This document provides a programmer's guide and reference for the SPiiPlus C library version 6.50. The guide begins with an introduction and overview of the library, describing its operation environment, communication capabilities, controller simulation support, and key features. It then covers using the library, including building applications, redistributing files, and registering the kernel mode driver. The bulk of the document is a reference for the C library functions, organized into sections for communication functions and service communication functions.