What is RS-485?
RS-485 is a serial communication protocol used for reliable data transmission over long distances and in electromagnetically harsh industrial environments.
It is particularly popular in automation and industrial applications because of its multi-device support, long cable length and noise-resistant signal transmission.
🧠 What does RS-485 do?
RS-485:
- Sends data serially (bit by bit) through differential signals over two (or more) wires
- Supports communication between one master and multiple slaves (multidrop)
- Is often used as the physical layer for protocols such as Modbus RTU
🔧 Key characteristics
| Feature | Description |
|---|---|
| Type | Serial, differential communication |
| Number of devices | 1 master and up to 32 (or more) slaves per bus |
| Topology | Bus structure with terminators at both ends |
| Cable length | Up to 1200 metres (at 100 kbit/s) |
| Speed | Up to 10 Mbit/s (at shorter distances) |
| Signal lines | A and B (differential pair), optional: GND |
| Standard | EIA-485 (synonym for RS-485) |
🏭 Typical applications
- Modbus RTU communication between PLCs, meters and HMIs
- Building automation (HVAC, access control)
- Industrial networks on machines and production lines
- Connection to Sensors, Actuators or I/O modules
- Legacy systems with serial communication
🔄 RS-485 vs. RS-232
| Feature | RS-232 | RS-485 |
|---|---|---|
| Number of devices | 1-to-1 connection | 1 master, up to 32 (or more) slaves |
| Distance | Up to 15 metres | Up to 1200 metres |
| Speed | Lower | Higher |
| Cabling | 3+ wires | 2 wires (differential) |
| Noise sensitivity | Higher | Lower (better noise resistance) |
⚙️ Connection tips
- Use twisted-pair cable for better noise immunity
- Add termination resistors at both ends of the bus
- Maintain consistent polarity (do not swap lines A and B)
- Avoid T-splices in cabling (preferably a linear structure)
📌 In summary
RS-485 is a robust and scalable communication protocol, ideal for serial networks in industrial automation. It is widely used as the physical layer for Modbus RTU and provides reliable communication over long distances and many devices.