While the Controller Area Network (CAN) bus is the standard automotive network, allowing cars to greatly reduce wiring harness size and weight, the protocol specification is also alive and growing in other areas. CAN transceivers are among the many analog building blocks that can strengthen designs in areas like factory automation and the IoT. You might even call them a hero of analog.

What Are the Different Kinds of CAN Buses?

A CAN 2.0A device protocol uses 11-bit identifiers, and one that uses 29-bit identifiers is commonly called CAN 2.0B. According to Wikipedia, the International Organization for Standardization released the CAN standard ISO 11898 in 1993, and it was later restructured into two parts. ISO 11898-1 covers the data link layer, while ISO 11898-2 covers the physical layer for high-speed CAN. ISO 11898-3 was released later and covers the physical layer for low-speed, fault-tolerant CAN.

The SAE J1939 standard is a widely used CAN protocol standard for trucks and buses. CAN bus is one of five protocols used in the on-board diagnostics (OBD)-II vehicle diagnostics standard, which has been mandatory for all cars and light trucks sold in the U.S. since 1996 and in Europe since 2001.

If Not CAN, What?

Alternatives to CAN include Local Interconnected Network (LIN) bus and Flexray. While LIN bus is even less inexpensive than CAN, it is not a full replacement. Rather, it provides a good alternative wherever low costs are essential and speed/bandwidth is not important. In automotive applications, LIN bus is typically used within sub-systems that are not critical to vehicle performance or safety. Flexray, in development since 2000, was used in a 2006 production BMW, and is now used in most German cars and SUVs. It enables faster 10Mbps data rates with deterministic time responses and is designed to be redundant. But, it is costly and much more complex than CAN and has only a 24m reach. There is also the MOST network that is used for infotainment systems. The very quick Ethernet connection is being proposed for infotainment and display applications in the car—and maybe more.

How CAN Makes a Difference on the Factory Floor

Luxury cars have as many as 150 automotive ECUs to control and regulate various functions, and every single new car now has at least 70 ECUs. These ECUs have to talk amongst themselves while controlling almost all aspects of the power train, the body, and the chassis. CAN connects each function to/from the various ECUs—from door locks to brakes, the accelerator pedal, and tail lights. Its popularity stems from very high reliability and simplicity—with low cost and low power.

CAN bus uses a simple 25Kbps to 1Mbps twisted-pair connection. Chips are now available to implement 2Mbps data. The bus will reach to 1000m at 50Kbps or 40m at 1Mbps. CAN’s vehicle popularity results from the elegant ways that developers have found to guarantee predictable, error-free communications in the safety-conscious automotive environment. And such characteristics can be put to very good use on the factory floor as well.

The fact that CAN is carried differentially on a single pair of wires means that it is able to reject common mode noise. The structure also reduces emissions since the electric fields of differential communications tend to cancel each other.

An industrial system using motors requiring large currents to start, stop, and operate is a good example of where CAN shines. The electromagnetic emissions from motors can cause large amount of errors in RS-485 or similar communications, while CAN is able to soldier through. On top of the physical reliability, CAN includes other features to ensure robustness, including cyclic redundancy checks (CRCs) and acknowledgements that enable error detection.

Operating as a bus topology requires devices to understand when they can transmit. CAN uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), which is also used in Ethernet. When a device wants to transmit its message, it has to sense whether the bus is free and not being used. Collision detection allows it to detect when another device on the bus tries to transmit at the exact same time. CAN uses priority mechanisms for bus management.

As CAN bus grew in popularity, different vendors created their own implementations. Each implementation had different networking layers, even though the physical layers were common. CANopen was devised to enable compatibility so that designers could pick CAN-enabled devices and know that they would interplay. It is most popular in embedded systems used for automation applications. In terms of the OSI model, CANopen implements the layers above and including the network layer.