Why RS-485 Requires Isolated Communication

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 speed, cable length, and reliability are unmatched by other standards.

When communication is needed in noisy environments and over considerable distances, the RS-485 communication interface is an excellent choice.

Why RS-485 Requires Isolated Communication

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 three reasons why isolation is necessary.

Device and Personal Safety — High Voltage Impact

RS-485 is used for communication between devices, and often, manufacturers are unaware of what types of devices their customers are connecting to their equipment.

If the other party uses a cheap resistor-capacitor voltage divider to step down 220V to 12V without complete isolation from the power grid, it can be very dangerous during testing, debugging, and use; or if the insulation of high-voltage equipment fails and high voltage appears on the RS-485 line, it poses serious risks to both equipment and personal safety.

Remote Reception Anomalies — The Impact of Potential Differences

In many practical applications, communication distances can reach several kilometers, and the nodes can be very far apart.Designers often directly connect each node’s reference ground to the local earth as the return ground for signals, which seems like a normal and reliable practice, but 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 ends of the earth. For example, in the figure below, due to the distance between A and B, the reference grounds are not at “0” potential, and the impedance of the ground wire is not “0”. Due to the current loop effect, the voltage at point A is Vs, while at point B it becomes Vc + Vs..

Data Anomalies/Device Damage — The Impact of Ground Loops

Some people might say:Since there is a potential difference between the grounds of the nodes, wouldn’t it be fine to connect the grounds of the two nodes with a wire?

This is a big mistake!This would only exacerbate the situation as this long wire would form a large ground loop with the earth!

As everyone knows from their student days, a closed loop in a changing magnetic field will generate current.50Hz AC power lines, large motors, etc., are sources of AC magnetic fields. If the bus runs 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, high-frequency noise, and other transient interferences can also be picked up by this giant “loop antenna,” causing communication anomalies.

When RS-485 Does Not Require Isolation

From the three reasons above, we can clearly see:

Ensure that our equipment does not connect to devices that may have high voltage or pose a risk of high voltage short circuits.
Ensure that the wiring of RS-485 does not exceed 100 meters.
Ensure that there are no strong magnetic, high voltage, or high-power devices on site.

END

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Why RS-485 Requires Isolated Communication

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