Arduino work on reading ALDL 160 baud data A resource I found useful for exploring more about ALDL: https://www.techedge.com.au/vehicle/aldl160/aldl_sw.htm

Arduino NANO, NPN transaitor, 10k resistor

• Digital pin 2 connected to collector of transistor (configured with internal pullup resistor)

• Base of transitor connect to ALDL port E with a resistor in series

• Ground connected to Emiter of transistor, ALDL port A, and ground of the Arduino.

• Optionally add an I2C header connected to 5V, GDN, A4 & A5 to attach a display.

Quick summary:

• Attach an interrupt to pin 2 on change
• If LOW (inverted by transitor) then note millis
• If HIGH, compare duration between change. < 2000us = 0; >2000us = 1
• Assemble bits into 9 bit bytes.
• 0x1FF (9 1s in a row) is the sync character
• Data until next sync is messages in format 0x0XX where XX is a data message per the ECM spec

IN PROGRESS

I have a 1990 GMC with a 5V 160 baud ALDL diagnostics port. After receiving a "service engine" light I set out the path of building a USB cable and using WinALDL (https://winaldl.joby.se/) to diagnose what was going on. (Code 32, EGR, had to replace a solenoid). After exploring the data in WinALDL I realized RPM and speed was reported by the ECM and I thought that would be useful display. I have a bunch of Arduinos and ESP32s kicking around and several 1602 and OLED displays.... I figured I must be able to connect a ucontroller to the data pin and extract that data on a display in near real-time.

I studied lots of source code repos and websites for inspiration:

• https://www.bot-thoughts.com/2018/01/decoding-gms-aldl-with-teensy-36.html
• https://www.techedge.com.au/vehicle/aldl160/aldl_sw.htm
• https://github.com/rchipka/Arduino-ALDL-160-baud
• https://github.com/MikeEckels/OBD1Scanner/tree/master/OBD1Scanner
• https://github.com/MikeEckels/OBD1Scanner/blob/master/OBD1Scanner/DataStreamDefs.h
• https://github.com/shimniok/ALDL160_logger
• https://winaldl.joby.se/aldlcable.htm