This is a cable I made to use to monitor my Syclone with DataMaster and FreeScan. Details, including the schematic, can be found at TechEdge. I am using their "two transistor sneak preview" circuit.
Externally, the cable consists of a DB9 connector with a CAT3 phone wire splitting into ALDL wires at one end, and a cigarette lighter plug at the other. The circuit needs 12V to operate, so a hot power source is needed. The cigarette lighter is the easiest way to do this, although running the wire to a different power source may work better for permanent installations. It's stated on the WinALDL page linked below that some computers will supply power on one of the serial pins. I don't believe this is a standard, so it may or may not work on your specific laptop. If you want to give it a shot, it's probably best to test it on any laptop you plan to use before snipping the lighter plug off. If you do try this, please let me know how it turns out. There is a ground wire in each end of the cable. Using ground from the ligher socket and a single wire for ALDL data is easiest, though grounding the second wire of the ALDL end (not pictured) at the ALDL connector itself may be easier for permanent installations, with only the power wire running elsewhere. Both ground wires run to the same place in the conversion circuit. Only one needs to be hooked up, though connecting both won't cause any problems.
I have successfully used this cable with DM 0D on my S-10, and with DM TT and FreeScan on my Syclone. From the TechEdge documentation, I believe it should work with any 160 or 8192 baud datastream, though I have not tested it with any other vehicles. See the WinALDL page for more information about connecting to other vehicles.
Here is a diagram of how I have my cable wired up to the TechEdge schematic. As you can see, both wires with white are ground and connect to the same point in the circuit. You can use either the lighter plug or ALDL ground, or both. The blue wire is for the ALDL data stream, and the orange wire is where the circuit gets its power.
I've received a few emails lately from people looking to build their own cables from this data. At the time I was building these cables, I worked at an electronics shop. I basically just showed the schematic to one of the techs and asked him what parts I needed. He grabbed the first parts he saw that would work. The diode I used is a 2.5amp 1Kvolt, which I'm pretty sure is major overkill for this circuit. The techs referred to the transistors as "9570's", though I'm not sure if that's a standardized part number or just our shop's internal number. I believe just about any NPN transistor would work, but I'm not an electrical engineer - I know basically enough to read a schematic and figure out how to put the parts together.
I got an email from Jerry Coleman with some info that may help some of you out. He listed 2N3904 (NPN general purpose switch/amp transistor) and 1N4148 (signal diode) as industry standard part numbers, which should be available at most electronics distributors or Radio Shack.
Many people don't like the fact that the data cables require the cigarette lighter to work properly. Unfortunately, every serial datalogging cable I've seen uses a conversion circuit that requires power. Neither serial nor parallel ports provide a direct power line. However, you can use a couple of the other signal lines to steal power. I researched this before making these cables, but gave up on it when I learned that this wasn't really standard, and may not work with all serial ports. Since then, I've converted my own cable to work off power stolen from the serial port, with no issues whatsoever. Dig also reported that he's never used external power with any of his cables, and has yet to encounter a laptop that won't properly power the cable.
If anyone else wants to mod a cable from me for this, I used 2.5A, 1000V diodes from Radio Shack (276-1114, it's a pack of 3), which are the same rating as what I used in the cable's circuit. Once again, they're probably overkill. Just scrape off the liquid electrical tape where orange wire goes into the resistor, and solder the striped end of the diodes there, and the other ends to pins 4 and 7 (nothing else in those pins). I was able to put them fairly close to the other stuff, so it all still fits inside the shell just fine.
You can find pictures and more details in this thread.
Here are some more links that may be helpful...
Andrew's GM ALDL Stuff
1993 S-10 Tahoe ext cab 4x4