During SIL, the plant and controller diagrams are executed entirely on the host computer. It is also common to include the sensor and actuator models as part of the plant diagram, at least initially. Sometimes they are neglected entirely implying they are ideal unity gain models with no dynamics. The controller is separately modelled and interfaced with the plant through the actuator and sensor interface signals. Initial design iterations often use a continuous-time model for the controller, which allows standard frequency domain methods to be employed for the design.

Once the design requirements are satisfied and adequate stability margins are achieved, the controller algorithm is converted to discrete time for further investigation of update time, multi-rate sampling, fixed-point implementations, time jitter, quantization, and time delays. The level of confidence in meeting the design requirements during the SIL phase depends significantly on the accuracy of the plant model. All design changes are made by modifying or augmenting diagrams. The automatically-generated code from a diagram is never directly altered.

Diagrams developed during the SIL phase are reused and possibly supplemented with additional models as development moves into the PIL and HIL phases.