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Signal Chain Basics #117: Uses of CAN-FD for longer payloads in industrial applications

Editor’s note: This month we bring you Signal Chain Basics #117 by author Abhijeeth Aarey, a systems engineer in the industrial interface group at Texas Instruments.

Introduction

The controller area network (CAN) interface standard has seen widespread adoption in a broad range of applications ranging from aerospace to industrial automation and embedded systems. Originally introduced around three decades ago, now referred to as classic CAN, this standard has been the foundation for many higher layer protocols and supports data rates of up to 1 Mbps. However, as modern systems grow in complexity and the trend for faster data rates continues, there is a need for the CAN standard to scale accordingly. CAN flexible data rate, or CAN-FD, was introduced in 2012 to address this need. This article takes a look at some of the key features of FD and how this could be beneficial for industrial applications.

CAN frame formats

To understand the changes in the new standard, we compare classic CAN format to CAN-FD in Figure 1.

Figure 1

Classic CAN versus CAN-FD frame formats with an 11-bit identifier.

Classic CAN versus CAN-FD frame formats with an 11-bit identifier.

The most significant difference is the extension of the data field from a maximum of 64 bits to 512 bits. As you will see later, this provides multiple benefits for systems using longer payloads. The control field is expanded to indicate new features such as extended data length and bitrate switching. The new cyclic redundancy check (CRC) algorithm is improved across the board and reduces the probability of undetected bit errors. This enhances the robustness of networks in large industrial installations with high ground-potential differences and also for noisy networks such as motor drive systems.

Data rates

During the arbitration phase at the beginning and end of each frame, the system can operate up to 1 Mbps. The speed is limited by various factors, including the transceiver’s loop timing. However, during the date phase, the transmitting node can use higher bitrates to transmit the payload. Here, the speed is limited only by the medium’s physical properties. The new standard is called FD since the data rate can vary between the arbitration and the data phase. Although 12 Mbps is possible in the data phase, speeds of 2-5 Mbps are more practical, owing to other system concerns, such as electromagnetic compliance (EMC). Applications involving flash programming or data downloads directly benefit from these higher data rates.

Larger payloads

Extending the payload size is useful in reducing overhead and improving data utilization efficiency. Figure 2 shows this behavior.

Figure 2

Data utilization with 11-bit identifier.

Data utilization with 11-bit identifier.

For small payload sizes, the new frame format introduces slightly more overhead, all else being equal. Intuitively, as the payload scales up, the efficiency improves. For example, with the corresponding maximum payload sizes considered for the respective formats, classic CAN achieves a data utilization efficiency of ~57 percent, whereas FD can achieve ~90 percent. This advantage is true regardless of the bit rates used . The need to implement a higher level transport protocol (TP), such as ISO 15765-2 (ISO-TP) to transmit multiple frames, is now significantly reduced because more data can be transmitted per frame with much less overhead. Position sensors in industrial automation systems can leverage this benefit to improve system responsiveness.

If higher data speeds are factored into the analysis, the overall throughput of the network is improved further. One can make a few intuitive conclusions from these observations. If a 1 Mbps classic CAN frame is used as a reference, then the same amount of data payload can be sent using 2 Mbps CAN-FD in half the time; or twice the data payload can be sent in the same time. As the bitrates are scaled up, FD advantages become more apparent. When both data rate and payload size are increased, then the average/overall bit rate gain inches closer to the theoretical bit rate gain for the data field alone. For example, with a 512-bit payload, 11-bit identifier, 1 Mbps arbitration speed and 4 Mbps data rate, you can achieve an average bit rate of ~3.5 Mbps overall. Similarly, for an 8 Mbps data rate, you can achieve an average speed of ~6 Mbps overall.

Conclusion

Larger payload sizes, increased protocol efficiency, improved error checking and a faster data rate all enable industrial networks to increase in size, length, robustness and efficiency. You can leverage these new features of CAN-FD to boost the performance of a wide variety of industrial applications.

About the Author

Abhijeeth Aarey is a systems engineer for Analog & Mixed Signal Circuits at Texas Instruments. He’s been in Industrial Interface where he designs exciting and challenging high-voltage products for Industrial and Automotive markets. He received his MSEE from the Arizona State University, Tempe, Arizona.

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