Terminal Resistor 120 Ohm, DB9, CAN Bus

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All CAN networks need two terminators; one at each end of the network. 

The CAN Terminator can be used where the CAN cable or CAN node at the end does not have internal termination. 

Why choose a 120Ω resistor?

What is impedance? In electricity, the resistance to the current in the circuit is often called impedance. The unit of impedance is ohm, which is usually expressed as Z, which is a complex number Z= R+i(ωL–1/(ωC)). Specifically, impedance can be divided into two parts, resistance (real part) and reactance (imaginary part). The reactance includes capacitive reactance and inductive reactance. The current obstruction caused by capacitance is called capacitive reactance, and the current obstruction caused by inductance is called inductive reactance. Impedance here refers to the mode of Z.

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.

Using two typical cables used in automobiles and twisting them into twisted pairs, the characteristic impedance can be obtained according to the above method, which is 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. Of course, it is also defined in the ISO 11898-2 standard.

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