RS-485 is a differential signaling standard for multi-drop serial communication. Two wires (A and B) carry complementary signals — receivers detect the voltage difference, giving excellent noise immunity on long cable runs. Up to 32 unit loads (nodes) can share a bus of up to 1200m.
| Topology | Multi-drop (bus) |
| Max nodes | 32 (standard UL) |
| Max distance | 1200m @ 100 kbps |
| Voltage | ±7V differential |
| Data rate | Up to 10 Mbps |
| Termination | 120Ω at each end |
Differential signaling: A-B > +200mV = logic 1 (mark), A-B < -200mV = logic 0 (space). Receivers reject common-mode noise that affects both A and B equally. End-of-line termination resistors (120Ω) prevent signal reflections on long cables. Direction is controlled by DE/RE pins on the transceiver IC.
Half-duplex requires direction switching. Termination required on long runs. Ground reference must be common. Maximum 32 standard loads per segment (extenders needed for more). Susceptible to ground loops in noisy environments.