Understanding the Voltage Level of RS-485 Communication

RS-485 is a widely used standard for differential signaling in communication systems. This article aims to provide a comprehensive understanding of the voltage levels associated with RS-485, ensuring clarity for professionals and enthusiasts alike.

Differential Signaling and Voltage Levels

RS-485 is a standard that defines a differential signaling system for balanced drivers and receivers. The core principle of RS-485 is that the information is transmitted by the difference in voltage between two signal lines, denoted as A and B. This differential signaling helps enhance the system's immunity to noise, allowing for longer transmission distances compared to single-ended signaling methods.

When discussing the voltage levels in an RS-485 system, it's important to understand that the standard does not specify exact absolute voltage values for the A and B lines. Instead, the key aspect is the differential voltage between the two lines. Typically, the A line is more positive than the B line when a logical high (1) is transmitted, and the B line is more positive than the A line when a logical low (0) is transmitted.

Logical Signal Levels

In practical terms, a logical high (1) is identified when the voltage difference between A and B is greater than 200 mV. Conversely, a logical low (0) is defined when the voltage difference is less than -200 mV. These thresholds ensure reliable communication in the presence of noise, which is a significant advantage of differential signaling.

Common-Mode Voltage Range

In addition to the differential signaling, RS-485 also specifies a range for the common-mode voltage. The common-mode voltage can vary within a range of 7V to 12V. This means that on top of the differential voltage, the common-mode voltage can be as low as -7V (below 0V) or as high as 12V (above 12V).

This wide range of common-mode voltage is crucial for RS-485, as it allows the system to operate effectively in environments with significant electrical noise. The ability to handle such a wide range of common-mode voltages is one of the key reasons why RS-485 is preferred in applications such as sensor networks, industrial automation, and other environments with high electromagnetic interference.

Detailed Signal Behavior

To illustrate the behavior of signals in an RS-485 system, consider the following example. Suppose the voltage on line A (A - lo) is less than the voltage on line B (B - hi) by more than 200 mV, the system interprets this as a logical 1. Conversely, if the voltage on line A is greater than the voltage on line B by more than 200 mV, the system interprets this as a logical 0.

Although the typical signal bias is from 0 to 5V, the definitive low (0) and high (1) statuses are determined solely by the 200 mV differential. This distinguishing characteristic makes RS-485 an efficient and robust communication protocol for a wide range of industrial and commercial applications.

Summary

RS-485 communication operates on a differential signaling principle, where the voltage difference between two lines (A and B) determines the logical status. The common-mode voltage range adds to the robustness of the system, enabling it to operate in environments with high noise levels. Understanding these voltage levels is crucial for anyone working with RS-485 communication systems, ensuring reliable and efficient communication in various applications.

Keywords: RS485, differential signaling, voltage levels