Any form of communication requires rules to help ensure that everyone stays on the same page. In electronics, these rules take the form of standards—widely applicable sets of design specifications published as recommendations by industry associations. When followed, these recommendations help engineered devices speak the same electronic language and thus achieve efficient, reliable communication.
RS-232 (the “RS” abbreviates “recommended standard”) was introduced in the 1960s as a standardized interface for serial communication. Though it remains useful for this purpose, alternatives such as RS-485 now exist and offer significantly enhanced performance. In this article, we’ll take a look at the most important differences between RS-232 and RS-485.
Point-to-Point vs. Multipoint
RS-232 is a point-to-point specification, meaning that one RS-232 device can communicate with only one other RS-232 device. Though with a bit of creativity, it’s possible to turn RS-232 into a “multidrop” network shared by more than two devices, the standard itself does not incorporate this functionality.
Because it’s a multipoint specification, RS-485 is much more flexible. Multiple RS-485 devices can communicate without any special modifications or interface circuits, as shown in Figure 1. An RS-485 driver must be able to sustain 32 “unit loads,” meaning 32 receivers with 15 kΩ input impedance.
Figure 1. This diagram conveys key characteristics of an RS-485 bus utilized by multiple transceivers. Image used courtesy of Analog Devices
Voltage Levels
The original RS-232 standard specified logic levels of +25 V and –25 V. It strains belief that an ordinary household serial interface would need 50 V of signal swing, but this was, after all, more than sixty years ago. Subsequent revisions to the standard lowered the signal swing to ±12 V and then to ±5 V. The voltage levels in RS-485 are much lower—it’s one of the most conspicuous differences between the two standards.
The diagram in Figure 2 depicts a logic-level data stream and the RS-232 version of that same data stream. Note that in addition to voltage-level conversion, the polarity is inverted. A +5 V logic-high becomes –5 V, while a 0 V logic-low becomes +5 V.
Figure 2. Logic-level data (top) and corresponding RS-232 signals generated by an RS-232 line driver (bottom). Image used courtesy of MIT
Single-Ended vs. Differential Signaling
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 the 0 V ground potential. RS-485 signals are differential, meaning that one information signal requires two complementary electrical signals. The receiver extracts information by comparing the two signals.
Figure 3 illustrates the difference between single-ended and differential signaling.
Figure 3. Single-ended and differential signals. Image used courtesy of All About Circuits
Signals generated by an RS-485-compliant driver have a minimum differential amplitude of 1.5 V; an RS-485 receiver has a minimum differential detection threshold of 200 mV. That way, there’s still sufficient margin for reliable detection of the digital data even if the signals degrade significantly as they travel from transmitter to receiver.
Figure 4 provides a visual representation of RS-485’s minimum driver and receiver amplitudes. The image source, a Texas Instruments application note titled “The RS-485 Design Guide,” is a good resource if you’re looking for detailed information on the standard.
Figure 4. Minimum amplitudes for an RS-485 driver and receiver. Image used courtesy of Texas Instruments
Signal Swing
The signal swing of an RS-485 bus is much lower than that of an RS-232 interface. This is an important benefit of RS-485, since smaller-amplitude signals allow for simplified circuit design and improved efficiency. Because the lower amplitudes are combined with differential signaling, they don’t increase the device’s susceptibility to EMI. In fact, RS-485 communication is more robust than RS-232 communication.
Higher data rates are another benefit associated with smaller-amplitude signals. The maximum data rate for RS-232 is about 1 Mbps. Theoretically, RS-485’s maximum is 10 Mbps—in practice, as Figure 5 shows, the limit is higher.
Figure 5. RS-485’s maximum data rate increases as cable length decreases. Image used courtesy of Analog Devices
Signal Encoding
RS-232 describes a complete solution for serial communication. It includes requirements for:
- Electrical characteristics.
- Signal characteristics.
- Connection schemes.
- Mechanical interfacing.
By contrast, RS-485 specifies only the electrical characteristics.
Neither of these standards define a signal encoding methodology. However, RS-232 typically uses the universal asynchronous receiver/transmitter (UART) signaling scheme, which defines start and stop bits, parity, and data encoding, among other things. RS-485 often makes use of UART as well.
As we see in Figure 6, one byte of UART data contains:
- A start bit.
- Eight data bits.
- A stop bit.
Figure 6. One byte of UART data. Image used courtesy of All About Circuits
If the receiver knows the transmitter’s data transfer rate, or baud rate, it can use an internal timer to correctly sample incoming data bits. UART communication doesn’t require an additional signal for organizing blocks of binary data. It doesn’t even require an external clock signal—the voltage levels are generated and interpreted using internal timers in the transmitter and receiver, which are configured for the same baud rate.
Key Takeaways
RS-232 and RS-485 have similar names and purposes, but they exhibit crucial differences in their specifications and the details of their implementation. Their performance characteristics are also very different, with RS-485 surpassing RS-232 in almost every way. 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.
Featured image used courtesy of Adobe Stock
