Terminology » Active vs Passive CAN Error Frame : Key Differences

Active vs Passive CAN Error Frame : Key Differences

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Introduction : When a node on the CAN bus detects an error, the protocol mandates transmission of an error frame; but the frame’s form depends on the node’s error state i.e. active or passive.

Active CAN Error Frame

In an “error-active” state, the node sends an active error flag (six dominant bits) to immediately notify all other nodes and abort the faulty transmission.
Active error frames ensure immediate error propagation and message abortion when critical failures occur.

Passive CAN Error Frame

In an “error-passive” state (after accumulating errors), the node sends a passive error flag (six recessive bits), which does not disrupt ongoing communication by other nodes.
Passive frames allow error affected nodes to continue communication without destabilizing the network.

Key differences

Feature	Error Active State	Error Passive State
Purpose	To actively enforce data integrity across the entire network.	To prevent a single faulty node from paralyzing the bus (Fault Confinement).
Error Flag Structure	6 Dominant Bits	6 Recessive Bits
Impact on the Bus	Actively destroys the current message on the bus, regardless of what other nodes are doing.	Cannot destroy a message on its own. It only signals the error locally. The bus is only affected if an Active node also sees the error.
Behavior After Transmission	Can immediately attempt to re-arbitrate and re-transmit its message after the error frame is complete.	Must wait for an additional “Suspend Transmission” period (8 recessive bits) after the Error Frame before it can participate on the bus again.
Node Condition	TEC ( Transmit Error Counter ) ≤ 127 and REC ( Receive Error Counter ) ≤ 127.	TEC > 127 or REC > 127.

Key Points

Every CAN node maintains two internal error counters viz. TEC and REC.

Transmit Error Counter (TEC): Increments when the node causes or detects transmission related errors.
Receive Error Counter (REC): Increments when the node detects errors in messages it is receiving.

These counters determine whether a node is in one of three states: Error Active, Error Passive, or Bus-Off.

Summary: Differentiating between active and passive error frames enables robust, fault aware CAN network behavior. This mechanism helps maintain network stability, fairness and resilience even under error conditions, preserving overall communication integrity.