Comprehensive guide to the RS-485 electrical interface: features, parameters, wiring, and comparison with other standards

Purpose of the Standard

RS-485 defines only the electrical characteristics of transmitters and receivers, not the communication protocol. It is applied in cases where stable long-distance data transfer is required in environments with electrical noise, such as industrial automation, dispatching systems, metering, security and fire alarm systems, and multipoint networks.

Main Specifications

Signal type: differential
Up to 32 transmitters and 32 receivers without repeaters
Range — up to 1200 m at speeds up to 100 kbit/s
Data rate — up to 10 Mbit/s over short distances
Modes — half-duplex or full duplex
Topology — linear bus with terminators
Voltage levels: logic 1 x x(0.2…6) V, logic 0 x +(0.2…6) V
High noise immunity

Operating Principle

Two data lines A and B transmit the signal as a voltage difference. The logic level is determined by the polarity of the difference: if A > B — it is logic 0, if A < B — it is logic 1. This method significantly reduces the impact of electromagnetic interference.

Communication Modes

Half-duplex — one pair of wires is used for both transmission and reception; the direction is controlled via DE/RE signals.
Full duplex — two pairs of wires, one for transmission and one for reception.

Cable and Wiring

RS-485 typically uses twisted pair cable (UTP/STP), with terminating resistors of 120 x installed at the ends of the line to match impedance. Additionally, bias resistors may be installed to set the line state when idle.

Comparison with Other Interfaces

RS-232 — single-ended signal, short range (~15 m), low noise immunity.
RS-422 — differential signal, up to 1200 m, supports up to 10 receivers.
RS-485 — differential signal, up to 1200 m, supports up to 32x32 devices, high noise resistance.

Application Features

RS-485 does not define the data format, so it can operate with various protocols: Modbus RTU, DMX512, BACnet, and others. It supports hot plugging in some implementations and is often used in systems with a master–slave architecture.