RS-485 wiring: topology, termination, biasing

Electrical basics

RS-485 (TIA/EIA-485) transmits differential signals on a twisted pair: the receiver evaluates the voltage between the lines (≥ 200 mV), which makes it robust against common-mode noise. Up to 32 unit loads per segment (more with ¼ or ⅛ unit-load transceivers); half duplex — one transmitter at a time.

Topology

Daisy chain (line): the cable goes from device to device. Stubs must be short — at high baud rates practically zero. Star wiring creates reflections and is the most common cause of "works on the bench, fails on site".

Termination

A resistor equal to the cable's characteristic impedance (typically 120 Ω for RS-485 pairs; PROFIBUS uses its own 150 Ω cable and active terminator) at both ends of the line absorbs reflections. Not at intermediate devices. Low-speed short buses sometimes work without termination — until they don't.

Biasing (failsafe)

When no device transmits, the line floats and receivers may read random bits. Bias resistors pull A and B to a defined idle state (in Modbus over serial line terms, a pull-up on D1 and pull-down on D0, typically in one place — often the master). Too many biased devices load the line; none causes framing errors between frames.

The common (0 V) conductor

RS-485 is differential but not floating: transceivers have a common-mode range (−7 … +12 V). Connect the signal common (Modbus "Common") between devices via the third conductor of the cable, or use isolated transceivers. Without it, potential differences between devices destroy communication or transceivers.

A/B naming confusion

The standard calls the lines A and B, but vendors label them inconsistently (A/B, +/−, D+/D−, D1/D0 in the Modbus spec where D1 = B = "+"). If nothing communicates, swapping the two data lines on one device is a legitimate test.

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