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RS-485 is a physical layer bus design standard used in industrial and instrumentation applications, and it has become one of the most widely used standard communication interfaces in the industry. This communication interface allows for multipoint bidirectional communication over a simple pair of twisted wires, and its noise suppression capability, data transmission rate, cable length, and reliability are unmatched by other standards. When communication is required in noisy environments and over certain distances, the RS-485 communication interface is an excellent choice.
So, do you know why RS-485 requires isolated communication?
During long-distance transmission, there are often interferences such as ground loops and transient voltages, making a reliable isolation design very important. Let’s discuss the three reasons why isolation is necessary.
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Device and Personal Safety—High Voltage ImpactRS-485 is used for communication between devices, and often, as manufacturers, we have no idea what type of devices our customers are communicating with. If the other party uses a few dollars’ worth of resistor-capacitor voltage reduction principles to step down 220V to 12V without any isolation from the power grid, it can be very dangerous during testing, debugging, and usage. Alternatively, if high-voltage equipment has insulation damage, high voltage on the RS-485 line can threaten both device and personal safety.
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Remote Reception Anomalies—Impact of Potential DifferenceIn many practical applications, communication distances can reach several kilometers, and the distance between nodes can be quite far. Designers often directly connect the reference ground of each node to the local earth as the return ground for signals, which seems like a normal and reliable practice. However, the earth is not an ideal “0” potential; it is also a conductor and has impedance. When large currents flow through the earth, there will also be potential differences across the earth where the current flows.For example, in the diagram below, due to the distance between A and B, the reference ground is not at “0” potential, and the impedance of the ground line is not “0”. Due to the current loop effect, the voltage at point A is Vs, while at point B it becomes Vc + Vs.
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Data Anomalies, Device Damage—Impact of Ground LoopsSome may say:Since there is a potential difference in the ground between nodes, can’t we just connect the grounds of the two nodes with a wire? This is a big mistake!Doing so will only exacerbate the situation; this long wire will form a massive ground loop with the earth!As we learned in school, a closed loop in a changing magnetic field will generate current. 50Hz AC power lines, large motors, etc., are sources of alternating magnetic fields. If the bus is close to or passes through these areas, the ground loop can generate currents of up to hundreds of amperes.The common-mode voltage generated by the current flowing through the ground loop will affect normal communication on the bus. In addition to stable magnetic field sources, surges from power lines, lightning strikes, and high-frequency noise can also be transient interferences picked up by this giant “loop antenna”, causing communication anomalies.
Under what circumstances can RS-485 operate without isolation?
From the three reasons above, we can clearly understand:
1. Ensure that our devices do not connect to potentially high-voltage devices or scenarios with high-voltage short-circuit risks.
2. Ensure that RS-485 wiring does not exceed 100 meters.
3. Ensure that there are no strong magnetic fields, high voltages, or high-power devices on site.
Disclaimer: The materials in this article are sourced from the internet, and the copyright belongs to the original author. If there are any copyright issues, please contact me for removal.
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