What Is CAN Bus? How Do Electronic Control Units Communicate in Vehicles?

What Is CAN Bus?

CAN stands for Controller Area Network. CAN Bus is a serial communication system that allows electronic control units and other devices to exchange data over the same network without requiring a central computer.

The technology was originally developed by Bosch to make wiring and communication between electronic control units in automotive applications more efficient.

A modern vehicle may contain dozens of electronic control units, and in some cases more than one hundred.

The engine control unit, automatic transmission control module, ABS/ESC system, airbag control unit, instrument cluster, climate control system, body control module, and driver assistance systems continuously exchange information with one another.

Connecting every control unit directly to every other module using separate wires would create an extremely complex electrical architecture.

CAN Bus allows multiple control units to share the same communication network.

Why Was CAN Bus Developed?

As the number of electronic systems in vehicles increased, so did the amount of information that needed to be shared between control units.

For example, engine speed information is not required only by the engine control unit.

The same information may also be used by:

• Automatic transmission control
• Instrument cluster
• Stability control system
• Cruise control
• Driver assistance systems

Through the CAN network, the engine control unit can broadcast this information, allowing other control units that require it to use the same message.

This architecture reduces the need for separate data wiring between individual electronic systems.

As a result, CAN technology can:

• Reduce the amount of wiring
• Simplify vehicle electrical architecture
• Enable fast data exchange between control units
• Support easier fault diagnosis
• Make it easier to integrate new electronic functions into the vehicle

How Does CAN Bus Work?

CAN is not based on device addresses in the traditional sense.

Instead of sending data directly to a specific receiver, devices on the network broadcast messages.

Each message has an identifier.

This identifier is used to indicate what kind of information the message carries and also plays a role in determining its priority on the network.

For example, one message may represent engine speed, another may contain wheel-speed data, and another may carry steering-angle information.

All compatible nodes on the network can physically receive the message.

Each control unit then determines, according to its software configuration, which messages it needs to process.

For this reason, CAN uses a message-based broadcast communication architecture.

What Are CAN High and CAN Low?

In a typical high-speed CAN physical layer, communication is usually carried over two conductors:

CAN High (CAN-H)
CAN Low (CAN-L)

Together, these two lines form a differential signal.

Instead of evaluating the voltage of only one line, the receiver measures the voltage difference between the two lines.

The Advantage of Differential Communication

The automotive environment is electrically noisy.

Ignition systems, electric motors, alternators, relays, power electronics, and other electrical loads can introduce interference into communication lines.

When similar external interference affects both CAN-H and CAN-L, the voltage difference between the two lines can remain largely preserved.

For this reason, differential signaling can provide greater resistance to electromagnetic interference than single-ended communication methods.

What Are Dominant and Recessive Bits?

In CAN communication, the terms dominant and recessive are used in addition to the conventional digital concepts of 0 and 1.

In general:

Dominant bit = Logical 0

Recessive bit = Logical 1

One of the important characteristics of the CAN network is that a dominant bit can override a recessive bit.

This behavior forms the basis of the arbitration mechanism used when multiple control units attempt to transmit data at the same time.

How Does CAN Arbitration Work?

Multiple control units on a CAN network may begin transmitting messages at the same time.

When this happens, the system must determine which message will be transmitted first.

CAN accomplishes this through a mechanism known as bitwise arbitration.

While each device transmits the identifier of its message, it simultaneously monitors the state of the bus.

If a control unit transmits a recessive bit but detects a dominant bit on the bus, it recognizes that another message has higher priority and stops transmitting.

The winning device continues sending its message.

How Is Message Priority Determined in CAN?

In CAN, a numerically lower identifier has a higher priority during arbitration.

This is because dominant bits appear earlier in lower identifier values.

For example, messages related to braking or safety systems can be assigned higher priorities in an appropriately designed network.

One of the key advantages of this mechanism is that the higher-priority message can continue transmitting without being corrupted, rather than both messages colliding and being retransmitted randomly afterward.

What Does a CAN Message Frame Contain?

CAN data is transmitted within a defined message frame.

A classical CAN data frame includes several major fields.

Identifier

The identifier determines the type of message and its arbitration priority.

Classical CAN systems can use either an 11-bit standard identifier or a 29-bit extended identifier.

Control Field

The control field contains information related to the structure of the message and the amount of data being transmitted.

Data Field

This is the section that contains the actual application data.

In Classical CAN, a single message can carry a maximum of 8 bytes of data.

CRC

The Cyclic Redundancy Check (CRC) is used to detect whether the message has been corrupted during transmission.

ACK

The ACK mechanism forms part of the acknowledgement process indicating that the message has been correctly received by at least one suitable node on the network.

A CAN frame also includes additional fields such as the start of frame, end of frame, and other control-related sections.

How Does Error Detection Work in CAN Bus?

One of the reasons CAN became widely used in automotive and industrial applications is its strong error-detection capability.

The CAN protocol uses several error-checking mechanisms together.

These include:

• CRC checking
• Bit monitoring
• Bit stuffing checks
• Frame format checking
• ACK checking

If a control unit detects an error during transmission, CAN error-notification mechanisms can be triggered on the network.

What Are Error Active, Error Passive, and Bus Off?

CAN controllers do more than simply detect communication errors.

They also maintain error counters to prevent a faulty device from continuously disrupting the entire network.

As the number of errors associated with a node increases, the device may transition into different error states.

Error Active

The device continues communicating normally and can actively signal detected errors.

Error Passive

When the error count reaches a certain level, the device's ability to actively signal errors on the network becomes restricted.

Bus Off

If a node generates too many transmission errors, the CAN controller can remove itself from the bus.

This mechanism helps prevent a malfunctioning electronic control unit from continuously disrupting communication across the entire CAN network.

Why Are Termination Resistors Used in CAN Bus?

High-speed CAN networks typically use a 120-ohm termination resistor at each physical end of the bus.

When the two 120-ohm resistors are considered in parallel, an appropriate resistance measurement between the CAN lines will typically show approximately 60 ohms.

The main purpose of the termination resistors is to reduce signal reflections along the transmission line.

At higher data rates, the wiring cannot be treated merely as a simple electrical connection; it must also be considered as a transmission line.

Incorrect termination can lead to:

• Communication errors
• Poor signal integrity
• Intermittent CAN faults
• Control units dropping off the network

For this reason, checking termination resistance is a common step in CAN network diagnostics.

What Is the Data Rate of CAN Bus?

CAN network speed varies depending on the application and network design.

In common high-speed implementations, Classical CAN can operate at data rates of up to 1 Mbit/s.

Not every network within a vehicle needs to operate at the same speed.

For example, networks used for powertrain systems that require faster data exchange may operate differently from body-electronics networks with lower bandwidth requirements.

As cable length increases, the maximum practical data rate may need to be reduced.

This is because signal propagation delay affects CAN arbitration and bit timing.

What Is CAN FD?

As the amount of data that needed to be transmitted in modern vehicles increased, some limitations of Classical CAN became more significant.

To address this need, CAN FD — CAN with Flexible Data-Rate was developed.

One of the main advantages of CAN FD is that it can carry more data within a single frame.

While Classical CAN can carry a maximum of 8 bytes of data, CAN FD can carry up to 64 bytes per frame.

CAN FD can also allow higher bit rates to be used during the data phase of a message.

The arbitration phase can take place at the nominal CAN bit rate, while compatible networks can switch to a higher speed during the data phase.

This can improve efficiency in modern vehicle architectures where software data, sensor information, and larger data packets need to be transmitted.

Are CAN Bus and OBD-II the Same Thing?

No.

CAN Bus and OBD-II (On-Board Diagnostics II) can be related, but they are not the same technology.

CAN is a communication network used between electronic control units.

OBD-II is a standardized diagnostic framework used to access vehicle diagnostic information.

In modern vehicles, OBD-II diagnostic communication can be carried over CAN.

For this reason, when a diagnostic tool is connected to the OBD connector, it may communicate with certain control units through the CAN network.

However, the vehicle's CAN network is not used only for OBD diagnostics.

Control units continuously exchange CAN messages during normal vehicle operation.

What Symptoms Can CAN Bus Faults Cause?

Because the CAN network connects many electronic control units, communication faults can produce a wide range of symptoms.

Examples include:

• Multiple warning lights appearing at the same time
• Inability to communicate with certain control units using a diagnostic tool
• Warnings from multiple systems appearing on the instrument cluster
• Electronic systems becoming temporarily unavailable
• Communication-related DTCs being stored

A CAN fault does not necessarily mean that the control unit itself has failed.

Possible causes can include:

• Broken wiring
• Short circuits
• Connector corrosion or oxidation
• Termination faults
• Power supply or ground problems
• A faulty control unit

For this reason, CAN faults require systematic electrical testing and network analysis.

What Is the Difference Between CAN Bus and LIN?

LIN (Local Interconnect Network) is a communication technology developed for lower-cost, lower-speed applications compared with CAN.

LIN is commonly used for systems such as:

• Power windows
• Seat controls
• Mirrors
• Climate-control actuators
• Simple body-electronics functions

These applications generally do not require high bandwidth.

CAN, on the other hand, is widely used in applications that require higher performance and reliability, including engine, transmission, braking, and more complex control systems.

In modern vehicles, CAN and LIN do not necessarily compete with one another. They can be used together for different functions within the same electronic architecture.

What Is the Difference Between CAN Bus and FlexRay?

FlexRay was developed for automotive applications requiring higher data rates and deterministic communication.

It has been used particularly in certain advanced chassis and control applications.

However, FlexRay is more complex and generally more expensive than CAN.

With the development of CAN FD and the growing adoption of Automotive Ethernet, the distribution of communication technologies within vehicle architectures is also changing.

What Is the Difference Between CAN Bus and Automotive Ethernet?

Modern vehicles generate increasing amounts of data because of cameras, radar sensors, advanced driver assistance systems, high-resolution infotainment systems, and centralized computing platforms.

The bandwidth requirements of these systems can be far greater than what traditional CAN networks can provide.

For this reason, Automotive Ethernet is becoming increasingly important in modern vehicle electronic architectures.

However, this does not mean CAN will disappear completely.

CAN and CAN FD remain highly suitable for control messages, sensor information, and many forms of ECU communication.

Future vehicle architectures are likely to use CAN, CAN FD, Ethernet, and other communication technologies together, with each serving different functions within the same vehicle.

How Is CAN Bus Used in Electric Vehicles?

CAN is not limited to internal combustion engine vehicles.

Electric vehicles also require extensive data exchange between electronic control systems.

For example:

• Battery Management System (BMS)
• Inverter
• Electric motor control system
• Vehicle control unit
• Charging system
• Thermal management system

may continuously exchange information with one another.

Battery SOC, temperatures, allowable charging and discharging power limits, and various system states can be shared over CAN or other in-vehicle network technologies.

For this reason, CAN continues to serve as an important communication infrastructure in electric vehicles.

What Are the Advantages of CAN Bus?

The main advantages of CAN technology include:

• Allowing multiple control units to share the same communication network
• Reducing wiring complexity
• Message-priority arbitration
• Strong error-detection capabilities
• Differential communication that is resistant to electromagnetic interference
• Error-management mechanisms that limit the impact of faulty nodes
• Broad ecosystem support across automotive and industrial systems

These characteristics are among the main reasons CAN has been used in vehicle electronics for decades.

What Are the Limitations of CAN Bus?

Although CAN is an extremely successful communication system, it has some limitations when faced with the growing data requirements of modern vehicles.

The 8-byte data field and limited bandwidth of Classical CAN are not sufficient for modern sensors that generate large amounts of data.

Information such as camera streams or high-volume radar data cannot be transmitted efficiently over Classical CAN.

In larger vehicle networks, correct network topology, termination, cable length, and timing also become increasingly important.

For this reason, modern automotive networks generally do not rely on CAN alone.

Instead, CAN operates as part of a broader in-vehicle communication architecture alongside technologies such as LIN, CAN FD, and Automotive Ethernet.

The Future of CAN Bus

Vehicle electronics are evolving away from traditional architectures containing large numbers of independent ECUs toward more centralized and zonal electronic architectures.

Next-generation vehicles are increasingly using high-performance central computers, zonal controllers, and high-speed Ethernet networks.

Despite this shift, CAN's role in automotive systems is not disappearing.

CAN FD extends the traditional CAN architecture with greater data capacity, while CAN-based communication continues to be used across a wide range of control systems.

CAN technology is also not limited to automotive applications. It is widely used in industrial automation, mobile machinery, agricultural equipment, medical devices, and various embedded systems.