What is RS485? Wiring, distance & devices explained

RS485 (also written EIA-485) is an electrical standard for serial communication over a twisted pair of wires, published by the Telecommunications Industry Association. It defines voltage levels and wiring rules only — not a message format. It has become the default physical layer for industrial field devices because it is cheap, tolerant of electrical noise, and can run long distances at moderate speed.

RS485 itself does not define what the bytes mean. A messaging protocol — most commonly Modbus RTU — runs on top of it to give the raw bytes structure: addresses, function codes, data, and error checking.

RS232 (the classic serial port) and RS485 solve the same basic problem — serial communication — but differ in every practical way that matters on a factory floor:

The differential signaling is the key difference: RS485 sends the same signal on two wires, inverted on one, and the receiver reads the difference between them. Electrical noise picked up equally on both wires cancels out — which is why RS485 survives being run next to VFD power cables and motor drives where RS232 would not.

An RS485 bus uses two signal wires, usually labeled A (or D-) and B (or D+), plus an optional common/ground wire. Every device on the bus connects to the same A and B lines — it is a shared bus, not point-to-point wiring like RS232 or Ethernet.

Only one device can transmit at a time. In a typical Modbus RTU setup this is managed by a single master (e.g., a PLC or gateway) that polls each slave device by address, one at a time — devices only speak when asked, which avoids collisions on the shared wire.

Three rules cause the majority of real-world RS485 problems when skipped:

• Termination.Put a 120-ohm resistor across A and B at each of the two physical ends of the bus — nowhere else. This matches the cable's characteristic impedance and stops signal reflections that corrupt data on longer runs or at higher baud rates.
• Biasing. When the bus is idle (nobody transmitting), the A/B lines can float and pick up noise, which receivers may misread as garbage data. A bias resistor network (pull-up on B, pull-down on A) at one point on the bus holds the idle state in a known logic level.
• Common reference.Run a ground/common wire alongside A and B, especially over longer distances or between devices powered from different supplies, to keep the common-mode voltage within the transceiver's working range.

RS485's maximum cable length depends directly on baud rate — faster signaling tolerates less cable and reflection before bits become unreadable:

Most industrial Modbus RTU installations run at 9,600–19,200 bps, trading speed for reliability over long cable runs across a plant. Figures are typical guidance, not a hard spec — actual distance depends on cable quality, termination, and the specific transceiver.

RS485 is designed for a daisy-chain (linear bus) topology — each device connects to the next, in a line, with termination only at the two ends. Star wiring (a central hub with wires radiating to each device) creates multiple unterminated stub ends and reflection points, and is a common cause of intermittent communication errors that are hard to diagnose.

If a star topology is unavoidable due to existing conduit runs, keep stub lengths as short as possible and consider a repeater or hub designed for RS485 star networks.

• Energy meters and power analyzers reporting kWh, current, and power factor
• Variable frequency drives (VFDs) exposing speed, load, and fault status
• PLCs and HMIs on a plant-floor field bus
• Environmental sensors — temperature, humidity, pressure transmitters
• Legacy CNC and machine controllers with a serial Modbus interface

Nearly all of these devices speak Modbus RTU on top of RS485 — it is the de facto standard messaging protocol for the bus.

RS485 devices cannot connect to a network or the internet on their own — they only understand the serial bus. A protocol gateway sits on the RS485 line as a Modbus RTU master, polls each device by address, and re-publishes the same data as Modbus TCP (over Ethernet) or MQTT (over WiFi), so SCADA software, an MES, or a cloud dashboard can read it without any change to the field wiring.

ThingConnect's MDIM-485 RS485-to-Ethernet converter and MDIM-485/WL RS485-to-WiFi converter are both built for exactly this: transparent Modbus RTU ⇄ TCP bridging, rated for unconditioned industrial panels (-40 to 75 °C), with surge and ESD protection on the RS485 line.