During HIL, the PIL is extended to include the plant hardware, sensors, and actuators. Often, however, in large systems, it may not be feasible or even possible to include the entire plant, all the actuators, and all the sensors. In these situations, some of the sensors, some of the actuators, and parts of the plant (as they become available) are included as HIL devices with the remainder simulated in models executing on the host.

Like in the PIL phase, automatic code generation of the control algorithm is loaded and executed on the target while the host is used to model the parts of the plant for which hardware is not available, as well as to interactively adjust parameters of the control algorithm and capture and present signal time history data.

The HIL phase always executes in real time. Even though the entire plant may not be actual hardware, the ability to test plant components (hydraulic pumps, lines, accumulators, actuators, electric servos, pneumatic actuators, and so on) in the controlled environment of the HIL improves the level of confidence in meeting the design requirements.