Modbus is an industrial communication protocol originally published by Modicon in 1979 for use with programmable logic controllers (PLCs). It has since become the most widely deployed industrial protocol in the world, supported by a vast range of sensors, PLCs, drives, energy meters, building controllers, and SCADA systems.

In short: Modbus comes in two main variants. Modbus RTU runs over serial connections (typically RS-485) and is found across legacy industrial equipment. Modbus TCP runs over Ethernet and is the same protocol re-encapsulated for IP networks. Modbus is a master-slave protocol: a single master device polls one or more slave devices, reading or writing data at specific register addresses. Industrial cellular routers from Teltonika and others can act as Modbus gateways, bridging RTU and TCP variants, and forwarding Modbus data over MQTT to cloud platforms.

Modbus's longevity in industry comes from its simplicity. The protocol is small, well documented, and patent-free. Almost any piece of industrial equipment built in the last 30 years either speaks Modbus natively or can be made to.

For IIoT deployments, Modbus is often the bridge between legacy plant equipment and modern cloud platforms. A Teltonika router on a plant network can poll Modbus RTU devices over RS-485, repackage the data as Modbus TCP or MQTT, encrypt it through a VPN, and deliver it to a cloud platform for analytics, dashboards, or AI-driven predictive maintenance. None of this requires changes to the original equipment.