The ECM produces the data signal at either a 12 Volt or 5 volt level that must be inverted and converted to RS232. One simple scheme is to feed the ALDL data into a single transistor level converter that inverts the data to produce a pseudo RS232 level (varies between 12 Volts and 0). This is connected to the receive data line of a serial port to be processed by the PC's UART.

R3
When you build the cable first try without the R3 resistor. That works for most vehicles. Some vehicles need a 10k resistor to start transmitting data but the resistor will change the ECM's mode. It will have a high idle and spark knock retard will be disabled. That is the reson why you should not use the R3 resistor if it works without it.

R4
With this resistor installed the interface will NOT work with fuel injected vehicles. This resisror should ONLY be used with some carbed vehicles that does not work with the standard interface. If you need this resistor then you probably have to use the 4800 baud setting in the configuration too.

On some computers it is possible to use the DTR pin on the serial port as 12 Volt source, and that is what I did.

If you want help building the cable:

http://www.aldlcable.com

e-mail winaldl@joby.se