What is RS-232?
RS-232 (Recommended Standard 232) is a standard for serial communication that is widely used to exchange data between computers and peripheral equipment, such as PLCs, Sensors, HMIs or modems.
The standard was developed in the 1960s and is mainly intended for point-to-point communication over short distances.
🧠 What does RS-232 do?
RS-232:
- Sends data bit by bit (serially) over a single cable
- Uses two devices: a DTE (e.g. a computer) and a DCE (e.g. a modem or PLC)
- Operates with voltage levels (e.g. +12V = logic 0, -12V = logic 1)
- Uses specific signal lines, such as TX (transmit) and RX (receive)
🔧 Key characteristics
| Feature | Description |
|---|---|
| Communication type | Serial, asynchronous |
| Point-to-point | Only 2 devices per connection (no bus structure) |
| Max. distance | Around 15 metres (depending on baud rate and cable quality) |
| Typical baud rates | 9600 – 115200 bits per second |
| Connectors | Usually DB9 or DB25 |
| Signal levels | ±3V to ±15V (typically ±12V) |
🧱 Common RS-232 signal lines
| Signal | Function |
|---|---|
| TXD | Transmit Data |
| RXD | Receive Data |
| GND | Signal ground |
| RTS/CTS | Handshake signals (optional) |
🏭 Applications of RS-232
- Communication between a PLC and an HMI
- Connection to industrial modems or data loggers
- Local communication with energy meters or instruments
- Connection to legacy machines or configuration tools
🔄 RS-232 vs. RS-485
| Aspect | RS-232 | RS-485 |
|---|---|---|
| Number of devices | 1-to-1 | 1 master, up to 32 slaves |
| Distance | Up to ±15 metres | Up to ±1200 metres |
| Speed | Lower | Higher possible |
| Cabling | 3 or more wires | 2 wires (differential) |
| Noise immunity | More susceptible | Better resistance to noise |
✅ Benefits of RS-232
- Simple and widespread
- No additional logic needed for simple connections
- Well supported in many devices
❗ Limitations
- Only one-to-one connection
- Susceptible to interference over long distances
- Outdated compared with modern protocols such as USB, Ethernet or RS-485
📌 In summary
RS-232 is a classic serial communication standard that is still used for simple, direct connections in industrial systems. It is ideal for short distances and legacy applications where reliability is important, but speed or scalability is less critical.