This article describes how one user took advantage of the published C-language code for ZCLIENT.C, the program that provides Dynamic Data Exchange (DDE) functionality with ZEMAX, to modify it so that the main utility functions provided in that code could be called from a user-provided FORTRAN function instead of having to use the C- or C++ language. It describes how a 32-bit Windows® executable can be built that combines a C-compiled ZCLIENT.OBJ object module with a FORTRAN user-function.

A useful starting place for the prospective user, as well as of course the relevant chapter in the ZEMAX manual, is the article on how to write a ZEMAX extension in C, where full details are given of the server-client relationship established by ZCLIENT and how the DDE interface works.