Modbus began as a serial protocol for Modicon PLCs and survived because it is trivial to implement: a master asks a slave to read or write registers at numeric addresses, and the slave replies. Modbus RTU runs over RS-485 serial links, often through a UART on the device; Modbus TCP wraps the same frames in TCP on port 502.
In IoT it is the lowest common denominator for talking to meters, drives, inverters and sensors: an edge gateway polls Modbus registers and republishes them as MQTT or OPC UA. The weaknesses are equally plain. There is no security, no data typing beyond 16-bit registers, no discovery and no event push, so every integration depends on a vendor’s register map and on polling. Those gaps are exactly what OPC UA was built to fill; see the industrial IoT guide.
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