Need Of Termination Resister in CAN network

Introduction to Termination Resistor in CAN Network

A termination resistor is an electrical component that is used to match the impedance of a transmission line to that of the connected load. It is typically used at the end of a transm

ission line to prevent signal reflections that can degrade the quality of the transmitted signal.

When a signal travels down a transmission line, it encounters impedance changes at every point where the line changes characteristics, such as where it connects to a load. When the signal reaches the end of the line, it encounters an impedance mismatch with the load, which causes some of the signal to be reflected back towards the source. This reflection can cause interference and distortion in the signal, reducing the overall signal quality.

A termination resistor provides a resistive load that matches the characteristic impedance of the transmission line, allowing the signal to be absorbed and preventing signal reflections. This helps to improve the signal quality and reduce noise and interference. Termination resistors are commonly used in digital communication systems, such as CAN, Ethernet and USB, as well as in analog audio and video systems.

Termination Resistor in CAN Network

In a CAN network, there are different ECU from the different suppliers that will be purchased by the Vehicle OEM and they will interface in their vehicle CAN network to add all the features. but the OEM doesn’t know about your ECU design and the circuit diagram, what is the internal and external resistance.

How they will manage all these is a headache for them. Always ask the supplier about this and connect again so many bugs will come again they will do the investigation which is not good and it will reduce productivity, more engineers will work for this. So to prevent this mostly all the protocols have their own physical layer design in which it defines all the electrical parameters.  

If you will see in case of I2C protocol or SPI protocol also they have a termination register. But in the case of CAN protocol, it is doing the same but somewhat more features are there as which is creating a vital role. The main reason is that most of the protocols are having very little distance or you can say these kinds of protocols are designed for onboard inter IC communication.

Even these are used in a single ECU nothing but a motherboard where the different types of IC like as external RAM, ROM, some power chip IC, CAN controller also. So like this on any board, there might be I2C, UART, SPI, etc. protocols are there to make the electrical data communication. but when in case of a vehicle, there will be a number of ECU even you can say 100+ ECU’s to control the different functionality. In this case, the CAN protocol which can be used up to 1km long.

In CAN network there is also another case that always all the ECU will on or off, and also you can not say. So in real-time how you will calculate the CAN bus network resistance. So to prevent this in each CAN network there should be terminal resistance to make the internal and external resistance balance as per the maximum power transform theorem. The maximum transform theorem defines that the internal resistance should be equal to external resistance to transfer the maximum power from the source to load without any loss.

But you can not say that there is no loss as the conductor we are using is a superconductor that will consume the zero power in transmission. Obviously there will be some loss but practically we are ignoring this since it is not possible to use the gold like superconductor for electrical energy transmission.

So basically since the CAN network is always used the BUS topology method for ECU to ECU communication in a CAN network, we need to measure all the ECU for total internal and external impedance for a particular CAN network. It is nothing like this is a single reason to use it but also there are so many reasons, so let us discuss all this below.

Theory and Cause for selecting 120 Ohm Resistor

The CAN network is having a long-distance as 40 meters and using the twisted pair cable for data transmission in the network. If you really want to go depth then please read the Transmission Line Theory. but mostly in this network, they used 120 Ohm according to the transmission line theory calculation. The basic concept is to transfer the data without any loss and noise and also not to create the reflection of VSWR (Voltage Standing Wave Ratio) due to more power.

So to prevent this the CAN network should follow the Maximum Power Transform Theory which tells that to transfer the maximum power from the source to load, the source resistance should be equal to the load resistance. So for this, once the Robot BOSCH designed and published after investigation as 120 Ohm is good for perfect data transmission and no need for re-investigation. But practically in the vehicle, it can vary in between 108 Ohm – 132 Ohm of CAN termination resistor for proper data transmission.

Working Principle of Termination Resistor in CAN Network

A termination resistor in a CAN (Controller Area Network) network is placed at each end of the network to prevent signal reflections that can cause data errors. The termination resistor works by providing an electrical load that matches the characteristic impedance of the transmission line, effectively absorbing any reflected signals and reducing their amplitude. This helps ensure that the data transmitted on the CAN network is received accurately and reliably by all nodes on the network. You can see the below images for how it connects in a network and works.

High-Speed CAN Termination resistor

The high-speed CAN network is a compulsory need of termination resistor as per the ISO 11898-2 standard. In High-speed, basically the termination resistor connected in each CAN transceiver. This required to handle the stub that is connecting in between the CAN transceiver and CAN bus. In the real in-vehicle network, they will be connecting that resistor in the DB9 connector for connection flexibility.

Mostly this resistor is connected inside the ECU in the vehicle, but in case of testing, you would have seen that the testers are supposed to connect a 120 Ohm CAN termination resistor in DB9 cable. Even if there should be two 120 Ohm resistor need to be connected in both the two ends of the CAN bus cable. This is required to reduce the noise and reflection of the signal that I have explained above.

Low-Speed or Fault-Tolerant CAN Termination resistor

The Low-Speed or fault-tolerant CAN bus is generally runs at the most maximum of 125 Kbps data rate. This kind of CAN bus is powered internally through a DC-DC converter. This CAN network is designed to withstand open, shorts, and incorrect loads on the CAN bus. Basically, if there will be any kind of faults will occur due to network inbalance load which needs to be recalculated and changed the termination resistor.

But run time is not possible, so that if any faults will occur, automatically the CAN bus will switch to a single line cable that doesn’t need any resistor, and automatically it will recover and started working. The below errors that can occur in most of the CAN network and for which the CAN bus might change to single line cable for recovery if:

• CAN_H cable interrupted.
• CAN_L cable interrupted.
• CAN_H short-circuited to +Ve terminal of the battery.
• CAN_L short-circuited to +Ve terminal of the battery.
• CAN_H short-circuited to Vcc.
• CAN_L short-circuited to Vcc.
• CAN_H short-circuited to ground.
• CAN_L short-circuited to ground.
• CAN_H and CAN_L mutually can be short-circuited.

Different Termonilogies used in this Topic Explained Below:

1. CAN Bus Termination Resistor: A component used in a Controller Area Network (CAN) to prevent signal reflections and ensure proper signal transmission.
2. CAN Bus Termination Resistor Value: The resistance value of the termination resistor that matches the characteristic impedance of the CAN bus.
3. CAN Network Termination Resistor: A resistor used to terminate a CAN network to prevent signal reflections and ensure proper signal transmission.
4. CAN Bus Termination Resistor Placement: The physical location of the termination resistor on the CAN bus, typically at the end of the bus.
5. CAN Bus Termination Resistor Calculation: The process of determining the appropriate resistance value for the termination resistor based on the characteristics of the CAN bus.
6. CAN Bus Termination Resistor Purpose: To match the impedance of the CAN bus and prevent signal reflections, ensuring proper signal transmission and preventing signal distortion.
7. CAN Bus Termination Resistor Function: To absorb and dissipate signal energy to prevent it from reflecting back into the bus, which can cause interference and signal degradation.
8. CAN Bus Termination Resistor Location: The physical position of the termination resistor on the CAN bus, which is typically located at the end of the bus.
9. CAN Bus Termination Resistor Importance: A necessary component in any CAN bus system to ensure proper signal transmission and prevent signal degradation or loss.
10. CAN Bus Termination Resistor Specification: The specific characteristics and requirements for the termination resistor, including resistance value, power rating, and physical dimensions.