Why does the CAN bus have two 120Ω termination resistors  ?

Both ends of the CAN bus must be connected to a terminal resistor to work properly. 

The terminal resistor should be the same as the impedance of the communication cable, and the typical value is 120 ohms. 

Its function is to match the bus impedance and improve the anti-interference and reliable performance of data communication.

The function of the terminating resistor is to absorb signal reflection and echo, and if the impedance is discontinuous and mismatched, signal reflection will occur, which will interfere with the transmitted signal.

If the terminal resistance is added to two separate lines, it is equivalent to an open loop state. This type of connection results in a more discontinuous impedance on the single wire, suddenly going to 0 at the end, causing the reflection to multiply.

The two 120 ohm resistors added by the high-speed CAN actually simulate the characteristic impedance generated on the transmission line when the wire harness is connected to infinity, not the actual impedance. 

This is a typical empirical value, and the specific value depends on the type of wire harness used. .

The reason why high-speed CAN needs to add a terminal resistance is that when high-frequency signals are transmitted, the signal wavelength is shorter than that of the transmission line, and the signal will form a reflected wave at the end of the transmission line, which will interfere with the original signal. 

Therefore, it is necessary to add a terminal resistance at the end of the transmission line to make the signal reach the transmission line. Does not reflect after the end.

The reason why the low-speed CAN does not add terminating resistance is that the wavelength is relatively long and the reflection and echo are weak. 

However, the control unit of the low-speed CAN system will have its own independent terminal resistance, which is not connected between the CAN-high wire and the CAN-low wire, but between each wire and the ground or +5V power supply

Premier Cable CO LTD,  www.premier-cable.net , is a cable factory based in China, we mainly make CAN Termination resistors, plugs and cables , IOT related cables

Our cable and adapters are fully 100% compatible with PEAK-System ,  IXXAT ， Kvaser ， HMS， CSS Electronics

We make : 

 
CAN-T Adapter

CAN Bus Cable 

CAN Lin Cable 

CAN Adapter Cable

Null modem crossover cable
 

DB9 to 9-Pin Terminal Block Adapter 

D-sub 9 pin 120 Ohm termination adapter

CAN Termination resistors, plugs and cables

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