EIA-485 (formerly RS-485, also TIA-485) is an OSI physical-layer electrical specification for a multipoint serial bus. It defines a balanced (differential) signalling scheme: data is carried as the voltage difference between the two wires of a twisted pair (commonly labelled A and B), so one polarity represents a logic 1 and the opposite polarity a logic 0. A receiver detects a valid signal once the differential voltage exceeds about 0.2 V, and works with a common-mode voltage anywhere between −7 V and +12 V. Because the data is read differentially rather than against ground, EIA-485 rejects common-mode noise well — that is what makes it a balanced interface, irrespective of the supply voltages used to drive the line (often a single 0–5 V supply).
EIA-485 specifies only the electrical characteristics of the drivers and receivers; it does not define a connector, a pinout, or any data protocol. Like EIA-422 it signals differentially over a twisted pair, so it can span long distances — up to about 1200 m (4000 ft) at lower data rates, with rates up to 10 Mbit/s over short runs (the product of bit-rate and cable length is limited). A standard driver supports at least 32 unit loads on one bus, and reduced-load transceivers allow many more nodes. The wires are best arranged as a single line (bus) of nodes, with a termination resistor across the pair at each end equal to the cable’s characteristic impedance (typically 120 Ω for twisted pair), plus bias resistors to hold the line in a known state when no driver is active. Without proper termination, reflections of the fast driver edges can create spurious transitions and corrupt data.
Notes
- EIA-485 is closely related to EIA-422: both use differential signalling, but EIA-485 adds three-state (tri-state) driver outputs so that multiple devices can share the same pair in a true multidrop bus.
- The standard does not mandate a connector; equipment vendors choose their own (screw terminals, RJ-45, DB-9, and others).