With the increasing demands for data transmission rates and bandwidth from industrial automation, intelligent manufacturing, and high-end equipment, the traditional CAN bus has shown performance bottlenecks in certain scenarios. To address this challenge, CANopen FD was developed – it perfectly combines the stability of the classic CANopen protocol with the high performance of the CAN FD physical layer, providing a powerful communication solution for modern embedded systems.

I. Origin and Development of CANopen FD

CANopen FD is not a completely new independent technology, but rather a significant evolution of the classic CANopen protocol.

1. Fundamentals

CANopen FD is developed based on the CAN FD (CAN with Flexible Data-Rate) physical layer protocol stack. CAN FD itself is a low-level enhancement of the traditional CAN 2.0 (classic CAN), significantly improving data transmission rates and efficiency while retaining the stable and reliable characteristics of the CAN bus through "bit rate switching" technology and larger data frames.

2. Launch Organization and Timeline

This technology was developed and standardized under the leadership of the international organization CiA (CAN in Automation). The core specification, "CiA 1301 - CANopen FD application layer and communication profile," was officially released around 2017, marking the entry of the CANopen protocol into the era of high-speed, high-capacity data transmission.

II. Scope of Use and Application Cases

CANopen FD is primarily used in applications with higher requirements for data throughput, real-time performance, and network load. Any field where classic CANopen is applicable can be upgraded to CANopen FD when performance needs improvement. Typical application cases include:

• High-end industrial automation: In scenarios such as multi-axis robot collaboration and precision CNC machine tools, high-speed synchronous data exchange at the microsecond level is required between multiple drivers and controllers. CANopen FD's 64-byte data frame can efficiently transmit multi-axis position, speed, and torque commands.

• Advanced Medical Equipment: CT scanners, MRI machines, etc., require high-speed transmission of large amounts of raw sensor data or control parameters. CANopen FD effectively reduces communication latency and improves equipment performance.

• Special Purpose and Engineering Vehicles: Modern agricultural machinery, fire trucks, excavators, etc., integrate numerous sensors, actuators, and controllers. CANopen FD can easily handle high-frequency data streams from subsystems such as GPS, cameras, radar, and hydraulic systems.

• Laboratory and Testing Equipment: In data acquisition and high-speed testing platforms, where large amounts of test data need to be recorded and transmitted in real time, CANopen FD's high bandwidth makes it an ideal choice.

III. Differences between the Protocol Stack and CANopen

CANopen FD inherits core concepts from CANopen such as the Object Dictionary, Network Management (NMT), and Heartbeat, but significantly enhances the implementation and capabilities of communication objects. The main differences are as follows:

1. Physical Layer Foundation: Revolutionary Improvement in Baud Rate and Data Capacity

This is the most fundamental difference between the two, directly determining the upper limit of bus performance.

(1) Baud Rate

• CANopen (based on classic CAN): Uses a single fixed baud rate. The entire data frame (arbitration, data, acknowledgment) is transmitted at the same rate (e.g., 125kbps, 250kbps, 500kbps, up to 1Mbps). The rate is strictly limited by the bus length (the higher the rate, the shorter the required bus length). • CANopen FD (based on CAN FD): Employs **Bit Rate Switching** technology to achieve dual-baud rate transmission:

◦ Nominal Bit Rate: The arbitration and control fields of the frame use a lower and more stable baud rate (e.g., 500kbps or 1Mbps) to ensure network stability and long-distance transmission capability, while remaining compatible with classic CAN;

◦ Data Bit Rate: The data and CRC fields dynamically switch to higher baud rates (e.g., 2Mbps, 5Mbps, or even higher), switching back to the normal rate after data transmission is complete.

• Advantages: Balances transmission distance and speed, significantly improving effective data transmission efficiency while ensuring network stability.

(2) Data Frame Capacity

• CANopen: Each frame can carry a maximum of 8 bytes of valid data;

• CANopen FD: Each frame can carry a maximum of 64 bytes of valid data, eight times the capacity of the former. 2. Upgrade of SDO (Service Data Object) – USDO (Universal Service Data Object)

This is one of the core innovations of CANopen FD in the protocol stack:

• CANopen: SDO is used for point-to-point configuration and parameter reading/writing, with low transmission efficiency and no support for routing;

• CANopen FD: Introduces the new USDO service, with four major advantages:

a. High-efficiency transmission: Optimized for 64-byte long frames, resulting in lower overhead when transmitting large amounts of data;

b. Routing capabilities: Natively supports cross-segment routing, simplifying node communication configuration in complex gateway networks;

c. Broadcast and multicast: Supports sending requests to multiple nodes simultaneously, enabling efficient batch configuration of devices or data querying;

d. Parallel communication: A single USDO server can handle requests from multiple clients simultaneously.

3. Enhancements to PDO (Process Data Object)

PDOs are used to transmit real-time, periodic process data:

• CANopen: Each PDO message maps up to 8 bytes of object dictionary data. Data exceeding 8 bytes requires multiple PDOs, increasing bus load and configuration complexity.

• CANopen FD: Each PDO message maps up to 64 bytes of data. A single PDO can transmit all critical status information of complex devices (such as servo drives) or simultaneously update multiple sensor values, significantly improving bus efficiency and real-time performance.

4. Extensions to EMCY (Emergency Message)

EMCY is used to broadcast error messages to the network when a device fails:

• CANopen: Emergency messages are limited to 8 bytes, resulting in limited diagnostic information.

• CANopen FD: Emergency messages can carry 64 bytes of data, allowing for more detailed diagnostic codes and field data to be reported during a fault, facilitating rapid root cause location.

IV. Conclusion

CANopen FD is not a subversion of CANopen, but rather a powerful upgrade in response to technological advancements. It retains CANopen's mature and reliable application layer design philosophy while fully unleashing the potential of the CAN FD physical layer through a dual-baud rate mechanism and larger data frames. For next-generation high-performance embedded systems limited by the bandwidth of traditional CAN bus, CANopen FD is an ideal communication protocol choice.