Why RS-485 Requires Isolated Communication

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RS-485 is a physical layer bus design standard in industrial and instrumentation applications, and 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 capabilities, data transmission rates, cable lengths, and reliability are unmatched by other standards.

When communication needs to occur in noisy environments and over considerable distances, the RS-485 communication interface is a great choice.

Why Does RS-485 Require 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, so a reliable isolation design is very important.

Let’s discuss the three reasons why isolation is necessary.

1

Equipment and Personal Safety — High Voltage Impact

RS-485 is used for communication between devices, and often manufacturers have no idea what type of device their customers are communicating with.

If the other party uses a few dollars worth of resistor-capacitor voltage reduction to step down 220V to 12V, with no isolation from the power grid, it can be very dangerous during testing, debugging, and use; or if high-voltage equipment has insulation damage, high voltage on the RS-485 line can threaten both equipment and personal safety.

2

Remote Reception Anomalies — Potential Difference Impact

In many practical applications, communication distances can reach several kilometers, and the distance between nodes can also be quite far.Designers often connect each node’s reference ground to the local ground as the return ground for signals, which seems like a normal and reliable approach, but the ground is not an ideal “0” potential; the ground is also a conductor and has impedance.

When large currents flow through the ground, there will also be a potential difference across the ground. For example, in the diagram below, since A and B are far apart, the reference ground is not at “0” potential, and the ground line’s impedance is not “0”. Due to the current loop effect, the voltage at point A is Vs, while at point B it becomes Vc + Vs..

3

Data Anomalies/Device Damage — Ground Loop Impact

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

This is a huge mistake!This would only worsen the situation, as this long wire would form a massive ground loop with the earth!

As you may know from your school days, a closed loop generates current in a changing magnetic field.50Hz AC power lines and large motors 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” and cause communication anomalies.

When RS-485 Does Not Require Isolation

From the three reasons above, we can clearly see:

• Ensure that our devices do not connect to equipment that may have high voltage or pose a short-circuit risk.

• Ensure that RS-485 wiring does not exceed 100 meters.

• Ensure that there are no strong magnetic, high voltage, or high power devices on-site.

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