Understanding the Use of 120 Ohm Resistor for CAN Termination

Controller Area Network (CAN) systems often employ 120-ohm resistors for termination purposes. This practice is rooted in the specific design characteristics and operational requirements of CAN communication. In this article, we will explore the reasons behind the use of 120-ohm resistors and their role in signal integrity, standardization, and physical layer design.

Impedance Matching and Signal Integrity

A CAN bus is essentially a transmission line, and like any other transmission line, it requires impedance matching at both ends for optimal performance. The CAN bus is designed as a twisted pair of wires, with a characteristic impedance of approximately 120 ohms. Using a 120-ohm termination resistor at each end of the bus helps to match this impedance, reducing signal reflections and ensuring that signals travel along the bus without significant distortion. Without proper impedance matching, signal reflections can occur at the end of the bus, leading to signal interference and potential data errors.

Proper Termination for Signal Integrity

A key factor in CAN systems is the minimization of signal reflections. When signals reach the end of the bus without proper termination, these reflections can interfere with the original signal, leading to data errors. By using 120-ohm resistors, the CAN system maintains signal integrity, ensuring that data is transmitted accurately and reliably over longer distances.

Standardization and Industry Compatibility

The CAN protocol, which was established by Robert Bosch in the 1980s, specifies 120-ohm termination as part of its standard. This standardization guarantees compatibility between different devices and systems utilizing CAN communication. Without this standardization, interoperability issues could arise, making it difficult for different manufacturers to integrate their devices seamlessly.

Physical Layer Design and Transceiver Optimization

The design of the CAN physical layer, including its transceivers, is optimized for a 120-ohm bus. This ensures that the transceivers can effectively drive the bus and receive signals without degradation. Using 120-ohm termination resistors aligns with this optimization, ensuring that the entire system operates efficiently and reliably.

Common Practice and Industry Adoption

The use of 120-ohm termination has become a common practice in the industry for CAN networks. This widespread adoption makes it a widely accepted norm, facilitating easier troubleshooting and the design of new systems. Industry professionals and engineers have come to trust this solution, and newer CAN systems are consistently designed with 120-ohm resistors for termination.

Comparison with Other Buses

Other communication buses also utilize terminating resistors, each with their own target impedance to ensure optimal performance. Here is a comparison:

Signal Standard Targeted Impedance (Ohms) CAN 120 USB 90 HDMI 95 IEEE 1394 (FireWire) 108 DisplayPort 100 VGA 75 DVI 95 PCIe 85 Ethernet Cat.5 100

As shown in the table, different communication protocols have different target impedances based on their specific requirements. This diversity in impedance values underscores the importance of understanding the underlying principles of impedance matching for each specific communication standard.

Overall, the use of 120-ohm resistors for termination in CAN systems ensures proper impedance matching, maintains signal integrity, adheres to industry standards, and promotes compatibility among devices. By following these best practices, engineers and professionals can build reliable and high-performing CAN networks.