What is a Can Termination Resistor

Are you questioning what is the CAN bus terminal resistor? Before we learn more about this type of resistor, let's have a glance at what is termination resistor first.

What Is Can Termination Resistor

Can terminal resistor is an indispensable part of automobile modern control system. It is one of the core components of a vehicle control system with its special function. 

Can terminal resistance effectively reduce the load on the line, reduce electromagnetic interference, suppress sudden noise interference, ensure the stable operation of the control system, and enhance the reliability of the vehicle control system. (An example of an application)

The Usage of Termination Resistors in Can Bus

There are mainly 3 purposes for using the termination resistor in Can Bus.

1. Ensure the correctness of the signal

CAN bus systems convey data using differential signaling. This implies that the data stream is encoded using the voltage differential between two wires, commonly referred to as CAN High and CAN Low.

Termination resistors are used to absorb signals at the ends of CAN bus cables to prevent signal reflections. These reflections might happen when the signal hits the end of the wire and has nowhere further to go. They can interfere with and corrupt data transmissions.

By terminating the bus with resistors that match the cable's characteristic impedance, the signal is absorbed and reflections are eliminated. This ensures that the data stream is sent precisely and dependably.

2. Reduce signal reflection and noise

The signal transmission rate on the CAN bus is up to 1Mbps, and the signal will be reflected many times during transmission.

If there is no terminal resistance, the signal will be reflected back and superimposed with the original signal, resulting in signal distortion. The function of the terminal resistance is to eliminate the reflection of the signal, reduce the signal noise, and ensure the stability of the signal.

3. Improve the anti-interference ability of the bus

CAN bus is generally used for real-time communication in harsh environments, such as automotive, industrial control, and other fields.

In these application scenarios, the bus will be affected by electromagnetic interference, voltage interference, and other interference sources, and the terminal resistance can improve the anti-interference ability of the bus to ensure the stability of communication.

How Many Termination Resistors Does a Can Bus Require

The answer to this question is two. A CAN bus network requires exactly two termination resistors, one placed at each end of the bus.

Having less than two Without termination resistors, signal reflections might occur, distorting data and causing problems.
Have more than two: Introducing more resistors upsets the desired impedance balance, resulting in signal quality concerns.

This can be checked easily by powering down all CAN devices and measuring the resistance between the "CAN High" and "CAN Low" wires. The measured resistance must be approximately 60 Ohm.

What Type of Terminating Resistors Are Used in the CAN Bus Wires

CAN bus terminal resistance is generally divided into two types: internal terminal resistance and external terminal resistance.

1. Internal terminal resistance

Internal terminal resistance refers to the resistance integrated in some CAN chips, which is generally located at the port of the chip.

The advantage of the internal terminal resistor is that it is easy to install and does not require additional devices, but its resistance value is usually large, usually 120 ohms, and may not meet some special application requirements.

2. External terminal resistance

External terminal resistance refers to the resistance added at both ends of the CAN bus, and its position is generally at both ends of the CAN bus. 

The resistance value of the external terminal resistor is generally 120 ohms, and the resistance value ranges from 100 ohms to 150 ohms. 

The advantage of external terminal resistors is that the resistance value is relatively stable and can meet the requirements of various applications, but additional devices are required and installed.

FAQS about Can Termination Resistor

We prepare 2 questions you may ask when using a termination resistor.

1. What are the factors to consider when choosing a termination resistor? 

（1）Bus length and topology

The length and topology of the CAN bus will affect the selection of terminal resistance. If the bus length is short or the topology is linear, the internal terminal resistance can be selected. If the bus length is long or it is a branch topology, you need to select an external terminal resistor.

（2）Working frequency

The working frequency of the CAN bus is generally 1Mbps, and the working frequency of the high-speed CAN bus can reach 10Mbps. 

When selecting the terminal resistance, it is necessary to consider the operating frequency of the bus, and the resistance value should conform to the characteristic impedance of the bus.

（3）Special application requirements

Some special application scenarios require the use of different terminal resistors, such as in high or low-temperature environments need to select a wide range of working temperature resistance; On the high-speed CAN bus, you need to choose a lower resistance value.

3. How to choose the resistance value of can termination resistor

The selection of the resistance value of the terminal resistance should be determined according to the characteristic impedance and bus length of the Can bus. 

In general, the characteristic impedance of the Can bus is 120 ohms, so the resistance value of the terminal resistance is also chosen to be 120 ohms.

When the bus length is short, you can choose a smaller terminal resistance value, such as 60 ohm or 30 ohm, to improve the transmission rate of the signal. When the bus length is longer, a larger terminal resistance value can be selected, for example,240 ohms or 480 ohms, to reduce signal reflection and attenuation.

Final Verdict

In summary, can termination resistor plays a very important role in practical applications. Therefore, the correct selection and installation of terminal resistance can ensure the stability and reliability of the CAN bus, thereby improving the performance of the whole system.