Automotive Semiconductors and Functional Safety
In CAN, what happens when two nodes start transmitting frames at the same time?
- AThe two frames collide and both are corrupted, so both nodes wait a random time before retransmitting
- BThe frame with the smaller ID value (higher priority) continues transmitting and the losing node stops and retransmits later, so the winning frame is not corrupted
- CThe frame with the larger ID value takes priority, and the frame with the smaller ID value is discarded and not retransmitted
Show answer and explanation
Answer: B. The frame with the smaller ID value (higher priority) continues transmitting and the losing node stops and retransmits later, so the winning frame is not corrupted
On a CAN bus, when 0 (dominant) and 1 (recessive) are driven simultaneously, 0 wins. A transmitting node compares each bit it sends with the bus state and, if it sent a 1 but sees a 0, judges that it has lost arbitration and stops transmitting. Since the ID is sent first, smaller ID values have higher priority, and the winning frame arrives intact. This is called non-destructive arbitration. Waiting a random time after a collision is the CSMA/CD method of early Ethernet.
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