What is a terminating resistor?

The resistance of the pair of communication lines is connected in parallel at both ends of the online network (on the two communication ports that are farthest apart). The termination matching resistance value depends on the impedance characteristics of the cable, regardless of the length of the cable. RS-485/RS-422 is usually connected by twisted pair (shielded or unshielded). The terminating resistor is generally between 100 and 140 Ω, and the typical value is 120 Ω. In actual configuration, on the two terminal nodes of the cable, that is, the nearest end and the farthest end, each terminal resistance is connected, and the node in the middle part cannot access the terminating resistor, otherwise communication errors will occur.

So what is the role of adding terminal resistance? In general, the terminating resistor is used to eliminate the signal reflection in the communication cable. According to the transmission line theory, the terminating resistor can absorb the reflected wave on the network and effectively enhance the signal strength, so that the long-distance communication can be realized.

There are two reasons for signal reflection:

(1) Impedance discontinuity: The signal suddenly encounters a small or no cable impedance at the end of the transmission line, and the signal will cause reflection at this place. The principle of such signal reflection is similar to the reflection of light from one medium into another. The method of eliminating this reflection is to connect a terminal of the same size as the characteristic impedance of the cable at the end of the cable. The resistor makes the impedance of the cable continuous. Since the transmission of the signal on the cable is bidirectional, a terminating resistor of the same size can be connected across the other end of the communication cable.

(2) Impedance mismatch: Another cause of signal reflection is the impedance mismatch between the data transceiver and the transmission cable. In the high-frequency circuit, when the frequency of the signal is high, the wavelength of the signal is very short. When the wavelength is short compared with the length of the transmission line, the superimposed reflection signal on the original signal will change the shape of the original signal, and the signal is made. Waveform distortion (depression or bulging). The reflection caused by this reason is mainly manifested in the fact that when the communication line is in the idle mode, the entire network data is confusing.

Then from the above understanding of the terminal resistance, we can explain the customer’s problem, that is to say, if the customer uses the UN 124XP terminal serial port long-distance 485 communication, it is necessary to access this resistor to ensure the smooth communication of long-distance communication, resistance The size is 120Ω.