The NET+OS 7.5.2 API documentation is integrated into the Digi ESP for NET+OS Help subsystem, and is accessible only after installation of NET+OS 7.5.2 with a valid license key.
In fact, the API documentation is a Windows Help file .chm, which you can download and browse through from here:
Unpack and store this locally under Windows.
If you open ApiReference.chm locally you will be able to browse through the NET+OS 7.5.2 API Reference Guide.
Jun 19, 2019