In practical industrial, power, automation, and instrumentation applications, the RS-485 bus standard is one of the most widely used physical layer communication protocols. It is particularly valuable in harsh electromagnetic environments, where ensuring reliable operation of data ports is critical. To meet these demands, compliance with relevant electromagnetic compatibility (EMC) regulations is essential. In this article, Shijian Company explores how to effectively protect RS-485 ports by leveraging the advantages of ADI’s agent line RS-485 chips and Bourns’ EMC protection devices, from theoretical analysis to real-world testing.
When designing an RS-485 port for EMC compliance, three key factors must be considered: electrostatic discharge (ESD), electrical fast transient (EFT), and surge immunity. These are defined by the IEC 61000-4 series standards. ESD refers to sudden discharges of static electricity between two objects at different potentials, while EFT involves rapid voltage spikes caused by switching operations. Surge, on the other hand, is typically associated with lightning strikes or power system faults, generating high-energy voltage transients. Designers must ensure that these disturbances do not compromise the integrity of the communication lines.
Angus Zhao, Deputy Director of Technical Support at Excelpoint, emphasizes that a well-designed RS-485 port must meet the requirements of ESD, EFT, and surge immunity. He explains that understanding these specifications is crucial when developing EMC solutions. For example, in China, GB/T 17626.2, GB/T 17626.4, and GB/T 17626.5 correspond to IEC 61000-4-2, IEC 61000-4-4, and IEC 61000-4-5, respectively. This article uses the IEC standards as a reference.
Electrostatic Discharge (ESD) is characterized by a sudden transfer of charge between two bodies with different potentials. The IEC 61000-4-2 standard defines four levels of ESD testing, ranging from Level 1 (least severe) to Level 4 (most severe). Each level corresponds to different environmental conditions, with higher levels being required in more demanding industrial settings.
Electrical Fast Transient (EFT) refers to short, high-frequency voltage pulses that can couple into signal lines through capacitive or inductive means. These transients are common in industrial environments due to relay contact bounce, switching of inductive loads, and similar events. The IEC 61000-4-4 standard outlines test levels for EFT, which range from Level 1 (good environment) to Level 4 (poor industrial environment).
Surge immunity is concerned with high-energy overvoltage events, such as those caused by lightning strikes or power system faults. The IEC 61000-4-5 standard defines specific waveforms and test levels for assessing a device's ability to withstand surges. Surges are generally more severe than ESD or EFT, as their energy levels can be several orders of magnitude higher.
To address these challenges, engineers often implement a two-tiered protection strategy: primary and secondary protection. The primary protection device, such as a gas discharge tube (GDT) or thyristor surge protector (TISP), absorbs and diverts most of the transient energy. Secondary protection, usually involving TVS diodes, provides additional safeguarding against any residual voltages or currents that may pass through the primary stage.
For example, in one solution, a combination of Bourns’ CDSOT23-SM712 TVS array, TBU overcurrent protection, and TISP surge protector is used to achieve robust ESD, EFT, and surge protection. Another solution incorporates a GDT to handle even higher surge levels, offering enhanced protection in extreme conditions.
These protection schemes have been tested and certified by third-party laboratories to ensure they meet IEC 61000-4-2, IEC 61000-4-4, and IEC 61000-4-5 standards. By carefully selecting components and designing circuits with coordination in mind, it is possible to create compliant, reliable RS-485 interfaces that perform well in challenging environments.
In conclusion, designing an EMC-compliant RS-485 port requires a thorough understanding of the relevant standards, the characteristics of the protection devices, and the application environment. With the right approach, it is entirely feasible to develop a robust and cost-effective protection solution that ensures long-term reliability in industrial settings.
TFT Display
TFT (Thin Film Transistor) is a thin film field effect transistor. The so-called thin film transistor means that each liquid crystal pixel on the Liquid Crystal Display is driven by a thin film transistor integrated behind it. This can Display Screen information at high speed, high brightness, and high contrast. TFT is an active matrix liquid crystal display.
The TFT-LCD liquid crystal display is a thin film transistor type liquid crystal display, also known as "true color" (TFT). TFT liquid crystal has a semiconductor switch for each pixel, and each pixel can be directly controlled by dot pulses, so each node is relatively independent and can be continuously controlled, which not only improves the response speed of the display, but also can be accurately controlled Display color gradation, so the color of TFT liquid crystal is more real.
In the fierce competition among many flat panel displays, why TFT-LCD can stand out and become the next-generation mainstream display is by no means accidental, it is the inevitable development of human technology and thinking mode. The liquid crystal has avoided the difficult light-emitting problem successively, and the light-emitting display device is decomposed into two parts by using the excellent characteristics of the liquid crystal as a light valve, namely the light source and the control of the light source. As a light source, brilliant results have been achieved in terms of luminous efficiency, full color, and life, and they are still being deepened. Since the invention of LCD, the backlight has been continuously improved, from monochrome to color, from thick to thin, from side fluorescent lamp type to flat fluorescent lamp type. The latest achievements in luminous light sources will provide new backlight sources for LCDs. With the advancement of light source technology, newer and better light sources will appear and be applied to LCDs. The rest is the control of the light source, the technology and process of the semiconductor large-scale integrated circuit are transplanted, the thin film transistor (TFT) production process has been successfully developed, the matrix addressing control of the liquid crystal light valve is realized, and the light of the liquid crystal display is solved. The cooperation of the valve and the controller enables the advantages of the liquid crystal display to be realized.
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