RS-485 Explained: Wiring, Protocols & Why It Still Dominates
RS-485 is the ancient (1983) but still ubiquitous differential serial standard that carries Modbus RTU, BACnet MS/TP, PROFIBUS DP, DNP3 over serial, and dozens of vendor-specific protocols. Despite Ethernet replacing it in new installations, RS-485 still dominates legacy infrastructure, building automation, and any application requiring 1,200 m cable runs over noisy industrial environments.
How RS-485 works
RS-485 is a differential serial standard — two wires (A and B, sometimes labeled D+ and D-) carry the signal as the difference between two voltages, typically ±5 V. The receiver detects the difference, not absolute voltage. This rejects common-mode noise picked up by both wires equally, which is why RS-485 thrives in industrial environments with motors, VFDs, and high-voltage switching.
- Multi-drop: up to 32 (standard) or 256 (extended drivers) devices on one segment
- Cable: 24 AWG twisted pair, 100-120 Ω characteristic impedance, shielded for industrial use
- Maximum length: 1,200 m at 9,600 baud; shorter at higher speeds (~30 m at 12 Mbps)
- Speeds: 1.2 kbps to 12 Mbps depending on cable length and quality
- Half-duplex typical — one wire pair, devices take turns transmitting (master/slave or token-passing)
- Full-duplex available with RS-422 (4 wires)
Wiring an RS-485 network correctly
- Daisy-chain topology — A to A, B to B, device to device. No stars, no Ts, no spurs longer than ~30 cm.
- Termination resistor — 120 Ω across A-B at both ends of the bus. Most industrial devices have a built-in termination jumper for the end devices.
- Biasing resistors — 470 Ω-1 kΩ from A to +5 V and from B to GND, somewhere on the bus (typically the master). Without bias, the bus floats during silent periods and receivers see noise.
- Common reference — connect device GND/COM together via a third wire OR via the cable shield (one end only — never both, to avoid ground loops).
- Shield — bonded to ground at one end only, typically at the master/PLC. Bonding both ends creates a ground loop that injects 50/60 Hz hum into the signal.
- Polarity — A to A, B to B. Check vendor documentation; some label as D+ / D- where D+ = A and D- = B, others reverse.
Common RS-485 mistakes
- No termination — works at low speeds and short distances; mysteriously fails at higher speeds or longer cables. Always terminate both ends.
- Star topology — multiple stubs from a hub. Reflections kill signal integrity. Use repeaters or daisy chain through device ports.
- Mismatched grounds — devices powered from different supplies without common reference. Differential signalling tolerates ±7 V common-mode but not unbounded; provide GND wire or cable shield.
- A/B reversed at one device — the network mostly works but errors increase. Use a multimeter: idle bus has A > B by 200 mV typically.
- Mixed termination jumpers — internal terminators left enabled on middle devices. Only end devices terminate.
- Wrong cable — using untwisted 4-conductor for the run. Use proper RS-485 cable with 100-120 Ω twisted pair (Belden 9841, 3105A, equivalents).
Protocols that ride RS-485
- Modbus RTU — by far the most common. Master-slave, 9.6/19.2/38.4/115.2 kbps typical. Universal across industries.
- PROFIBUS DP — Siemens-led fieldbus, 1.5/12 Mbps. Predecessor to PROFINET, still common in legacy European plants.
- BACnet MS/TP — building automation. HVAC controllers, lighting, fire alarms.
- DNP3 (serial) — utility SCADA over serial radio links. Still common in remote oil & gas and electric distribution.
- SLMP / MELSOFT — Mitsubishi proprietary serial protocols.
- DH-485 — legacy Allen-Bradley SLC-500 / MicroLogix protocol.
- Vendor-specific — every drives manufacturer (ABB, Yaskawa, Allen-Bradley PowerFlex, Siemens) has a serial protocol.
RS-485 vs RS-232 vs Ethernet
| Aspect | RS-232 | RS-485 | Ethernet |
|---|---|---|---|
| Signalling | Single-ended | Differential | Differential |
| Distance | 15 m | 1,200 m at 9.6 kbps | 100 m / segment |
| Speed | Up to 115 kbps | Up to 12 Mbps | 100 Mbps – 10 Gbps |
| Devices | 2 (point-to-point) | 32-256 | Unlimited (per segment) |
| Noise immunity | Poor (industrial) | Excellent | Excellent |
| Use today | Legacy, console ports | Legacy + new building automation | Default for new installations |
Frequently asked questions
What is RS-485?
RS-485 is a differential serial communication standard, ratified in 1983, that uses two wires (A and B) to carry signals as a voltage difference. The differential signalling rejects common-mode noise, making RS-485 ideal for industrial environments. Up to 32 devices on a single segment, 1,200 m at 9.6 kbps, supporting protocols like Modbus RTU, PROFIBUS DP, BACnet MS/TP, and DNP3.
How do I wire RS-485?
Use a daisy-chain topology — A to A, B to B, device to device, no stars or stubs longer than 30 cm. Add a 120 Ω termination resistor at both ends of the bus (most devices have a built-in jumper). Add biasing resistors (470 Ω-1 kΩ) from A to +5 V and B to GND somewhere on the bus, typically at the master. Connect a common ground reference between devices. Bond the cable shield at one end only.
Why does RS-485 need termination?
Termination resistors (120 Ω at each end of the bus) match the cable's characteristic impedance, preventing signal reflections that corrupt data at high speeds or long distances. Without termination, RS-485 may work at low speeds and short cables but fails mysteriously at higher speeds. Always terminate both physical ends of the bus, even on short installations.
What is the difference between RS-232 and RS-485?
RS-232 is single-ended (one signal wire plus ground), point-to-point only, limited to ~15 m and 115 kbps. RS-485 is differential (two-wire signal), supports 32+ devices on one bus, runs up to 1,200 m at 9.6 kbps or shorter at 12 Mbps. RS-485 has dramatically better noise immunity for industrial use; RS-232 is mainly used for console ports and legacy point-to-point links.
Is RS-485 still relevant in 2026?
Yes — RS-485 is still the dominant fieldbus in legacy installations and in many new building automation projects. Modbus RTU over RS-485 powers millions of industrial devices, BACnet MS/TP runs in nearly every commercial HVAC system, and PROFIBUS DP remains in service in European plants. New greenfield installations increasingly use Ethernet (Modbus TCP, PROFINET, EtherNet/IP), but RS-485 remains the default for distributed simple devices and building automation.