Serial Communication
RS-232 vs RS-485: Serial Communication Basics
Serial communication remains essential in industrial environments. RS-232 and RS-485 are the two main standards you'll encounter. Understanding their differences helps you connect devices correctly and troubleshoot communication issues.
RS-232: Point-to-Point
RS-232 was designed for connecting a single device to a computer — think terminal to modem. It uses voltage levels of ±3V to ±15V relative to ground. RS-232 is inherently point-to-point: one transmitter, one receiver. Maximum cable length is officially 15 meters, though shorter runs are more reliable. Common connectors are DB-9 and DB-25.
RS-485: Multi-Drop Networks
RS-485 was designed for industrial multi-drop networks. It uses differential signaling — the voltage difference between two wires — making it highly noise-immune. RS-485 supports up to 32 devices on a single bus at distances up to 1,200 meters. It's half-duplex (two wires) or full-duplex (four wires). This makes it ideal for protocols like Modbus RTU.
Wiring Considerations
RS-232 uses separate TX and RX lines with a common ground. Wiring is straightforward but requires attention to null modem vs. straight-through configurations. RS-485 uses a twisted pair (A and B or D+ and D-) that must be properly terminated with 120Ω resistors at each end of the bus to prevent reflections. Improper termination is the most common RS-485 problem.
When to Use Each
Use RS-232 for simple device configuration, short-distance programming connections, or legacy equipment with RS-232 ports. Use RS-485 for production communication, multi-device networks, longer distances, or noisy environments. Many devices support both — PLCs often have RS-232 programming ports and RS-485 communication ports.
Quick Comparison
| Feature | RS-232 | RS-485 |
|---|---|---|
| Topology | Point-to-point | Multi-drop bus |
| Max Devices | 2 (1 TX, 1 RX) | 32 units |
| Max Distance | 15m (50 ft) | 1,200m (4,000 ft) |
| Signaling | Single-ended | Differential |
| Noise Immunity | Low | High |
| Common Use | Programming ports | Modbus RTU networks |
Key Takeaways
RS-232 is for simple point-to-point connections at short distances
RS-485 is for industrial networks with multiple devices and longer cable runs
RS-485 requires proper termination — 120Ω resistors at both ends of the bus
Differential signaling makes RS-485 much more noise-immune than RS-232
Many PLCs and devices support both standards for different purposes
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