CAN Protocol Basics Quiz

Study Notes

CAN Bus: A two-wire, high-speed serial communication network used for reliable data exchange between electronic control units (ECUs) in automobiles.
CAN High (CAN_H) and CAN Low (CAN_L) are the two wires used in CAN communication.
Data Transfer: CAN transmits data in frames containing information fields, such as identifier, control, data, and CRC.
Terminated with 120ohm resistor: To prevent reflections and ensure reliable signal transmission.
Message-Based: CAN uses a message-based communication structure, where any node can send a message with a unique identifier.
Multi-master Protocol: Any node can access the bus, enabling multiple devices to communicate simultaneously.
Broadcast Capability: Messages can be sent to one, many, or all nodes on the network.

CAN Frame Format:
- Start of Frame (SOF): Single dominant bit (0) indicates the beginning of a frame.
- Arbitration Field: Identifies the message, containing the 11-bit or 29-bit identifier and RTR bit.
- Control Field: Contains the Data Length Code (DLC) and reserved bits for future expansion.
- Data Field: Carries the actual data being transferred (0 to 8 bytes).
- CRC Field: Contains the CRC sequence and delimiter for error checking.
- ACK Field: Two-bit field for acknowledgment.
- End of Frame (EOF): Seven recessive bits (1) mark the end of the frame.
- Interframe Space: Three recessive bits separate frames for timing and synchronization.

Data Frame: Transmits actual data with a dominant RTR bit (0).
Remote Frame: Initiates the transmission of data by another node with a recessive RTR bit (1).
Error Frame: Transmitted when a node detects an error during communication.
Overload Frame: Signifies that a node is too busy to process messages immediately.

Purpose: Improves data transfer reliability by providing error detection.
Mechanism: A checksum calculation based on the message data is transmitted with the frame. The receiver compares the calculated CRC with the transmitted CRC for error detection.
CAN uses a 15-bit CRC.

ACK Slot: A dominant bit transmitted by all nodes that successfully receive a frame.
ACK Delimiter: A recessive bit following the ACK slot.
Purpose: Ensures all nodes received the frame correctly.

High CAN: For real-time, critical applications like engine control, braking, powertrain, and chassis systems.
CAN Low: For non-critical functions such as door operation and window controls.
CAN FD (Flexible Data): For high-speed, critical real-time applications with extended data bandwidth.

CAN Node: An ECU that participates in the CAN network.
CAN Controller: An interface within an ECU that manages communication with the CAN bus.
CAN Transceiver: A circuit that interfaces between the ECU and the physical CAN bus, converting logic levels to voltage levels (dominant and recessive states).

Function: Acts as a central interconnect for various field buses in vehicles.
Purpose: Enables communication between different bus types (CAN, LIN, FlexRay) and connects to external systems like telematics control units and OBD scan tools.

Advantages over other communication protocols:
- Low cost: Two-wire system reduces hardware complexity.
- High speed: Supports data rates up to 1 MBit/s.
- Multi-master operation: Enables shared access to the bus by multiple devices.
- Broadcast capability: Messages can be sent to one or many nodes.
- Error handling: Reliable message delivery with CRC and ACK mechanisms.
- Flexibility: Nodes can be easily connected or disconnected.
- No message collisions: The arbitration mechanism prevents simultaneous transmission conflicts.
- Standardized: Widely adopted with industry standards like SAE (Society of Automotive Engineers).

Dominant State (Logical 0): Indicates a strong signal, typically represented by a lower voltage level on the CAN bus.
Recessive State (Logical 1): Indicates a weak signal, typically represented by a higher voltage level on the CAN bus.
Wired-AND Operation: When multiple nodes transmit data concurrently, the CAN bus reflects the dominant (lowest) voltage level, effectively performing a logical AND operation.

Function: Delineates frames and ensures proper synchronization between transmissions.
Data and Remote Frames: Separated by an interframe space from preceding frames, regardless of the type.
Error and Overload Frames: Not preceded by an interframe space.