CAN

A controller area network (CAN) is a serial communication protocol that allows electronic control units (ECUs) to exchange data across a vehicle without a central computer. Instead of point-to-point wiring between every sensor and actuator, CAN uses a shared twisted-pair cable backbone, typically running at 250 kilobits per second on low-speed circuits or 500 kilobits per second on high-speed circuits in modern vehicles. Each device (the engine control module, transmission controller, anti-lock brake module, and so on) listens to and broadcasts messages onto this common bus simultaneously.

CAN was introduced in the early 1990s by Bosch as a way to reduce wiring harness complexity and weight. A single car now contains dozens of ECUs: replacing individual sensor wires with message traffic on a pair of wires cut hundreds of metres of copper per vehicle. The protocol uses a message-based architecture where each transmitted frame contains an identifier, data bytes, and error-checking bits. Any node can transmit when the bus is free, but if two nodes transmit at once, the one with the lower-priority identifier backs off automatically. This collision resolution happens in microseconds and requires no central arbiter.

Variants and physical layout

High-speed CAN typically runs on a shielded twisted pair at 500 kbps and handles critical control systems like the powertrain. Low-speed CAN often uses four-wire (two twisted pairs) at 125 kbps and manages body functions such as lighting and door locks. Fault-tolerant CAN variants exist for aerospace and industrial use, but automotive applications rely mainly on the ISO 11898 standard. The network is terminated at both ends with 120-ohm resistors to prevent signal reflections on the twisted pair.

Failure modes in CAN systems are well understood: an open-circuit wire prevents all communication on that bus; a short to battery or ground corrupts all frames; a stuck transmitter (one ECU continuously broadcasting) will lock the bus. Modern vehicles run multiple independent CAN networks to isolate critical systems. A failed body-control CAN might disable window motors but must never affect engine or brake operation. Technicians diagnose CAN faults using a scope or CAN analyzer that reads frame traffic in real time, checking both message identifiers and data payload against known specifications.

CAN remains the global standard for automotive electrical communication because it is robust, simple, and inexpensive to implement. Each ECU requires only a small transceiver chip (an MCP2551 or equivalent) and a small amount of firmware to handle the protocol logic. The result is that a single twisted pair now carries dozens of sensor inputs, control commands, and diagnostic signals that would have needed thousands of individual wires in older vehicles.