During the CAN bus debugging process, I encountered a problem where message transmission failed. Many engineers face similar issues, and it's often unclear why this happens. In this discussion, we'll explore the reasons behind CAN message transmission failures.
To better understand why CAN messages might fail to transmit, let’s first examine what a properly formatted CAN message should look like. Table 1 presents a standard data frame packet, which represents a typical correctly transmitted CAN message.
| Identifier | RTR | IDE | DLC | Data Field | CRC | ACK Slot | ACK Delimiter | EOF |
|------------|-----|-----|-----|------------|-----|----------|---------------|------|
| 11 bits | 1 | 0 | 4 | Variable | 15 | 2 | 1 | 7 |
Figure 1 shows the standard data frame format, illustrating how these components come together.
The CAN bus operates as a broadcast communication system. To ensure all nodes on the bus correctly receive messages, the sender requires each receiving node to acknowledge receipt before sending. This is why the acknowledgment (ACK) segment exists within the message.
The ACK segment in a CAN frame consists of 2 bits: the ACK Slot and the ACK Delimiter. After transmitting a message, the sender sends two recessive bits in the ACK field. If the receiving node successfully decodes the message, it responds by sending a dominant bit during the ACK Slot.
In the event that no ACK response is detected (i.e., the response remains recessive), the sender triggers an error flag, increases its error counter by 8, and automatically resends the message. If multiple retries still result in no ACK response, the error counter reaches 128 (indicating 16 error frames), and the node transitions into an error passive state, as shown in Figure 3.
What causes ACK segment errors? Possible reasons include:
- Only one valid node exists on the bus. In such cases, the node attempting to send cannot receive an ACK response because there are no other nodes to respond. Consequently, the node repeatedly attempts to resend the message, leading to increasing error counts.
- Baud rates are mismatched or nodes are not initialized properly, preventing ACK responses from being generated.
- Issues with the bus wiring, such as short circuits, open circuits, or reversed connections, disrupt communication and prevent ACKs.
- Nodes connected to the high-speed CAN bus may be using fault-tolerant low-speed CAN, causing mismatches in communication protocols.
When debugging a CAN bus and encountering message transmission failures, always verify whether any of the above issues are present. Using the right tools can significantly simplify the diagnostic process. Figure 4 demonstrates how the CANScope from Zhiyuan Electronics helps identify error frames and their corresponding waveforms. This tool also supports comprehensive testing across the physical layer, data link layer, and application layer, providing valuable assistance to CAN engineers.
For instance, Figure 5 illustrates a CANScope test project, highlighting its capabilities in diagnosing and resolving complex CAN bus issues.
In conclusion, understanding the intricacies of CAN communication and employing the right tools can help mitigate many common pitfalls. Whether troubleshooting hardware issues or analyzing protocol behavior, careful attention to detail ensures reliable CAN bus operation.
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