RS-485
Differential serial over a twisted pair.

RS-485 is defined by TIA/EIA-485-A (the standard maintained by the Telecommunications Industry Association). It specifies the electrical characteristics of generators and receivers for balanced digital multi-point transmission systems. RS-485 does not define a higher-level protocol — only the physical layer; standards like Modbus RTU, Profibus, and BACnet ride on top.

The bus uses differential signaling on a twisted pair: the receiver compares the voltage between the two conductors rather than each conductor against ground. This gives strong immunity to common-mode noise and ground-potential differences between nodes, which is why RS-485 dominates industrial control wiring.

Two wiring topologies are common: half-duplex (2-wire), which uses one differential pair for bidirectional traffic with a turnaround between transmissions, and full-duplex (4-wire), which uses two pairs for simultaneous send and receive. The standard rates up to 32 standard unit loads on one bus; bus-friendly transceivers (1/4 UL or 1/8 UL) extend this to 128 or 256 nodes. Maximum data rate trades off against cable length per the standard's recommendation curve.