RS-485 and Modbus

RS-485 sends data as the difference between two wires (A/B), so it rejects noise and works over 1200 m with up to 32 standard loads. It is half-duplex: a driver-enable pin switches the transceiver (MAX485) between sending and receiving. Terminate both ends with 120 Ω.

Modbus RTU is a simple controller/peripheral protocol: each request has a device address, a function code (03 = read holding registers, 06 = write single register), data and a CRC-16. It is everywhere in PLCs, energy meters and VFDs.