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RS-485 is a physical layer bus design standard in industrial and instrumentation applications, which 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 twisted pair of 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 considerable 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, so a reliable isolation design is very important. Let’s discuss the three reasons why isolation is necessary.
Device and Personal Safety—High Voltage Impact
RS-485 is used for communication between devices, and many times, as manufacturers, we have no idea what type of equipment our customers are communicating with. If the other party uses a few bucks worth of resistive-capacitive voltage reduction principles to reduce 220V to 12V, with no isolation from the power grid, it can be very dangerous during testing, debugging, and use. Alternatively, if high-voltage equipment has insulation damage, and high voltage is present on the RS-485 line, it poses a threat to both equipment and personal safety.
Remote Reception Abnormalities—Potential Difference Impact
In many practical applications, communication distances can reach several kilometers, and the distance between nodes can be quite far. Designers often connect the reference ground of each node to the local earth as the return ground for the signal. While this seems like a normal and reliable approach, 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 a potential difference across the earth at the current’s path.For example, in the figure below, due to the distance between A and B, the reference ground is not at “0” potential, and the ground wire’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.
Data Abnormalities, Device Damage—Ground Loop Impact
Some might say: If there is a potential difference in the earth between nodes, couldn’t we just connect the grounds of the two nodes with a wire?This is a huge mistake! Doing so would only make the situation worse; this long wire would form a massive ground loop with the earth!
As you may know from your school days, a closed loop in a changing magnetic field generates current.50Hz AC power lines and large motors are sources of AC magnetic fields. If the bus runs close to or through these areas, the ground loop can generate currents reaching hundreds of amperes.
The common-mode voltage generated by currents flowing through the ground loop can disrupt normal communication on the bus. In addition to stable magnetic field sources, transient interferences such as surges from power lines, lightning strikes, and high-frequency noise can also be 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 see:
1. Ensure our devices do not connect to potentially high-voltage equipment 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, high-voltage, or high-power devices on site.
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