busoff

Bus-off is a fault condition in Controller Area Network (CAN) where a node’s CAN controller stops transmitting and effectively disconnects from the bus after accumulating too many errors. Each CAN node maintains two 8-bit error counters: Transmit Error Counter (TEC) and Receive Error Counter (REC). The node starts in the error-active state; with errors the counters increase. When the Transmit Error Counter reaches a critical threshold (often 256), the controller transitions to the bus-off state, preventing the node from driving the CAN bus. While bus-off, the node cannot participate in arbitration or transmit, and the fault is isolated to that device, while other nodes may continue to communicate.

Causes of bus-off include faulty hardware, wiring or termination problems, excessive electrical noise, faulty transceivers, or software that generates protocol violations or invalid bit sequences. Bus-off serves to protect the network by preventing a single faulty node from continuously disrupting communication.

Recovery typically requires a reset or reinitialization of the CAN controller, or power cycling of the device. After reset, the node may attempt to rejoin the bus if the fault condition is cleared and the controller reinitializes. Some CAN controllers implement a recovery delay or automatic restart behavior after a bus-off event.

In practice, bus-off is one end of CAN’s error-state model, which also includes error-active and error-passive states. Understanding bus-off is important for diagnosing network faults and ensuring system reliability in automotive and industrial CAN applications.