This digital electronics text focuses on "how to" design, build, operate and adapt data acquisition systems. The material begins with basic logic gates and ends with a 40 KHz voltage measurer. The approach aims to cover a minimal number of topics in detail. The data acquisition circuits described communicate with a host computer through parallel I/O ports. 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. All circuits and software are thoroughly tested. Construction details and troubleshooting guidelines are included. This book is intended to serve people who teach or study one of the following: digital electronics, circuit design, software that interacts outside hardware, the process of computer based acquisition, and the design, adaptation, construction and testing of measurement systems.