ALDL cable
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The ALDL Cable
The ALDL (Assembly Line Diagnostics Link) Cable is an interface that connects the ALDL port on your vehicle to the serial or USB (via an adapter) on your computer. The purose of this cable is to relay sensor data, trouble codes, and general diagnostic health information to your PC so that you can better understand what your car is doing.
Building the ALDL Cable
MAX 232 Method
You will need:
- 1 x MAX232 chip MAX232ACPE-ND
- 1 x 1K @ .25W resistor 1.0KQBK-ND
- 1 x 1N4148 diode 1N4148DICT-ND
- 5 x 4.7uF @ 25V capacitors 493-1056-ND
- 1 x printed circuit board
- 1 x 9 pin male D connector
- Misc wire
The parts can be bought from http://www.digi-key.com.
Two Transistor Method
This is the simplest form of the circuit. It performs quite well most applications. This circuit requires that you have a source for the 12V DC power. This comes from either the cigarette lighter plug or from directly wiring the ALDL cable to a DC source in the vehicle. In some circumstances, you are able to power the circuit from the PC's serial port. However, most laptops do not generate compliant voltages on the serial port, and thus the option is not recommended when the vehicles power source is readily available.
The transistors used in this circuit are generic NPN transistors. 2N3904, 2N2222, or similar will work just fine. This circuit is not particularly sensitive to the gain of the transistors, they are operating well into saturation, and the baud rate is slow enough that switching times are not particularly important either. You can use surface mount versions of these generic transistors so that everything fits nicely in the DB9 backshell, for your RS-232 port. The diode is also a generic low-power switching diode such as the 1N914 or 1N4148. All the resistors are 10K, and again, the circuit is quite tolerant of component variations. If you have a supply of 5K or 20K resistors, these will work also. Try not to get too far afield from these values or the switching times will suffer.
