RS-232, RS-422 and RS-485 serial communication

RS-232, RS-422 and RS-485 are a related family of standards for serial data communication, in which the bits of each character are sent one after another over a small number of wires rather than in parallel. They define the electrical signalling — voltage levels, wiring and drive characteristics — rather than the meaning of the data, so many higher-level protocols run on top of them. All three are commonly used with asynchronous framing, where each byte is wrapped in a start bit, optional parity bit and one or more stop bits, and both ends must agree on a bit rate (baud).

RS-232 (also called EIA/TIA-232) is the classic PC serial-port standard. It is single-ended: each signal is a voltage measured against a shared ground, using bipolar levels in which a negative voltage represents a logic 1 (mark) and a positive voltage a logic 0 (space). Because it is referenced to ground and uses fairly large voltages, RS-232 is limited to short, point-to-point links of roughly 15 metres at modest speeds, typically over DB-9 or DB-25 connectors with separate transmit and receive lines.

RS-422 and RS-485 instead use differential (balanced) signalling, sending each bit as the voltage difference between two wires. A receiver reads that difference and rejects noise picked up equally on both wires, which allows much longer cables (on the order of 1200 metres) and higher data rates. RS-422 drives one transmitter to many receivers, while RS-485 supports true multi-drop networks with many transceivers sharing a two-wire bus, usually half-duplex. These properties make RS-485 a workhorse of industrial automation and building control, underpinning protocols such as Modbus, Profibus and the DMX512 lighting standard.

Frequently asked questions

What is the main difference between RS-232 and RS-485?

RS-232 is single-ended, short-range and point-to-point, while RS-485 is differential and supports long cable runs and many devices on one multi-drop bus.

How far can an RS-232 link reach?

About 15 metres (50 feet) at standard rates. The practical distance falls as the baud rate rises, because cable capacitance distorts the signal.

Can I connect an RS-232 device directly to an RS-485 device?

No. They use different electrical levels and signalling, so you need a converter or a transceiver that translates between the two standards.