Shulou(Shulou.com)06/01 Report--

This article shows you how the electromagnetic compatibility design of the RS485 interface in EMC is. The content is concise and easy to understand, which will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

1. Introduction to RS485 API

RS-485 uses balanced transmission and differential reception to realize communication, and the detailed electrical parameters of its interface are as follows:

Interface definition: a, B

Operating voltage:-7V-12V

Signal rate: 10Mb/s

Interface cable: twisted pair shielded cable

Routing mode: according to the actual situation, the maximum length is 1000m.

2. EMC design requirements

RS485 is used for communication between equipment and computer or other equipment. In application and air-conditioning products, it is often mixed with power supply, power signal, etc. The RS485 interface design may affect EMC problems as follows:

1. Radiation emission problem: the RS485 interface will bring out the interference inside the veneer and form external radiation through the power cord, resulting in radiation emission test exceeding the standard.

2. Anti-interference problems: RS485 will be affected by external interference will lead to communication anomalies, such as pulse group interference, radio frequency interference in air-conditioning products will lead to abnormal communication due to compressors, fans, solenoid valves and other interference sources; because the RS485 wiring is long, there may be outdoor wiring, so lightning protection design should be considered

3. Principle design of RS485 interface.

1. D1 and D2 are gas discharge tubes, which are mainly used to discharge common mode surge energy. the breakdown voltage of gas discharge tube is 300V and the flow rate is 1kA.

2. D3, D4 and D5 are semiconductor discharge tubes, which are mainly used to discharge common mode and differential mode surge energy. Semiconductor discharge tube type BS0300N-C has a turn-on voltage of 40V and a flow rate of 100A.

3. R1 and R2 choose a resistance of 10 Ω, (no comma) 1 to 2W, because the starting voltage of the semiconductor discharge tube is lower than that of the gas discharge tube, in order to ensure the smooth conduction of the gas discharge tube, the discharge and amplification energy must be increased by the resistance (R1, R2) to ensure that most of the energy is discharged through the gas discharge tube, and the semiconductor discharge tube provides fine protection.

4. C1 and C2 capacitors provide a low impedance reflux path for interference, which can effectively reduce the external common mode interference current.

The capacitance can be adjusted according to the actual situation, from 22pF to 1000pF, and the typical value is 100pF.

5. The common mode inductor can attenuate the common mode interference, suppress the internal and external interference of the veneer, improve the anti-interference ability of the product, and at the same time reduce the external radiation through the signal cable. the common mode inductor selects the device with an impedance of 2200 Ω when using 100MHz.

6. In the actual test, if there is an EMC problem in the interface, the above components can be adjusted, and the specific component adjustment parameters refer to the "Standard for selection of commonly used electromagnetic compatibility Devices".

4. PCB design of RS485 interface

PCB Design Essentials:

1. The layout of components should be compact, and the position of components should be arranged according to the direction of signal flow.

2. Follow the principle of protection before filtering

3. When designing the ground, it is necessary to ensure that the protective ground is isolated from the digital ground.

4. The signal alignment should be parallel and isometric.

5. When laying copper in PCB, pay attention to the protective devices and common-mode inductors hollowed out directly below.

The above content is what the electromagnetic compatibility design of RS485 interface in EMC is like. Have you learned the knowledge or skills? If you want to learn more skills or enrich your knowledge reserve, you are welcome to follow the industry information channel.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.