significant implications for system design:
- Data Packaging: Information larger than 8 bytes must be split across multiple CAN messages. This requires implementing protocols (like CANopen or J1939 TP) to handle segmentation and reassembly.
- Efficiency: For small data packets, CAN is very efficient. For transmitting large data blocks, the overhead of multiple frames and transport protocols can become significant.
- Message Design: Engineers must carefully design their CAN messages to fit essential data within the 8-byte limit. This might involve scaling values, using bit flags, or prioritizing critical data.
Examples of Data Transmission
Here are some simplified examples of how data fits into the 8-byte limit:
- Engine Temperature: A single 2-byte integer representing temperature can easily fit.
- Multiple Sensor Readings: Four 2-byte sensor values (8 bytes total) can potentially fit in one message.
- Vehicle Identification Number (VIN): A 17-byte VIN cannot fit in a single message and requires segmentation.
Breakdown of a Standard CAN Data Frame
While the data field is the payload, the complete CAN data frame includes other fields that contribute to the overall message size on the bus (but aren't part of the user's data). Understanding this helps differentiate the payload limit from the total transmission size.
| Field | Description | Size (approx.) |
|---|---|---|
| Start of Frame | Indicates the beginning of the message | 1 bit |
| Arbitration Field | Message ID and Remote Transmission Request | 12 or 32 bits |
| Control Field | Data Length Code (DLC) - specifies data size | 6 bits |
| Data Field | The actual payload (0 to 8 bytes) | 0 to 64 bits |
| CRC Field | Cyclic Redundancy Check for error detection | 15 bits |
| ACK Field | Acknowledgment slot | 2 bits |
| End of Frame | Marks the end of the message | 7 bits |
| Interframe Space | Space between messages | 3+ bits |
Note: Sizes are approximate and can vary slightly based on CAN standard (Standard vs. Extended ID) and specific implementation.
The 8-byte limit specifically refers to the Data Field.
Summary
In conclusion, when asking about the CAN bus message size, you are typically referring to the data payload capacity. Based on the standard, this is limited to a maximum of 8 bytes per message.
