Modbus
Modbus is a simple, open industrial protocol introduced by Modicon in 1979 for reading and writing registers in controllers. Modbus TCP runs it over Ethernet on port 502 and Modbus RTU over serial lines; both are still everywhere because nearly every device speaks them.
Modbus has no authentication and no encryption. Any host that can reach port 502 on a device can read its registers and, unless the device is configured read-only, write them. The Modbus/TCP Security specification adds TLS, but adoption in installed equipment is rare.
Its simplicity is why it persists: power meters, drives, weigh scales, and remote I/O all expose Modbus, which makes it the protocol most likely to be crossing an unsegmented boundary.
Why it matters in cement, mining, and aggregates
In aggregates and ready-mix plants, scale systems and drives often speak Modbus to the batch controller across the same network the office uses. Monitoring for unexpected Modbus write commands, and segmenting port 502 away from the corporate side, are two of the highest-value low-cost controls at a small plant.