How does CAN arbitration influence which device's PGN is placed on the CAN bus when multiple devices transmit?

How does CAN arbitration influence which device's PGN is placed on the CAN bus when multiple devices transmit?

• A. Arbitration uses time slots scheduled by a master device.
• B. Arbitration uses the message ID (which encodes the PGN priority); the message with the highest priority (lower numeric ID) wins arbitration and is transmitted first.
• C. Arbitration uses the order devices connected to the bus.
• D. There is no arbitration; all devices transmit simultaneously.

Answer: (B)

When multiple devices try to send at the same time, CAN uses hardware arbitration based on the message ID that encodes the PGN priority. Each transmitter puts the ID bits on the bus, and CAN represents bits as dominant (0) or recessive (1). If a transmitting node sees a recessive bit on the bus while another node is actively driving a dominant bit for that same position, the higher-priority message (the one with the lower numeric ID) continues, and the lower-priority node stops transmitting. This means the message with the highest priority, indicated by the lowest ID value, wins arbitration and is placed on the bus first. In NMEA 2000, the PGN is encoded into the CAN ID, so the device with the highest PGN priority (lowest numeric ID) wins and its PGN goes onto the CAN bus. The other devices back off and may retry after the bus becomes free. The other ideas don’t fit because CAN isn’t time-slotted or master-scheduled, and there is no rule based on the order of devices physically connected. There is indeed arbitration, which prevents simultaneous transmissions.

When multiple devices try to send at the same time, CAN uses hardware arbitration based on the message ID that encodes the PGN priority. Each transmitter puts the ID bits on the bus, and CAN represents bits as dominant (0) or recessive (1). If a transmitting node sees a recessive bit on the bus while another node is actively driving a dominant bit for that same position, the higher-priority message (the one with the lower numeric ID) continues, and the lower-priority node stops transmitting. This means the message with the highest priority, indicated by the lowest ID value, wins arbitration and is placed on the bus first. In NMEA 2000, the PGN is encoded into the CAN ID, so the device with the highest PGN priority (lowest numeric ID) wins and its PGN goes onto the CAN bus. The other devices back off and may retry after the bus becomes free.

The other ideas don’t fit because CAN isn’t time-slotted or master-scheduled, and there is no rule based on the order of devices physically connected. There is indeed arbitration, which prevents simultaneous transmissions.