RS-485
also: TIA-485 · EIA-485 · RS485
A differential electrical standard for long, noisy, multi-device serial links; the physical layer under Modbus RTU, BACnet MS/TP and many industrial devices.
RS-485 sends each signal as the difference between two wires, which rejects much of the electrical noise found in buildings and plants. It allows many devices on one twisted pair and long cable runs. Application notes commonly cite up to 32 standard "unit loads" per bus (more with reduced-load transceivers) and cable runs of up to about 1,200 m (4,000 ft) at lower data rates, with achievable speed falling as length grows.
RS-485 defines only the electrical layer; protocols such as Modbus RTU and BACnet MS/TP run on top. Most installations are half-duplex, with devices taking turns on one pair.
Where it bites: termination and biasing resistors, daisy-chain (not star) wiring, ground potential differences between distant devices, and surge protection on long or outdoor runs. Field wiring mistakes are the most common reason "the network is flaky".
Founder takeaway
If your product connects to building or industrial equipment by wire, plan RS-485 with proper termination, isolation and surge protection from the start.
engineer review pending