CAN Bus Explained: Nodes, Frames, and ECUs
- Influx Technology
- Jul 31
- 5 min read
Understanding CAN bus is essential whether you're developing vehicles, monitoring industrial systems, or working on embedded electronics. As one of the most widely used communication systems in engineering, it powers everything from engine diagnostics to medical device telemetry.
This guide covers what CAN is, how it works, and why it still matters today. You’ll also learn how it applies to practical tasks like data logging, fault diagnosis, and system validation.
CAN Bus Explained
The Controller Area Network (CAN) is a system that enables different components of a vehicle or machine to share information. Introduced in the 1980s, it was developed to reduce the complexity of wiring and improve communication reliability between control units.
Rather than running separate wires for each connection, CAN allows devices like the engine control unit(ECU), anti-lock braking system (ABS), and more to send and receive messages over a shared data line. This approach simplifies wiring and enables fast, reliable messaging without needing a central computer.
Today, CAN is used far beyond automotive. You’ll find it in industrial automation, medical equipment, and off-highway vehicles. Many development and diagnostic tools are built to interface directly with CAN systems, helping engineers capture real-time data non-intrusively, ideal for design validation, troubleshooting, and testing workflows.
Common Applications
CAN bus is used across industries that rely on reliable, real-time communication between electronic systems. Common applications include:
Vehicle diagnostics
Reading fault codes, monitoring live parameters, and validating ECU behaviour in development and service environments.
Fleet telematics
Logging GPS, speed, engine load, and fuel usage data to the cloud for operational insights and predictive maintenance.
ECU calibration and flashing
Real-time tuning and firmware updates during development and testing, often involving complex synchronisation across multiple ECUs.
Medical device integration
Synchronising sensors and actuators in high-reliability applications, such as robotic surgical systems and patient monitoring equipment.
Agricultural and off-highway equipment
Coordinating hydraulic, engine, and sensor systems across rugged environments with high electrical noise.
What Does a CAN Bus Look Like?
A CAN bus is a shared twisted two-wire system that allows multiple electronic control units (ECUs), sensors, actuators, and data loggers to exchange messages without the need for a central controller.
Each connected device is called a node - any unit capable of sending, receiving, or listening to messages on the network is a node. All nodes connect in parallel, meaning they share the same two signal wires: CAN High and CAN Low.
While the standard recommends green for CAN Low and yellow for CAN High, this isn’t always followed in practice.
These two wires carry differential signals, where the same data is transmitted in opposite voltage directions. When CAN High increases, CAN Low decreases. Each node measures the difference between the two voltages, which makes the system highly resistant to electrical noise, an essential feature in harsh environments like vehicles and industrial machines.
To maintain signal quality, 120-ohm resistors are placed at both ends of the bus. These prevent electrical reflections that could corrupt data, especially at higher communication speeds.
How ECUs Read and Respond to CAN Messages
When a message is broadcast on the CAN bus, every node in the network receives it; however, not every node needs to respond.
The process begins with the ECU continuously listening to all traffic on the bus. When a message arrives, the ECU checks the message’s identifier to determine if it is relevant to its function. If the ID is relevant, the ECU accepts the message and processes the data; if not, the ECU simply ignores it.
Once the message is accepted, the ECU can take multiple actions. It might update an internal parameter, such as storing the latest RPM reading, trigger a specific response like activating a cooling fan, or prepare a reply to send its data back onto the bus.
This allows each ECU to make independent decisions based on shared information, resulting in a system that is scalable and efficient, without the need for a central controller.
Physical Connections to a CAN Bus
There is no single connector type for CAN (Controller Area Network). The physical interface you use with a CAN bus often depends on your environment, whether you are in a test lab, on a production line, or working with compact embedded systems.
In research and development (R&D) and diagnostics, connectors like DB9 and open screw terminals are frequently used. These connectors are easy to attach and remove, and they are well-supported by various development tools. This makes them ideal for use in test benches, prototypes, and vehicle trials.
In production systems, the choice of CAN connectors is typically influenced by factors such as space, environmental conditions, and industry standards. This could involve using:
OBD-II ports in automotive service tools,
M12 connectors in industrial automation,
JST connectors in drones, 3D printers, and compact electronics.
The physical connector can vary from large and robust for harsh environments to highly compact for embedded systems. However, the CAN protocol remains the same.
When working with any new system, always check the documentation to confirm the wiring and termination details. Some connectors may include power or shielding pins, while others might only expose the CAN High and CAN Low connections.
What Is CAN Bus Baud Rate?
The baud rate defines how fast data is transmitted on the CAN bus. It’s measured in bits per second
(bps).
A higher baud rate allows faster communication, which is important for time-sensitive applications like real-time sensor monitoring or rapid ECU coordination. However, higher speeds are more sensitive to noise and signal reflections, especially over longer cable lengths.
The baud rate must be the same across all nodes on the network. If even one device is out of sync, communication errors will occur.
See our in-depth guide: CAN baud rate and timing
CAN Message IDs
Every message on the CAN bus has a unique identifier called an ID. This number serves three main purposes:
It defines the priority of the message - lower ID numbers have higher priority.
It helps engineers (not the bus itself) know what kind of data the message carries, based on how the system was designed.
It allows each receiving device to check whether the message is relevant and decide whether to process it or ignore it.
These IDs are usually written in hexadecimal, a compact format used to represent binary values more easily.
For example:
0x100 is a hexadecimal number
It's just a shorter way of writing the binary value 0001 0000 0000
That binary form is what the CAN hardware uses to determine message priority during arbitration.
You don’t need to memorise hexadecimal, just know that each message has a number, and the lower the number, the sooner it gets through if two messages try to send at once.
Why CAN Bus Is Still Relevant
Despite the emergence of high-bandwidth protocols such as Automotive Ethernet and FlexRay, the CAN Network remains a foundational technology in many industries. Why?
Real-time responsiveness – CAN is built for live communication, ensuring critical messages are delivered without delay.
Low complexity – Its simple architecture makes it easy to implement, validate, and maintain, even in cost-sensitive or resource-constrained environments.
Noise immunity – Differential signalling and bit-level arbitration make CAN exceptionally robust in electrically noisy conditions.
Widespread standardisation – Based on ISO 11898, CAN is a globally trusted standard across automotive, industrial, medical, and off-highway applications.
Rather than being replaced, CAN often works alongside newer technologies. It continues to serve a critical role in diagnostics, sensor data exchange, safety systems, and embedded control, especially where reliability and timing are non-negotiable.