Architecture of CAN Bus System | Master CAN Interview Questions (2026)

Architecture of CAN Bus System: The Controller Area Network (CAN) bus system is a popular communication protocol widely used in automotive and industrial applications. It allows multiple electronic control units (ECUs) to communicate with each other efficiently over just two wires. In this article, we will explain the basic architecture of a CAN bus system in simple terms, making it easy for beginners to understand.

Learn the basic architecture of a CAN bus system in this beginner-friendly guide. Understand key components like ECUs, CAN-High and CAN-Low wiring, message framing, and arbitration. Perfect for anyone starting with automotive or embedded communication protocols.

Architecture of CAN Bus System

What is a CAN Bus System?

A CAN bus system is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate without a host computer. It is especially useful in environments with high electrical noise, like cars, where reliable communication between multiple ECUs is essential.

Key Components of CAN Bus Architecture

1. Nodes (Electronic Control Units – ECUs)

Each device or node in a CAN bus system contains:

• A Microcontroller (MCU) that processes data.
• A CAN controller that manages message framing, error detection, and protocol handling.
• A CAN transceiver that converts data to and from the physical bus lines.

Nodes can both send and receive messages on the bus, making CAN a multi-master system.

2. Bus Lines: CAN-High and CAN-Low

Communication takes place over two wires called CAN-High (CAN-H) and CAN-Low (CAN-L). These wires carry differential signals, meaning one wire carries the inverse of the other. This helps the system resist electrical noise and improves data integrity.

3. Termination Resistors

At both ends of the CAN bus, 120-ohm resistors are connected to prevent signal reflections and ensure reliable communication.

Layers in the Architecture of the CAN Bus

CAN bus architecture follows international standards defined by ISO 11898. It primarily works on two layers of the OSI model:

• Physical Layer (ISO 11898-2): Defines wiring, signal voltage levels, and data rates. The twisted pair of CAN-H and CAN-L cables operate at speeds up to 1 Mbps (Classical CAN) or higher in CAN FD.
• Data Link Layer (ISO 11898-1): Handles message framing, arbitration (which node gets to transmit when multiple nodes start transmitting simultaneously), error detection, and message acknowledgment.

How CAN Bus Communication Works

• Message Framing:
  Messages, or frames, consist of an identifier, data length, data payload (up to 8 bytes in Classical CAN), CRC for error checking, and control bits.
• Arbitration:
  When multiple nodes send messages simultaneously, the one with the highest priority (lowest identifier number) wins the bus without data loss. This process is called arbitration.
• Error Handling:
  CAN automatically detects errors and retransmits faulty messages, ensuring reliable communication.

CAN Bus Physical Design and Topology

The standard CAN bus uses a linear topology, where all nodes are connected along a single pair of wires with termination resistors at each end. This layout reduces reflections and interference.

CAN Protocol Block Diagram — Overview

The CAN bus follows a multi-master bus architecture, which means that any node (or ECU) connected to the bus can send or receive messages at any time. It also has built-in error handling to keep communication reliable.

The main part of the CAN system is called the CAN core. This core acts like a memory-mapped device to the host processor and is responsible for sending and receiving CAN messages (called frames). It can be integrated easily with the host using standard interfaces like APB (Advanced Peripheral Bus) or AHB-Lite (Advanced High-performance Bus Lite), depending on performance and power needs.

Key Parts Inside the CAN Core

• Acceptance Filters: These help the system decide which messages should be accepted or ignored, making sure the ECU only processes relevant data.
• Interrupts: Programmable signals to alert the processor about new messages or errors.
• Buffers: Memory areas for storing messages before sending or after receiving.
  • There are two types of transmit buffers:
    • Primary Transmission Buffer (PTB): Holds high-priority messages.
    • Secondary Transmission Buffer (STB): Stores lower-priority messages.
  • Buffers work in a First-In-First-Out (FIFO) manner in priority mode.

CAN Bus Layered Architecture

The CAN communication protocol is defined by the ISO 11898 standard and has two main layers:

1. Data Link Layer:
   This layer manages how data is sent between nodes, error checking, and message prioritization. It has two sublayers:
   • MAC (Medium Access Control): Controls which node can use the bus at a given time and handles framing and error detection.
   • LLC (Logical Link Control): Handles flow control, error recovery, and acceptance filtering.
2. Physical Layer:
   This layer defines how the data is physically transmitted over the wires — including voltage levels, timing, and connectors. It consists of:
   • PCS (Physical Coding Sublayer): Encodes and decodes the bits on the bus.
   • PMA (Physical Medium Attachment): Handles framing and synchronization of bits.
   • PMD (Physical Medium Dependent): Deals with actual transmission and reception of bits on the wire.
   • MDI (Medium Dependent Interface): The physical connection to the network medium (cables).

How CAN Bus Works — Data Transmission and Arbitration

• CAN Frames:
  Data is sent in packets called frames. Each frame includes a unique identifier (priority), control bits, data payload, and error-checking codes.
• Multi-Master and Arbitration:
  Multiple nodes can try to send data at the same time. To avoid collisions, CAN uses arbitration — a method that prioritizes messages with lower identifier numbers. The node with the highest priority continues to send, while others wait.
• Bit Monitoring:
  While sending data, each node listens to the bus to make sure the bit they sent matches what is on the bus. If not, it means another node has a higher priority message, so the lower priority node stops transmitting.

Example of Arbitration in the Architecture of the CAN Bus

Imagine two ECUs want to send messages simultaneously:

• ECU 1 has an identifier 0x234
• ECU 2 has an identifier 0x352

CAN uses wired-AND logic, meaning a dominant bit (0) overrides a recessive bit (1). When both ECUs transmit the third bit, ECU 1 sends 0 and ECU 2 sends 1. The bus reads 0 (dominant). ECU 2 detects the difference and stops transmitting, letting ECU 1 continue because it has higher priority.

Standard CAN Frame Format

Here’s what a typical CAN message frame looks like (in the standard 11-bit format):

Field	Description
SOF (Start of Frame)	Indicates the beginning of a message (1 bit)
Identifier	Defines message priority (11 bits)
RTR (Remote Transmission Request)	Distinguishes between data and remote frames (1 bit)
Control Field	Contains user-specified flags
IDE (Identifier Extension)	Indicates if frame is standard (11-bit) or extended (29-bit)
DLC (Data Length Code)	Specifies number of data bytes (4 bits)
Data Field	Contains up to 8 bytes of actual data
CRC (Cyclic Redundancy Check)	Error-checking code (15 bits)
ACK (Acknowledgement)	Confirms correct reception (2 bits)
EoF (End of Frame)	Marks the end of the message (7 bits)

Summary Architecture of the CAN Bus

CAN Bus Element	Description
Nodes (ECUs)	Devices with MCU, CAN controller, and transceiver
CAN-High and CAN-Low	Twisted pair differential signaling wires
Termination Resistors	120 Ω resistors at bus ends to prevent reflections
Physical Layer (ISO 11898-2)	Defines wiring and signaling specifications
Data Link Layer (ISO 11898-1)	Manages message framing, arbitration, and error handling
Arbitration	Priority-based bus access control

Conclusion

The CAN bus system is a simple yet powerful communication protocol enabling multiple devices to communicate over a shared two-wire bus efficiently. Its architecture combines robust physical wiring, standardized messaging, and smart arbitration techniques to ensure data integrity in noisy environments like vehicles. Understanding its basic components and working principle is essential for anyone diving into embedded systems or automotive electronics.

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