What Are RS-485 / RS-232?
RS-232 and RS-485 are communication standards that carry UART serial data with specific voltage levels and electrical rules. They make logic-level UART suitable for long distances.
RS-232
It's an old but common standard. It uses a single-ended signal (relative to ground), usually works between two devices over a short distance (~15 m). It's the classic serial port of computers.
RS-485
It uses a differential (difference between two lines) signal; so it's very robust against noise, reaches up to 1 km and allows many devices (multi-drop) on a single line.
| Property | RS-232 | RS-485 |
|---|---|---|
| Signal | Single-ended | Differential |
| Distance | ~15 m | ~1 km |
| Devices | 2 | Many |
The Need for a Transceiver
Both need a transceiver chip (e.g. MAX232, MAX485) that converts the logic-level UART signal to the standard levels. A termination resistor is placed at the ends of an RS-485 line.
Where Is It Used?
RS-232: legacy device/PC connections, modems, debugging. RS-485: industrial automation, Modbus networks, building automation and long-distance sensor networks.
Half and Full Duplex
RS-485 usually works in two-wire half-duplex: the same line transmits and receives in turn, so direction control is needed. In the four-wire full-duplex version, transmit and receive are on separate pairs and allow simultaneous communication.
Common Mistakes
- Mixing levels: feeding an RS-232 level directly to an MCU causes damage.
- Termination: if the resistor is missing on RS-485, reflections occur.
- Direction control: if RS-485 is half-duplex, the transmit/receive direction must be driven correctly.
Frequently Asked Questions
Why does RS-485 go a long distance?+
What's its relationship to Modbus?+
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