CAN bus

[1] A controller area network (CAN) or CANBUS is a vehicle bus standard designed to allow efficient communication primarily between electronic control units (ECUs). Initially developed to reduce the complexity and cost of automobiles electrical wiring via multiplexing, the CAN bus protocol has since been adopted in various other contexts, such as 3D printing. This broadcast-based, message-oriented protocol ensures data integrity and prioritization through a process called arbitration, allowing the highest priority device to continue transmitting if multiple devices attempt to send data simultaneously, while others back off. Its reliability is enhanced by differential signaling, which mitigates electrical noise. If one side of the differential signal is lost, the system can switch to one wire communication. This ensure data communication for critical systems. Common CAN protocol versions are CAN 2.0, CAN FD, and CAN XL which vary in data rate abilities and max data payload sizes.

Upsides CANBUS replaces big heavy bundles of dedicated wiring with via simple 2-wire twisted pair, drastically reducing vehicle weight, cost, and complexity while allowing many electronic control units to share data instantly. [D1] It's also cheap.

It has multimaster architecture as it has many nodes that can broadcast on the bus without a centralized controller managing traffic.

CAN H and CAN L

CAN H (CAN High) and CAN L (CAN Low) are the 2 dedicated data lines forming a CAN bus. They use differential signaling (the voltage difference between the 2 wires) to provide high electrical noise immunity in cars and industrial environments.

IO measuring 1 wire's volage relative to ground, a CAN transceiver reads teh differential voltage between CAN H and L (Vdiff = VCAN H - VCAN L) to determine the binary logic states making the signal immune to electric noise.

A CAN bus frame's binary values 0 and 1 represent physical electrical states on the bus, known respectively as dominant and recessive states:

Bit 0 (dominant state)

A logic bit 0 value represents the dominant state is when both CAN H > CAN L and is represented at ~2.5 V on CAN H and CAN L = 0 V.

E.g., CAN H = 3.5 V and CAN L = 1.5 V, then the differential voltage is 2 V. The transceiver actively pulls CAN H higher and CAN L lower. A dominant bit overrides a recessive bit on the network, the key for bus arbitration.

Electrical behavior: A node actively drives current through the network, creating a high differential voltage (2 V) between the CAN H and CAN L lines.
Bus dominance: If 1 node sends a 0 while another sends 1, the 0 overrides the 1 on the physical wires, ensuring high-priority messages win arbitration without interrupting communication.

Bit 1 (recessive state)

A logic bit 1 value represents the recessive state, is when both CAN H and CAN L = 2.5 V. So the differential voltage is 0 V. Both wires are at the same baseline voltage, and no device actively drives the bus.

Electrical behavior: No node actively drives the bus. Output transistors are switched off, and the termination resistors passively pull the differential voltage down to neutral (≈0 V).
Idle/Passive State: The bus defaults to a recessive state (all 1s) when no node is actively transmitting data or during mandatory frame delimiters like the End of Frame (EOF).

Types

CAN bus is classified as 3 main types based on identifier size and data transfer capabilities:

CAN 2.0A uses 11-bit Message Identifiers (Standard CAN).
CAN 2.0B uses 29-bit Message Identifiers (Extended CAN), allowing for a significantly larger range of unique message IDs.
CAN FD (Flexible Data-Rate) allows for higher data transfer rates and larger payloads compared to standard CAN 2.0.

Transmission speeds

A 125 kbps is considered as low speed.

A 1 Mbps is considered as high speed.

A 15 Mbps is Flexible Data-Rate (CAN FD)

Frame structure

CAN bus uses 4 frame types to manage communication, data transfer, and error handling across the network:

The data frame is the main frame used to send actual message payloads (up to 8 bytes in standard CAN or 64 bytes in CAN FD) from a transmitter to one or more receivers.
The remote frame is sent by a node to explicitly request data from another node on the bus. It contains the target message identifier but no data payload (the RTR bit is set to recessive / 1).
The error frame is broadcasted immediately by any node that detects a transmission error (such as a CRC failure or bit mismatch). It intentionally violates bit-stuffing rules to interrupt current bus activity so all nodes discard the corrupted message and the sender can retransmit.
The overload frame is sent by a receiving node currently too busy to process incoming data. It requests an extra delay between the current and next Data/Remote frame to allow the node's processor to catch up.

Frames fields

A standard CAN Bus data frame, the most common frame type, consists of 7 main fields: Start of Frame (SOF), Arbitration Field, Control Field, Data Field, Cyclic Redundancy Check (CRC), ACK Field, and End of Frame (EOF).

Start of frame (SOF) is 1 dominant bit (0) that alerts all nodes to an incoming message and synchronizes their internal clocks.
The arbitration field has a the 11-bit (or 29-bit) message identifier (ID) bit to indicate the message priority for the node with the lowest numeric ID wins control of the bus without collision. It uses the RTR (Remote Transmission Request) the frame from a reote request. The field's name refers to that CAN bus does bus arbitration, the automated decision-making process resolving conflicts if 2+ nodes try to transmit messages simultaneously.

arbitrator (general): settles dispute between competing parties to decide who wins

The control field uses the data length code (DLC) to specify how many bytes of data are in the payload (0-8 bytes).
The data field has the actual data payload being transmitted (0 to 8 bytes / 0 to 64 bits).
The CRC field has a 15-bit Cyclic Redundancy Check code and a delimiter bit to detect data corruption during transmission.
The acknowledge (ACK) field is a 2-bit field where any receiving node that correctly decodes the frame overwrites the transmitter's recessive bit with a dominant bit (0) to signal successful receipt.
The end of frame (EOF) consists of 7 consecutive recessive bits (1s) indicating the message is complete, followed by an Inter-Frame Space before a new frame can start.

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CAN bus

CAN H and CAN L

Bit 0 (dominant state)

Bit 1 (recessive state)

Types

Transmission speeds

Frame structure

Frames fields

References

My documents

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