Terminal Resistor 120 Ohm, DB9, CAN Bus 

D-Sub 9 pin 120 Ohm termination adapter

CAN Termination resistors, plugs and cables

120 Ohm for high speed CAN system

CAN Bus adaptor with a 9-pin D-Sub

120 Ohm for high speed CAN systems according to ISO 11898-2 Male to female

Between CAN High (pin 7) and CAN Low (pin 2), there is a built-in 120 Ohm CAN Bus termination resistor.

D-Sub 9 pin 120 Ohm termination adapter is a CAN Bus adaptor with a 9-pin male D-Sub connector at one end and a 9-pin female D-Sub socket at the other. 

Why is the terminating resistor 120 ohms ?

Through the reasons for signal reflection, we can conclude that the resistance of the terminal resistor depends on the impedance of the cable.

The characteristic impedance of any cable can be obtained experimentally.

One end of the cable is connected to a square wave generator, the other end is connected to an adjustable resistor, and the waveform on the resistor is observed through an oscilloscope.

Adjust the resistance value of the resistor until the signal on the resistor is a good square wave without ringing, and the resistance value at this time can be considered to be consistent with the characteristic impedance of the cable.

Typical cables used in the CAN bus, twist them into twisted pairs, and the characteristic impedance can be obtained according to the above method to be about 120Ω, which is also the resistance value of the terminal resistance recommended by the CAN standard, so this 120Ω is measured. It is not calculated, it is calculated based on the actual wiring harness characteristics.

This is also the recommended resistance value in the CAN standard