Born in cars, the Controller Area Network now links drives, sensors and robots in machines and medical devices. Its clever arbitration lets many nodes share two wires without collisions or lost priority messages.
CAN is a robust multi master serial network built for noisy environments. On top of it, CANopen and DeviceNet add the device profiles and services that industrial automation needs.
What Is CAN Bus?
CAN bus, or Controller Area Network, is a multi master serial bus defined in ISO 11898 that uses a differential pair, CAN_H and CAN_L, and message based arbitration so any node can transmit while priority is always respected. It is a form of serial communication designed for harsh electrical environments.
Texas Instruments explains in its CAN introduction that the bus has two states, dominant and recessive. A dominant bit always overwrites a recessive one, which is the key to arbitration.
The network is a line with a twisted pair cable and 120 Ω resistors at both ends. Stubs to each node must stay short.
Industrial protocols such as DeviceNet and CANopen run on the same physical layer.
How CAN Bus Arbitration Works
Because the lowest identifier wins, message priority is built into the ID. No time is wasted on collisions, unlike classic Ethernet.
Error detection includes CRC, bit stuffing checks and acknowledgements. Faulty nodes switch themselves off the bus after repeated errors.
Bit Rate vs Cable Length
| Bit Rate | Typical Maximum Length |
|---|---|
| 1 Mbit per s | About 40 m |
| 500 kbit per s | About 100 m |
| 250 kbit per s | About 250 m |
| 125 kbit per s | About 500 m |
| 50 kbit per s | About 1000 m |
CAN bus length is limited because every node must see each bit within the same bit time during arbitration. Transceiver and cable delays set the practical limits shown.
Compared with RS485, CAN adds arbitration and error handling in hardware, which simplifies software.
7 Essential CAN Bus Facts
On a healthy CAN bus, measuring about 60 Ω between CAN_H and CAN_L with power off confirms both terminators are fitted. Around 120 Ω means one is missing.
Terminations matter on every bus type, as shown in ControlNet termination. Missing resistors cause reflections and error frames.
CANopen Basics
Process data objects for fast real time data.
Service data objects for configuration.
Network management states.
Every parameter has an index.
CSS Electronics notes that a CANopen network supports up to 127 nodes. One PDO frame carries data that would need four frames through SDO transfers.
CiA 301 was first released in 1994, and drive profiles such as CiA 402 followed. Servo drives from many makers use them.
Bus Load Formula
Example:
2000 messages per second, about 125 bits per frame with stuffing
Bit rate 500 kbit per s
Load = 2000 × 125 ÷ 500000 × 100 = 50 percent
Keep average load below about 60 to 70 percent so low priority messages are not delayed too long. Burst loads matter as well.
Bus Load Calculator
A classic 8 byte frame with an 11 bit ID is about 111 bits before stuffing. Use 125 to 135 bits for planning.
- Hardware arbitration without collisions.
- Strong error detection.
- Low cost transceivers.
- Works well in noisy machines.
- Short payloads in classic CAN.
- Length falls as speed rises.
- Needs careful termination.
- Lower bandwidth than Ethernet.
For higher bandwidth motion networks, compare with EtherNet/IP vs EtherCAT.
TI Introduction to CAN PDF
CANopen Simple Intro Video
CAN Bus FAQ
Related Articles
- DeviceNet Protocol
- RS232 vs RS485
- Types of Communication Protocols
- Communication Protocols in PLC
- Industrial Network Protocol Courses
External References
- Introduction to CAN, Texas Instruments
- CANopen Explained, CSS Electronics
- Controller Area Network, Wikipedia
What We Learn Today
- CAN bus uses dominant and recessive bits for collision free arbitration.
- Terminate both ends and match bit rate to cable length.
- CANopen adds PDOs, SDOs and profiles for automation devices.