OBD II UART Hookup Guide

Contributors: Toni_K
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Board Overview

On-Board Diagnostics, Second Generation (OBD-II) is a set of standards for implementing a computer based system to control emissions from vehicles. It was first introduced in the United States in 1994, and became a requirement on all 1996 and newer US vehicles. Other countries, including Canada, parts of the European Union, Japan, Australia, and Brazil adopted similar legislation. A large portion of the modern vehicle fleet supports OBD-II or one of its regional flavors.

Among other things, OBD-II requires that each compliant vehicle be equipped with a standard diagnostic connector (DLC) and describes a standard way of communicating with the vehicle’s computer, also known as the ECU (Electronic Control Unit). A wealth of information can be obtained by tapping into the OBD bus, including the status of the malfunction indicator light (MIL), diagnostic trouble codes (DTCs), inspection and maintenance (I/M) information, freeze frames, VIN, hundreds of real-time parameters, and more. You can read more about the OBD-II protocol here.

STN1110 is an OBD to UART interpreter that can be used to convert messages between any of the OBD-II protocols currently in use, and UART. It is fully compatible with the de facto industry standard ELM327 command set. Based on a 16-bit processor core, the STN1110 offers more features and better performance than any other ELM327 compatible IC. ScanTool has some great resources for the STN1110 available on their website, including:

Board Schematic

The OBD-II UART board has both the STN1110 and the MCP2551 chips populated on it, allowing the user to access both CAN and OBD-II protocols. The schematic can be viewed/downloaded here.

The STN1110 is the main controller chip on the board. This communicates with the CAN, ISO and J1850 transceivers. Voltage on the board is regulated to both 5V and 3.3V for all of the components to function properly. The board is powered from the DB9 connector.

Board Pin Out

There are two different connection points on the board. The first, on the outside edge of the board, is a 6-pin connector that is compatible with an FTDI board. However, only the TX, RX and GND pins are connected on this header, to allow for UART communication.

There is a second 8-pin header close to the DB9 connector. This allows the user to tap into the VBAT line, the CAN bus, the LINE bus and the J1858 bus, along with the common ground pin.

Now that we now about the board itself, let's move on to hooking it up!