Differences Between RS-232 and RS-485

Any form of communication requires rules to help ensure that everyone stays consistent. In electronics, these rules take the form of standards—a widely applicable set of design specifications published as recommendations by industry associations. Following these recommendations helps engineering equipment use the same electronic language, enabling efficient and reliable communication.

RS-232 (with “RS” standing for “recommended standard”) was introduced in the 1960s as a standardized interface for serial communication. While it is still useful for this purpose, alternatives such as RS-485 now exist, offering significantly enhanced performance. In this article, we will explore the most important differences between RS-232 and RS-485.

RS-232 is a point-to-point specification, meaning that one RS-232 device can only communicate with another RS-232 device. Although it is possible to creatively transform RS-232 into a “multipoint” network shared by more than two devices, the standard itself does not include this feature.

In contrast, RS-485 is a multipoint specification, making it more flexible. Multiple RS-485 devices can communicate without any special modifications or interface circuits, as shown in Figure 1. RS-485 drivers must be able to withstand 32 “unit loads,” meaning 32 receivers with a 15kΩ input impedance.

The original RS-232 standard specified logic levels of +25 V and -25 V. The claim that a typical home serial interface requires a signal swing of 50V is hard to believe, but that was over sixty years ago. Subsequent revisions of the standard reduced the signal swing to ±12V and then to ±5V. The voltage levels in RS-485 are much lower, which is one of the most significant differences between the two standards.

Figure 2 describes the logic level data flow and the RS-232 version of the same data flow. Note that in addition to the voltage level conversion, the polarity is also reversed. A +5V logic high level becomes -5V, while a 0V logic low level becomes +5V.

Typical logic level signals and RS-232 signals are single-ended, meaning that one information signal requires one electrical signal. The electrical signal is referenced to a ground potential of 0V. RS-485 signals are differential signals, meaning that one information signal requires two complementary electrical signals. The receiver extracts the information by comparing the two signals.

Figure 3 illustrates the difference between single-ended signals and differential signals.

The minimum differential amplitude of the signals generated by RS-485 compatible drivers is 1.5V; the minimum differential detection threshold for RS-485 receivers is 200mV. This ensures that even if the signal significantly degrades during transmission from the transmitter to the receiver, there is still enough margin to reliably detect digital data.

Figure 4 provides a visual representation of the minimum driver and receiver amplitudes for RS-485.

The signal swing of the RS-485 bus is much lower than that of the RS-232 interface. This is an important advantage of RS-485, as the smaller amplitude signals can simplify circuit design and enhance efficiency. The lower amplitude, combined with differential signaling, does not increase the device’s sensitivity to EMI. In fact, RS-485 communication is more robust than RS-232 communication.

A higher data rate is another benefit associated with smaller amplitude signals. The maximum data rate for RS-232 is about 1Mbps. Theoretically, the maximum for RS-485 is 10Mbps—actually, as shown in Figure 5, this limit is even higher.

RS-232 describes a complete solution for serial communication. It includes the following requirements:

In contrast, RS-485 specifies only electrical characteristics.

Neither of these standards defines a signal encoding method. However, RS-232 typically uses a Universal Asynchronous Receiver/Transmitter (UART) signaling scheme, which defines start bits and stop bits, parity, and data encoding, among others. RS-485 also frequently uses UART.

As shown in Figure 6, a byte of UART data contains:

If the receiver knows the data transmission rate or baud rate of the transmitter, it can use an internal timer to correctly sample the incoming data bits. UART communication does not require additional signals to organize binary data blocks. It doesn’t even need an external clock signal—using internal timers in both the transmitter and receiver configured for the same baud rate to generate and interpret the voltage levels.

RS-232 and RS-485 have similar names and uses, but they exhibit significant differences in specifications and implementation details. Their performance characteristics also differ greatly, with RS-485 surpassing RS-232 in almost every aspect. While RS-232 is a convenient and satisfactory interface for certain applications, RS-485 is a superior, future-oriented solution for serial communication.