Understanding CAN Protocol – A Simple Technical Guide
Hello and welcome to MAE Learning. In this blog, we will understanding CAN Protocol (Controller Area Network). This protocol is widely used in vehicles. It is also used in industrial systems. It allows many electronic units to talk to each other using a shared network. This blog explains CAN in clear and simple terms, without heavy technical language.
1. What is the CAN Protocol?
CAN is a communication system that lets electronic control units (ECUs) exchange data over a common bus.
It was designed for vehicles but is now used in factories, robots and machines.
Key ideas:
- No central controller
- Message-based communication
- Very reliable, even in noisy environments
2. Types of CAN Messages
CAN supports different message types:
- Data Frame – Carries actual data
- Remote Frame – Requests data from another node
- Error Frame – Reports an error on the bus
- Overload Frame – Adds delay when a node is busy
3. High-Speed CAN vs Low-Speed CAN
| Feature | High-Speed CAN | Low-Speed CAN |
| Speed | Up to 1 Mbps | Up to 125 kbps |
| Usage | Engine, braking | Windows, mirrors |
| Fault tolerance | Lower | Higher |
| Cable length | Shorter | Longer |
4. What is SAE J1939?
SAE J1939 is a vehicle communication standard based on CAN. It is mainly used in trucks, buses and heavy vehicles.
Main features:
- Uses 29-bit identifiers
- Supports large networks
- Defines message format and meaning
5. What is CANopen and Its Purpose?
CANopen is a higher-level protocol built on CAN.
Purpose:
- Makes CAN easier to use
- Standard way to control devices like motors and sensors
- Widely used in industrial systems
6. How Does CAN Arbitration Work?
Arbitration decides who can talk first on the bus.
- Each message has an identifier
- Lower identifier = higher priority
- Nodes send bits while listening
- If a node loses, it stops sending
Why it matters:
It avoids data collision without stopping the network.
7. Hard Synchronization vs Soft Synchronization
- Hard synchronization
Happens at the start of a message frame - Soft synchronization
Small timing adjustments during data transfer
Both keep all nodes in sync.
8. Functional vs Physical Addressing
- Physical addressing
Message sent to one specific node - Functional addressing
Message sent to all nodes with a certain function (like diagnostics)
9. Sending More Than 8 Bytes of Data
Classic CAN allows only 8 bytes per frame.
To send more:
- Multi-frame messages
- Transport protocols (like ISO-TP)
- CAN FD (supports up to 64 bytes)
10. What is OBDII?
OBDII stands for On-Board Diagnostics.
It allows:
- Reading vehicle fault codes
- Monitoring engine health
- Emission control checks
11. Why Diagnostic Standards Are Important
Diagnostic standards:
- Ensure compatibility
- Make repairs easier
- Reduce cost and time
- Improve safety and emissions control
12. Can Two Nodes Use the Same Header at the Same Time?
Yes, if they send the same identifier and data.
- The bus sees it as one message
- No error occurs
13. CAN Physical Layer Voltage Levels
CAN uses two wires:
- CAN_H
- CAN_L
| Bus State | CAN_H | CAN_L |
| Recessive | ~2.5V | ~2.5V |
| Dominant | ~3.5V | ~1.5V |
14. What Happens If Two Nodes Send the Same Identifier?
- Both transmit together
- Arbitration sees no conflict
- Message is sent successfully
15. Verification vs Validation in CAN Systems
- Verification
“Did we build it right?” - Validation
“Did we build the right thing?”
Both are needed for safe and reliable systems.
16. Bit Rate vs Baud Rate
- Bit rate – number of data bits per second
- Baud rate – number of signal changes per second
In CAN, they are often the same, but not always.
17. Active Error Mode vs Passive Error Mode
- Active error mode
Node sends error flags normally - Passive error mode
Node sends quieter error signals to avoid disturbing the bus
18. Role of the CAN Transceiver
The CAN transceiver:
- Connects the controller to the bus
- Converts digital data to voltage signals
- Protects against electrical noise
19. Why Differential Voltage Is Used
CAN uses difference between two wires, not absolute voltage.
Benefits:
- Strong noise resistance
- Works well in vehicles
- More reliable communication
20. Termination Resistors in CAN
- Two 120-ohm resistors
- Placed at both ends of the bus
- Reduce signal reflection
- Improve signal quality
21. How Message Filtering Works
Each node:
- Checks message identifiers
- Accepts only required messages
- Ignores the rest
This reduces CPU load and improves speed.
