This book is a digital electronics text focused on 'how to' design, build, operate and adapt data acquisition systems. The book is intended to serve people whose goals include teaching or learning one or more of the following: digital electronics, circuit design for computer expansion slots, software which interacts with outside hardware, the process of computer based data acquisition, and the design, adaptation, construction and testing of measurement systems. The fundamental idea of the book is that parallel I/O ports (available for all popular computers) offer a superior balance of simplicity, low cost, speed, flexibility and adaptability.