RS485 is often used in our equipment due to its convenient wiring and long transmission distance (theoretically, it can transmit 1200 meters). However, various problems often occur in practical applications. If it is distinguished according to the frequency of occurrence, it can be divided into four types:

The first is common mode interference. Common mode interference often refers to the common signal interference that is simultaneously loaded on each input signal interface section. Common mode interference is transmitted between the signal line and the ground, and belongs to asymmetric interference.

1) Judging from the interference source, the interference generated by lightning, nearby arc, nearby radio station, motor, frequency converter or other high-power radiation devices on the cable is common mode interference.

2) Judging from the frequency, common mode interference is mainly concentrated above 1MHz.

Methods for eliminating common mode interference include:

(1) Shielded twisted pair shall be adopted and effectively grounded

(2) Where there is a strong electric field, galvanized pipe shall be used for shielding

(3) When wiring, keep away from the high-voltage line, let alone bundle the high-voltage power line and signal line together for wiring

(4) Do not share the same power supply with the equipment that is easy to cause interference such as electric control lock

(5) Linear regulated power supply or high-quality switching power supply (ripple interference less than 50mV)

(6) Use differential circuit

The second is matching interference. The reason is that the system load matching is unreasonable. For example, the signal line is too long, there are too many (long-distance) star wires, too many loads, and there is no system matching resistance, etc., which leads to the unstable operation of the system.

The terminal matching resistor is used in the case of long-distance or high baud rate, and is not necessary for communication. It can be compared with not using it to achieve the best effect. The terminal matching resistance value depends on the impedance characteristics of the cable and is independent of the length of the cable. RS-485 is generally connected by twisted pair (shielded or unshielded). The terminal resistance is generally between 100 and 140 Ω, and the typical value is 120 Ω. In the actual configuration, one terminal resistor is connected to the two terminal nodes of the cable, i.e. the nearest end and the farthest end, while the node in the middle part cannot access the terminal resistor, otherwise, communication error will occur.

The third is hardware failure. Generally, there is a history of high voltage (lightning strike and leakage) in the line, which causes problems in the RS485 chip or isolation module of individual equipment of the system and affects the overall reception.

Generally, the method of disconnection and elimination is adopted for detection.

The fourth is line fault, such as local short circuit and one open circuit of signal line, which often makes the system work locally or normally, but the work is unstable.

Generally, the resistance is measured or eliminated one by one.