PROFINET defines three device roles: IO Controller (equivalent to the PLC master), IO Device (equivalent to a slave: sensor, actuator) and IO Supervisor (engineering PC used for diagnostics and configuration).

The three standardised roles are: IO Controller, which runs the PLC program, reads inputs and writes outputs (Siemens S7, Rockwell ControlLogix, etc.); IO Device, the field equipment such as sensors, actuators, drives or remote IO modules; and IO Supervisor, the engineering PG/PC used for commissioning, debugging and diagnostics (TIA Portal, STEP 7, etc.). A single PROFINET network may host several IO Controllers using the shared device architecture.

Remember that the IO Supervisor is not required for cyclic operation; once the project is downloaded, the Controller and Devices run autonomously and the Supervisor only re-attaches for maintenance.