FreeScan is an ALDL Scan Tool for various GM based ALDL vehicles
When I owned my Lotus Esprit GT3 MY1997, I was concerned with the lack of diagnostic scan tools available. Therefore, I took apart the ECU, unplugged the MemCal, read in the binary code, disassembled it with IDA Pro and found out how the diagnostics worked.
What helped was having a friendly dealer who offered me a few supervised hours sniffing the protocol conversation with the car's ECU while certain diagnostic tasks were performed.
The result was FreeScan was born and can be seen on my website HERE
In this era of pre-OBD-II protocols, each manufacturer had their own way of talking and interfacing to the ECU. Whereas OBD-II is a 12V protocol, ALDL is a 5V protocol. They also use their own type of interface connector too with a non-standard 8197 baud signalling speed.
This project is what kick-started my (ECUFix)(https://www.ecufix.com/shop/) business where years later I did the same thing to my Audi S4's ECU which was somewhat more complicated... Anyway, I used to sell ALDL interfaces but you could still hack one of the USB FTDI FT232R ISO OBD-II / VAGCOM / Fiat ECU Scan / IAW Scan2 OBD-II interfaces to give you the +-5V signalling required - oh and it's USB too since trying to find an RS232 port on a computer is very difficult nowadays.