This document provides a programmer's guide and reference for the SPiiPlus C library version 6.50. The guide describes how to use the C library to communicate with SPiiPlus motion controllers over various communication channels like serial, Ethernet, and PCI. It gives an overview of the library concepts and functions. Key functions allow opening communications, sending and receiving data, performing transactions with the controller, and closing connections. Revision details are provided for version 6.50.