This article explores an isolated digital interface that adheres to the RS-485 standard, which remains one of the most widely used protocols for data transmission in industrial and commercial applications. It defines the RS-485 common-mode voltage range (CMVR) to explain how signals should be properly isolated within the transceiver. Additionally, it discusses how power supply paths and local controller circuits can experience significant common-mode voltages.
Isolation is a critical technique used to prevent current from flowing between two communication points while allowing data and power signals to pass through. This not only protects sensitive electronic components from high voltage damage but also maintains signal integrity by eliminating ground loops caused by large ground potential differences.
Over the past decade, regulatory requirements have evolved, mandating isolation in data transmission systems for equipment operating in harsh environments. There's a growing trend toward multi-channel isolation technology, leading to more advanced isolation components. These are commonly found in telecommunications, industrial networks, medical systems, sensor interfaces, motor control, and metering applications.
The article focuses on an isolated RS-485 interface and explains the CMVR defined by the standard, which is -7V to +12V. It illustrates how this range includes driver output common-mode voltage, ground potential difference, and longitudinal coupling noise. The receiver processes only differential signals within this range, rejecting common-mode components using internal dividers and a differential comparator.
For higher common-mode voltages, such as ±25V, transceivers are redesigned with improved driver output transistors and higher divider ratios. For even higher voltages, galvanic isolation barriers are necessary to protect the transceiver from dangerous levels of voltage.
Figure 3 demonstrates an isolated data link where the receiver is separated from its local controller. Isolation involves both power and data lines, with isolated DC/DC converters transforming grounded microcontroller power rails into floating power rails for the transceiver. Data path isolation is achieved through a digital isolator, ensuring safe and reliable communication.
In Figure 4, the common-mode equivalent circuit shows how the isolation barrier, with a very high resistance, effectively decouples the receiver from the ground, preventing common-mode voltages from affecting signal integrity. This ensures stable operation regardless of the common-mode voltage level, making the system more robust and reliable.
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