Modbus

Modbus organizes a device's data as numbered registers and coils that a client can read or write. Its simplicity and openly published specification, maintained by the Modbus Organization, made it a common language for meters, inverters, drives, chillers, PLCs and sensors. Modbus RTU runs over RS-485; Modbus TCP runs over Ethernet.

The catch is that Modbus says nothing about what each register means. Every device has its own register map, scaling and units, documented by its manufacturer with varying care. Integration work is mostly reading those maps and testing against real hardware.

Modbus has no built-in security: anyone on the network can read or write registers. Connected deployments need network segmentation or a secured gateway.