Скачать или смотреть Understanding Arbitration in CAN Bus: Why Higher CAN ID Nodes Often Lose

Learn why nodes with `higher CAN IDs` consistently lose arbitration in CAN Bus and find solutions to optimize your CAN protocol performance.
---
This video is based on the question https://stackoverflow.com/q/74261995/ asked by the user 'Ashfin' ( https://stackoverflow.com/u/17848953/ ) and on the answer https://stackoverflow.com/a/74275812/ provided by the user 'Lundin' ( https://stackoverflow.com/u/584518/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Node with higher ID always losing arbitration in CAN Bus

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Arbitration in CAN Bus: Why Higher CAN ID Nodes Often Lose

The Controller Area Network (CAN) is a robust communication protocol widely used in automotive and industrial applications. As a beginner in the CAN protocol, you may have some questions about how it operates, especially regarding node arbitration. One common query is: What happens when two nodes are continuously sending CAN frames? Does the node sending the frame with the higher CAN ID always lose arbitration?

In this guide, we'll explore this question in detail, breaking down the arbitration process in CAN Bus and explaining the underlying principles of how nodes interact on the network.

The Basics of CAN Bus Arbitration

Before diving into the specifics of why nodes with higher CAN IDs may lose arbitration, it’s essential to understand what arbitration entails.

What is Arbitration in CAN Bus?

Arbitration is the process through which multiple nodes share access to the communication bus.

The CAN protocol uses a method known as non-destructive bitwise arbitration to resolve conflicts when two or more nodes try to transmit messages concurrently.

In this method, each node compares the bits it is transmitting with the bits being sent by others. If a node detects a recessive bit (logic high) while it is transmitting a dominant bit (logic low), it will cease transmission because the dominant bit wins.

The Role of CAN ID in Arbitration

CAN ID (Identifier) plays a crucial role in determining which node gets access to the bus.

Nodes with lower CAN IDs will always have higher priority over nodes with higher CAN IDs during arbitration.

This priority is key in ensuring that more critical messages can be sent without delay.

The Question: Do Higher CAN IDs Always Lose?

Yes, generally, nodes with higher CAN IDs tend to lose arbitration. Here’s how the process unfolds:

Initial Arbitration: When two nodes attempt to send a message, they initially look for a recessive idle state on the bus. The node with the lower CAN ID will dominate the arbitration.

Winning the Frame: The node with the lower ID sends its data frame first, and once the transmission is complete, the bus switches to three recessive bits for the Inter-frame Space (IFS).

Continuous Transmission: If these nodes continue to send frames in quick succession, the one with the lower CAN ID will consistently win subsequent rounds of arbitration as long as the bus remains active.

Important Considerations

RTR/SRR Bits and IDE: While your understanding is largely correct, it is also essential to remember that arbitration is influenced by several factors, including the RTR (Remote Transmission Request) bit, the SRR (Substitute Remote Request) bits, and IDE (Identifier Extension) bits. For example, an 11-bit frame (standard CAN) will always take precedence over a 29-bit frame (extended CAN) if the identifiers are the same.

Bus Load Considerations: If you notice very high bus load (100% utilization), it could indicate issues within your network. Continuous high utilization may result from network errors or misconfigured nodes that could affect overall communication.

Conclusion

In conclusion, understanding how arbitration works in the CAN Bus is vital for anyone working with this protocol. Nodes with higher CAN IDs generally lose arbitration due to the prioritization of lower identifiers. Remember to also consider other factors, such as bit types and overall bus conditions, to optimize your CAN network's performance effectively.

If you're facing issues in your applications or have queries regarding optimization techniques, feel free to share your experiences in the comments below!