The stimulus feature provides a means for a DLL to cause an event in the ESL program that calls it. (Dynamic link libraries are discussed in What is a DLL?.)  For instance, a DLL that provides an interface to a hardware device might need to alert an ESL program when a certain condition arises on the device.

Functions and subroutines that reside in a stimulus library are no different from ordinary external functions and subroutines. They must be declared before they are referenced, and the references must match the declaration. In order to allow a stimulus library to invoke responses in a program that references it, however, the library must be declared as a stimulus in the ESL program.