This device reads the data stream from GM OBD1 vehicles communicating at 8192 baud, with the 12-Pin ALDL connector. This is sometimes also referred to as "OBD 2.1". Using this project, a simple "dataStream" can be defined to provide a "mapping" of all the byte positions for each available sensor reading on the vehicle. The one provided in this project is specifically for a 1994 Chevy S-10 Pickup. The dataStream was created based off the A100.ads binary. Other ".ads" definition files can be found here

The heart of this scanner is an arduino UNO microcontroller paired with a simple three-transistor circuit. More information on how the circuit works can be found here. All credit for this circuit goes to these people. I also created a custom PCB, however, this is untested.

The basic approach of this project revolves around the dataStream. This defines all byte mappings of each sensor so the program can properly decode them. Once a dataStream is defined, we must construct a command. A command is a five byte long message sent to the vehicle to initiate a conversation. The first byte is the message ID, the second is the length, then the mode type, an unknown value, and finally the two's compliment checksum of the entire command.

//message ID, length, mode type, unknown, 2's compliment checksum
unsigned char mode1Cmd[] = {0xF4, 0x57, 0x01, 0x00, 0xB4};

Finally, we can simply pass the command, it's length, and a buffer to the "getAldlData()" function, process the data, and display it over the serial monitor with software serial.

unsigned char rawDataStream1[DATA_STREAM_LEN];
unsigned char rawDataStream2[DATA_STREAM_LEN];

// Not recommended to call both mode1 and mode2 commands consecutively, but it does work.
getAldlData(mode1Cmd, mode1CmdLength, rawDataStream1);
processMode1Data(rawDataStream1, &stream1);
displayData1(&stream1);