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.

Transistor Interface

Transistor Interface Schematic
Note: Anyone with suggestions please send them in!


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.