Discrete and Process Automation

A conversation about communication standards

15 May 2024
Key characteristics of an RS-485 bus utilized by multiple transceivers. Source: Analog Devices

Industry associations establish recommended standards (RS) to enable data acquisition and control systems to speak the same electronic language. RS-232 was introduced in 1960 as a standardized interface for serial communication and remains useful for this purpose, but alternatives such as RS-485 now offer significantly enhanced performance.

What are the differences?

RS-232 is a point-to-point specification: one RS-232 device can communicate with only one other RS-232 device. By contrast, RS-485 is a multipoint specification that allows multiple devices to communicate without any special modifications or interface circuits. The RS-485 driver must be able to sustain 32 unit loads, meaning 32 receivers with 15 kΩ input impedance.

Typical logic-level signals and RS-232 signals are single-ended — one information signal requires one electrical signal. RS-485 signals are differential — one information signal requires two complementary electrical signals. The receiver extracts information by comparing the two signals.

A major benefit of an RS-485 bus is its considerably lower signal swing relative to that of an RS-232 interface. The smaller-amplitude signals allow for simplified circuit design and improved efficiency. As the lower amplitudes are combined with differential signaling, they don’t increase a device’s susceptibility to electromagnetic interference.

Higher data rates are another benefit associated with smaller-amplitude signals. The maximum data rate for RS-232 is about 1 Mbps, and the maximum for RS-485 is 10 Mbps.

[Learn more about these communication standards on GlobalSpec]

With these performance capabilities, RS-485 communication is more robust than RS-232 communication. Though RS-232 can be a convenient and satisfactory interface for certain applications, RS-485 is a superior, more future-proof solution for serial communication.

To contact the author of this article, email GlobalSpecEditors@globalspec.com

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