PC Scan Tool Interface Details
Interface to connect GM ALDL to PC Parallel (Printer) Port
My program uses the printer port and a very simple 2 wire cable. Some use
a special hardware device that has a serial interface for the computer.
In my case, I didn't want to have to build any special hardware.
It only works on older GM vehicles with C3I injection modules which
output data at a tortoise-like 160 bits per second.
on the Parallel Port:
I used ground (25) and the 'Busy' pin 11. Any of the printer status pins
will do really.
on the ALDL:
E-Serial Data...for scan tools
Since I didn't want to use ANY hardware, I connected pin 25 to E, and
11 to A, which is ass-backwards from and electrical viewpoint, but from a
practical viewpoint, it works :)
(well, as long as your car and your computer don't share a common ground,
like running a laptop plugged into the lighter socket maybe)
Warning: My ECM outputs 5 volts which the Parrallel Port is happy with. Some models output 12 volts and that's too high
and is even more likely to fry something. If yours sends 12 volts, consider using the circuit below.
Note: Anyone with suggestions please send them in!
Homebrew Automotive Mailing List
A simple 1-transistor interface, thus isolating the two machines.
I soldered this together in the parallel cable head, and it works just
fine. I did bring the ground of the ALDL connector to the ground of the
parallel port with no problems.
Since the parallel port doesn't bring +5V to the printer for any reason, I
tapped the +5V off the mouse port. Worked for me. But its more hassle for
others, so I drew my schematic to utilize one of the in/out control pins,
which your program would need to make "high" at all times you want to read
For those people who have the Diacom software, here is a schematic diagram of
an interface cable which may be compatible. I have received letters from a few
people who claim to have built it and verify that it works.
Car to PC Interface details
Note: It's not my diagram and I don't use Diacom software. Try it at your own risk.
If diacom is trademarked, servicemarked, or watermarked then that's their business,
I have nothing to do with it.
Newer 8192 bps format:
The newer vehicles are at least 8192 bits/second, for those you can
probably make a fairly simple serial cable with a voltage level-shifting
chip. I dont have one and so I dont care.
Why do you need the V.L.S. chip in the later ECM and not the C3?
The newer 8192 bps format can be read by a conventional serial port if you
pull some timing tricks, making the software's job much simpler, and you
don't need a fast computer.
The ALDL on my car outputs 5 volts or 0 volts dc. Serial data needs to be
+12/-12 or sometimes +5/-5. That's what the level shifter chips can do.