I too am gonna add to this old thread, though my story is different.
My brother wanted me to build an ALDL cable so he can diagnose my VT Commodore and so I have built the given circuit in this thread that involves the ATMEGA8 chip and MAX232 chip, but there are some grey areas with that circuit. Firstly and most importantly, what is the frequency of the crystal that connects between pins 7 and 8 on the ATMEGA8 chip??? And is the frequency value of the crystal universal for all Commodores or does it vary for different models? I have also gotten the OBDII plug as well but am unsure of the pin connections, does anyone know which connects to what pin?
If anyone who has built the ATMEGA8 version of the circuit can help me I would most appreciate it.
Cheers
Troy