Modbus
The world's most common industrial protocol — invented in 1979 by Modicon for PLCs. Still standard on factory floors and in solar panels / inverters.
Variants: Modbus RTU (RS-485 binary), Modbus ASCII (text), Modbus TCP (port 502, most modern). Master/slave model (now client/server terminology). Addressing spaces: Coils (1-bit outputs), Discrete Inputs, Holding Registers (16-bit RW), Input Registers (16-bit RO). Brutally simple — no encryption, no auth — which makes it a perfect target for OT attacks. Powers millions of industrial gadgets because it costs nothing to implement.