PLCs are the workhorses of automation. They are programmed in the languages of IEC 61131-3, such as ladder logic and structured text, and execute the program repeatedly, reading digital and analog inputs, computing, and setting outputs within a guaranteed cycle time. Fieldbuses such as Modbus and PROFINET connect them to drives, sensors and SCADA.
For the IoT the PLC is usually the data source, not the thing to replace. Modern controllers expose their variables over OPC UA or MQTT, and an edge gateway reads them without touching the control program. Keeping that read path separate from the write path is a basic safety and security principle on any plant network.
Part of the IoT Glossary. Spotted an error or a missing term? Tell us.