Unit Testing in industrial automation refers to the process of individually testing small, isolated parts (or “units”) of an automation control system — such as a PLC function block, logic routine, or software module — to ensure that each performs correctly before integrating it into the larger system.

Definition: Unit Testing is the verification of the smallest testable part of control logic — for example, a PLC function block, a control sequence, or a piece of structured text code — to confirm it works as intended independently of the rest of the automation system.