Your ESL code should never explicitly call the conversion routine. Rather, whenever the ESL compiler encounters a reference in your program to the field for which a conversion routine is specified, it generates code to call that conversion routine at runtime. Because your ESL code never explicitly calls the conversion routine, you do not declare your external conversion routine as an ESL function or subroutine.

A field defined with a using clause in the structure is always stored in the external format. When the ESL compiler encounters a read reference to the field, it generates code to call the conversion routine to convert the field to the ESL type and store it at a location within ESL memory. For a write reference, the ESL compiler generates code that calls the routine at runtime to convert the data to the external type and store it in the field.