CAN bus termination resistor D-sub 9 pin male female connector  

120 Ohm termination resistor CAN H and CAN L

D-sub 9 pin 120 Ohm termination adapter

 
CAN bus terminator 

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

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

CAN terminating resistor

Terminating resistors are used to eliminate signal reflections in communication cables. During communication, there are two reasons for signal reflections: impedance discontinuity and impedance mismatch

The impedance is discontinuous, and the signal suddenly encounters the cable at the end of the transmission line with little or no impedance, and the signal will cause reflections at this place.

The principle of this signal reflection is similar to the reflection of light.

 
Terminal resistance is divided into single resistance and double resistance. RS-485 bus terminal matching mostly adopts double resistance parallel terminal matching. If single resistance method is adopted, the parallel resistance value of the load terminal is equal to or similar to the characteristic impedance of the transmission line, so that the static current will be reduced. It will be very large, and it requires high current driving capability, and is rarely used.

Double-resistor parallel terminal matching is because the parallel value of the two resistors is equal to or similar to the characteristic impedance of the transmission line, and each resistance is larger than the characteristic impedance of the transmission line, so the current requirement is not high.

However, whether it is a single resistor or a double resistor is matched in parallel, it will always bring DC power consumption and reduce the bus load capacity.

Generally, the characteristic impedance of the 485 bus transmission line is 120 ohms, and two 120 ohm resistors are used as the terminal matching resistors of the 485 bus.

Since the parallel resistance of the 485 bus will lead to DC power consumption, it is generally recommended that the terminal resistance should not be connected when the transmission distance does not exceed 300 meters and the transmission rate is low. It is only considered when there is signal reflection in the transmission and the communication is unstable.