What is CANopen? What is the difference between CANopen and CAN bus?

May 7, 2024

CANopen is a communication protocol that emerged from the development of the CAN (Controller Area Network) bus initially created by the German company Bosch in the 1980s. Recognized for its reliability and communication performance, the CAN bus was widely adopted in both automotive and industrial applications.

CAN

The ECAN bus utilizes twisted pair cables for signal transmission and features a multi-master structure, allowing each node in the network to compete for bus access based on priority. It also includes mechanisms for lossless bit arbitration and error detection.

As the CAN bus protocol became established, several application-specific communication protocols emerged, but they lacked standardization. Addressing this issue, various companies and organizations began to explore and promote the development of an open, standardized high-level communication protocol—eventually leading to CANopen.

In 1992, the CAN in Automation (CiA) organization was established, dedicated to promoting the development and standardization of the CANopen protocol. Composed of companies from the automotive and industrial sectors, CiA aimed to broaden the application of CANopen across various domains. The first international standard for CANopen, ISO 11898-2, was released in 1994, laying the groundwork for standardization of the protocol.

ISO 11898-2 defined the physical and data link layers of the CAN bus, with CANopen being developed on top of this standard. As CANopen standards were established, the protocol found extensive applications in industrial automation and automotive electronics.

CANopen

Manufacturers began adopting the CANopen protocol to interconnect devices, enhancing system scalability and interoperability.

CiA continued to develop more CANopen standards, with the most significant being CiA DS (Device Specification) and the DS301/DS302 standards. DS301 defines the basic features of the CANopen protocol, while DS302 specifies the particular requirements for CANopen devices.

By the 2010s, CANopen had expanded into the medical equipment sector, facilitating networking and collaborative operation among various medical devices such as monitors, ventilators, and infusion pumps. This network connectivity allows these devices to work together, share data, and respond to patient needs in real time.

Key technical details of CANopen include:

1. Object Dictionary: A crucial component in a CANopen node, it is a table containing all configurable parameters and variables of the node, serving as the device’s "identity card" detailing all its functions and characteristics.

2. Communication Objects: These are used for data transmission within the network, including Process Data Objects (PDOs) and Service Data Objects (SDOs).

3. PDOs: Used for real-time data exchange, supporting rapid and efficient communication essential for real-time control. PDOs can be configured to automatically transmit upon specific events, ensuring timely data updates during critical moments.

 

4. SDOs: Used for transmitting non-real-time data, such as configuration parameters or diagnostic information, allowing for point-to-point data transfers. SDOs handle data that does not require real-time updating and can operate in a blocking mode, meaning other SDO transfers cannot proceed until one is completed.

  1. Node State Machine: Defines the various states of a node and the transitions between them, such as initialization, stopping, and operational states.
  2. Heartbeat and Node Guarding: These mechanisms monitor the status and health of nodes on the network. Nodes periodically send heartbeat messages to indicate their normal operation status. If a node’s heartbeat is not received within a predefined time, the system considers the node to have failed.

In practice, configuring devices is a crucial step. System integrators need to configure each node's object dictionary based on specific application requirements, often involving setting up PDO mappings, communication parameters, and various timers. Network planning should consider topology, cable lengths, and node distribution.

Additionally, considerations for bus termination resistors are necessary to ensure signal integrity. Each node has a unique node ID, and these IDs must be assigned to avoid conflicts and ensure that all nodes can correctly join the network at startup. Nodes are able to report their error states and can provide additional fault information through SDOs or special error control objects. The system needs to be able to respond to these errors and take appropriate recovery actions.

CANopen is more than just a data transmission protocol; it offers a comprehensive set of mechanisms for managing and monitoring the entire network. As technology advances, CANopen continues to evolve, adapting to new application scenarios and requirements. Whether in traditional industrial automation or the growing sectors of smart devices and the Internet of Things, CANopen continues to play a vital role.

 

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