The features and amenities we've come to expect from our automobiles are achieved through onboard electronic control units which are connected together by bus systems for the exchange of data (on-board communication). Off-board communication technologies support diagnostic communication between external test equipment (OBD scan tools, HiL test systems, flash stations, workshop testers, etc.) and automotive control units. This volume explains the basics of communication principles, protocols, and various bus systems such as CAN, LIN, FlexRay, and MOST. A detailed description of the diagnostic protocol UDS (Unified Diagnostic Services) is followed by the structure of external test equipment based on the ASAM MCD system, the ISO specification of MVCI (Modular Vehicle Communication Interface), and the ODX format (Open Diagnostic Data Exchange) in minute detail. For clarity, we have included several practical examples from various stages of the process chain, including protocol development, hardware-in-the-loop systems, reprogramming by flash download to a PDA-based OBD Scan Tool, and workshop testers.