While many protocols for vehicle diagnostics have been developed over time, such as KWP 2000, ISO 15765 and K-Line, automotive OEMs and suppliers have agreed on Unified Diagnostic Service (UDS) as a standard protocol to ensure universal compatibility sex. Diagnostic protocols are defined in the ISO-14229 standard, which automotive OEMs follow to provide a general-purpose computer system that can be used to diagnose any vehicle.
Figure 1 UDS incorporates and integrates other automotive diagnostic protocols such as ISO 15765.
Modern cars have 40 to 150 ECUs, each of which performs specific functions such as electronic fuel injection (EFI), engine control, door locks, brakes, and window operation. This in turn makes it difficult to test and diagnose vehicle systems in the event of a malfunction.
The UDS protocol allows a diagnostic tester (client) to control diagnostic functions on the vehicle ECU (server) over a serial data link. UDS requests are sent to controllers that provide positive or negative responses. A diagnostic test tool connected to the ECU retrieves the fault code and displays it using a graphical user interface (GUI).
The UDS software stack stores the fault code for each problem in the vehicle in the ECU memory and transmits it to the client when required. The UDS stack also allows engineers to perform ECU flashing and reprogramming when necessary.
Figure 2 The UDS protocol stack enables users to specify feature sets.
The ISO 14229 UDS protocol is based on the Open Systems Interconnection (OSI) model, which uses layers 5 (session layer) and 7 (application layer) of the OSI model. It is worth mentioning here that the CAN protocol works at layer 1 (physical layer) and layer 2 (data link layer) of the OSI model.
