This article focuses on an isolated digital interface that adheres to the RS-485 standard, which remains one of the most widely used data transmission protocols in industrial and communication systems. It defines the RS-485 common-mode voltage range (CMVR) to explain how a transceiver’s signal should be properly isolated. Additionally, it discusses how the power supply path and local controller circuit can be exposed to significant common-mode voltages.
Isolation is a critical technique that prevents current from flowing between two points in a communication system while still allowing data and power signals to pass through. This protection is essential for preventing damage to sensitive electronics caused by high voltages. Isolation also helps maintain signal integrity by eliminating ground loops that can occur due to large differences in ground potential between devices.
Over the past decade, industry regulations have evolved, requiring equipment operating in harsh environments to implement isolation in their data transmission systems. There is now a growing trend toward multi-channel isolation technology, which enhances system reliability and performance. These applications are commonly found in telecommunications, industrial networks, medical devices, sensor interfaces, motor control systems, and metering equipment.
The article explores an isolated RS-485 digital interface, focusing on the CMVR defined by the RS-485 standard, which ranges from -7V to +12V. This range includes the driver output common-mode voltage (VOC), the ground potential difference (GPD) between the driver and receiver, and longitudinal coupling noise (VN). Understanding this range is crucial for ensuring reliable communication over long distances or in noisy environments.
In non-isolated RS-485 systems, the receiver processes only differential signals within the specified CMVR, rejecting any common-mode components. This is achieved through internal voltage dividers and a differential comparator, which amplifies only the useful signal. However, when dealing with higher common-mode voltages, such as ±25V, the transceiver must be redesigned to handle these conditions, often involving higher resistance values and improved driver output capabilities.
For very high common-mode voltages—often reaching hundreds of volts—galvanic isolation becomes necessary. This type of isolation physically separates the transceiver from the power and signal paths, protecting both the system and the connected devices.
Figure 3 illustrates an isolated RS-485 data link where the receiver is electrically separated from its local controller. The power rails are converted using an isolated DC/DC converter, creating floating power lines for the transceiver. Data isolation is handled by a digital isolator, ensuring safe and reliable signal transfer.
As shown in Figure 4, the isolation barrier introduces a very high resistance (RISO = 10^14 Ω) in series with the receiver's common-mode resistance (RCM = 10^5 Ω). This configuration ensures that the majority of the common-mode voltage drops across the isolation barrier, effectively eliminating its impact on the receiver. The isolated ground (GND2-ISO) tracks the receiver input voltage, preventing any risk of exceeding the maximum input voltage rating. Meanwhile, the isolated power supply (VCC2-ISO) remains stable regardless of the common-mode voltage level.
By incorporating galvanic isolation, RS-485 systems can operate reliably in challenging environments, offering enhanced safety, stability, and performance. This makes them ideal for use in a wide range of industrial and commercial applications.
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