CAN Bus Arbitration

Overview

CAN bus arbitration defines how multiple ECUs can start transmitting at the same time without causing data collisions. It ensures deterministic bus access by resolving priority bit by bit during transmission.

Prerequisites

• Basic understanding of CAN frame IDs
• Difference between dominant and recessive bits

Concept Breakdown

Priority via Identifier

Lower numerical CAN IDs have higher priority. This priority is enforced directly on the bus during transmission.

Dominant vs Recessive Bits

• Dominant (0) overwrites recessive (1)
• All nodes monitor the bus while transmitting

Step-by-Step Logic

1. Multiple ECUs detect an idle bus
2. Each ECU starts transmitting its identifier
3. When a recessive bit is overwritten by a dominant bit, the ECU loses arbitration
4. The losing ECU stops transmitting immediately
5. The winning ECU continues without interruption

No data is corrupted. No retry delay is needed.

Common Mistakes / Misconceptions

• "Arbitration causes collisions" → ❌ Arbitration prevents collisions.

• "Higher ID means higher priority" → ❌ Numerically lower IDs win.

Practical Implications

• Real-time guarantees for safety-critical ECUs
• Bus load planning depends on ID assignment
• Attackers can abuse arbitration via high-priority frames

References

• ISO 11898-1
• Bosch CAN Specification 2.0
• Vector CAN Technical Overview