Schematic of a GM ALDL -> PC converter

[Robert J. Korn]

John Faubion wrote:
> 
> Mike if this works then why are there not more people doing this? The
> software should be easy and the cable should be easier. Additionally if its
> that easy, why did Diacom spend the time to write software to adapt to a
> parallel port? 8{)
> jfaubion at beaches.net

Depending on your point of view :

A) PROFIT & GREED.
B) To protect their BIG MONEY investment in obtaining the codes from GM.
C) Government Conspiracy

ECM is 0 to 5v parallel port is also.
No level converters are needed, just a pullup resistor to +5V.
This makes the cable simpler and the software a few lines larger,
and sometimes speed sensitive. 

Writing software is a one time expense so larger is OK.
Hardware is a per customer expense, must keep it down.

But thats enough about the manufacturer.

Simplest for the diy'er is to use a max232 chip in a box.
Then you can use almost any com port.