RS485

RS-485, originally EIA-485, is a widely used electrical standard for balanced serial communication on multipoint differential buses. It employs twisted-pair wiring and differential signaling to improve noise tolerance and allow long cable runs in industrial environments.

The physical layer supports two-wire, typically half-duplex operation, though four-wire variants allow full-duplex. It is designed for multipoint networks, with multiple transmitters and receivers connected to a single pair. Nodes are arranged on a bus topology; star configurations are discouraged due to reflections. End-of-line termination resistors of about 120 ohms are placed at both ends of the main cable, and biasing resistors may be used to define a known idle differential state when no device is driving the line.

Electrical characteristics emphasize robustness: RS-485 uses differential signaling on a balanced pair, providing good noise rejection and long reach. Most implementations limit the bus to 32 unit loads, though many modern transceivers are low-unit-load types enabling larger numbers of devices.

Speeds and distances trade off with length: lower data rates permit longer runs (theoretically up to long distances at very low rates), while higher data rates require shorter cable lengths, commonly tens to hundreds of meters for Mbps ranges.

Applications: RS-485 is a common physical layer for industrial networks and building automation. It underpins protocols such as Modbus RTU, Profibus DP, BACnet MS/TP, and others. Practically, reliable operation relies on proper termination, biasing, shielding as needed, and adherence to recommended cable routing and stubbing practices. Isolation options are used to protect equipment and improve noise immunity in harsh environments.