ALDL

[frederic.breitwieser]

>Does anybody know how diacom talks to the parallel port on a PC for
>ALDL monitoring?

Sure Andy, its pretty easy actually... though difficult to duplicate unless 
you have decent programming skills, which I do not unfortunately.

1980's GM ALDL 160 baud:
The data is spewwed out of the ECM with different length pulse widths, and the 
diacom software, through the parallel port, monitors the pulse width, then 
determines the value, assembling the data in your PC so it can display the 
information on the screen.  A simple, software UART at its finest.

There is a schematic floating around on the internet that contains several 
2n2222 NPN transistors, a dime a dozen, which convert the ALDL voltages to 
something the parallel port likes.  I managed to build the circuit in about an 
hour, and I did in fact recieve data from my 148 ECM, the one out of the 86/87 
Buick Grand National, as well as several of the FWD 3.8L Buicks (like the 
LeSabre).  The error codes I got were that the RPM was 0, the MAF failed, as 
well as other sensors.  Of course, the ECM was sitting on the garage floor not 
connected to anything, so this makes sense.

Anyhoo, hope that helped.

Frederic