RS-485 — Industrial Multi-Drop Signaling

RS-485 is the physical layer backbone of industrial communication. When you see a factory floor with dozens of sensors, motor drives, and PLCs all connected on a single twisted pair cable — that cable is almost certainly RS-485.

What Problem Does RS-485 Solve?

RS-232 and TTL UART are single-ended: they measure voltage relative to a common ground. Over long cables, ground potentials shift, noise couples into the signal, and the bit is corrupted.

RS-485 uses differential signaling: it sends the same bit on two wires simultaneously — one inverted. The receiver looks at the difference between the two wires, not the absolute voltage. Noise that couples onto both wires equally cancels out.

flowchart LR classDef se fill:#fee2e2,stroke:#dc2626,color:#7f1d1d classDef diff fill:#dcfce7,stroke:#16a34a,color:#14532d classDef noise fill:#fef9c3,stroke:#ca8a04,color:#713f12 SE["Single-ended (RS-232)\nMeasures: Wire vs GND\n+12 V = 0, −12 V = 1\nNoise on wire = corrupted bit"]:::se DIFF["Differential (RS-485)\nMeasures: A wire vs B wire\nA > B by ≥200 mV = '1'\nB > A by ≥200 mV = '0'\nNoise on both wires = cancels out"]:::diff N["Electrical noise\ncouples equally\nonto both wires"]:::noise N -->|"corrupts single-ended"| SE N -->|"cancels in differential"| DIFF

The RS-485 Standard — Key Numbers

Property	Value
Signaling	Differential (A and B wires)
Logic threshold
Max devices on one bus	32 unit loads (256+ with modern transceivers)
Max cable length	1200 m at 100 kbps
Max data rate	~10 Mbps at short distances
Topology	Multi-drop bus (daisy-chain)
Duplex	Half-duplex (2-wire) or full-duplex (4-wire)
Termination	120 Ω at each end of the cable

Half-Duplex vs Full-Duplex

Most RS-485 systems use half-duplex (two wires: A and B). Only one device transmits at a time. The master controls who speaks when.

sequenceDiagram participant M as Master participant S1 as Slave 1 participant S2 as Slave 2 participant S3 as Slave 3 Note over M,S3: Half-duplex: only one device drives the bus at a time M->>S1: Request (master enables TX, disables RX) Note over M: Master switches back to RX S1->>M: Response (slave 1 enables TX, others listen) M->>S2: Request S2->>M: Response M->>S3: Request S3->>M: Response

Full-duplex uses four wires (TX+, TX−, RX+, RX−) and allows simultaneous sending and receiving — but is only practical for a single master talking to a single slave. Multi-drop almost always uses half-duplex.

The Bus Topology — Daisy-Chain, Not Star

RS-485 must be wired as a daisy-chain (linear bus). Stubs and star topologies cause signal reflections that corrupt data.

flowchart LR classDef device fill:#dbeafe,stroke:#2563eb,color:#1e3a5f classDef term fill:#fef9c3,stroke:#ca8a04,color:#713f12 classDef wire fill:#f1f5f9,stroke:#94a3b8,color:#334155 T1["120 Ω\ntermination"]:::term M["Master\n(PLC / PC)"]:::device S1["Slave 1\nSensor"]:::device S2["Slave 2\nDrive"]:::device S3["Slave 3\nMeter"]:::device T2["120 Ω\ntermination"]:::term T1 --- M --- S1 --- S2 --- S3 --- T2

The 120 Ω termination resistors at each end absorb the signal energy and prevent reflections. Without them, signals bounce back and forth on the cable causing ghost bits.

Why RS-485 is Called a "Hardware Protocol"

RS-485 defines only the physical layer:

What voltages appear on the A and B wires
How many devices can connect
How long the cable can be
What impedance the cable should have

RS-485 says nothing about:

What bytes mean
How to address a specific device
What commands exist
Whether a packet was received correctly

That is the job of a software protocol running on top of RS-485. This is the same relationship as:

Hardware layer	Software protocol on top
RS-485	Modbus RTU, PROFIBUS, DMX512, BACnet MS/TP
RS-232	Modbus RTU, AT commands (modems), NMEA (GPS)
Ethernet cables	TCP/IP, UDP
WiFi radio	TCP/IP, UDP

The hardware layer is the road. The software protocol is the traffic rules and language used on that road.

RS-485 Transceiver — The DE/RE Pin

Every RS-485 node needs a transceiver chip (e.g., MAX485, SN75176) that connects the microcontroller's UART TX/RX to the A/B differential bus. Because the bus is half-duplex, only one device drives it at a time. The transceiver has two control pins:

DE (Driver Enable) — HIGH to transmit
RE̅ (Receiver Enable, active-low) — LOW to receive

In firmware, before sending you pull DE HIGH (and RE̅ HIGH to disable receiver), transmit your bytes, then switch back: DE LOW, RE̅ LOW to receive the reply.

flowchart LR classDef mcu fill:#dbeafe,stroke:#2563eb,color:#1e3a5f classDef chip fill:#fef9c3,stroke:#ca8a04,color:#713f12 classDef bus fill:#dcfce7,stroke:#16a34a,color:#14532d MCU["MCU\nTX pin\nRX pin\nDE/RE pin (GPIO)"]:::mcu TR["MAX485\nTransceiver\nDE: enable drive\nRE: enable receive"]:::chip BUS["RS-485 Bus\nA wire (+)\nB wire (−)\nGND"]:::bus MCU -->|"TX"| TR TR -->|"RX"| MCU MCU -->|"DE/RE\ncontrol"| TR TR <-->|"differential\nsignal"| BUS

RS-485 vs RS-232 vs UART — Quick Comparison

	UART (TTL)	RS-232	RS-485
Voltage levels	0 / 3.3 V or 0 / 5 V	±12 V single-ended	±(0.2 to 6) V differential
Max devices	2	2	32–256
Max distance	~1 m	~15 m	~1200 m
Noise immunity	Low	Medium	Very high
Topology	Point-to-point	Point-to-point	Multi-drop bus
Direction	Full-duplex	Full-duplex	Usually half-duplex
Protocol included?	No	No	No
Typical industrial use	Debug, sensors on board	PLC programming port	Field bus (Modbus, PROFIBUS)

Key Takeaway

RS-485 is not a communication protocol — it is a physical layer standard that solves two problems UART cannot: long distances and multiple devices on one bus. Any protocol that needs those properties runs on top of RS-485.

The most common one you will encounter in industrial and building automation is Modbus RTU — which is exactly what comes next.

→ Modbus RTU — Software Protocol on RS-485