Making Aldl Cable To Connect PC To OBD1 Ecu

I've always wanted to connect my laptop to my GMC van OBD1 based ECU , a little research on the internet lead me to a simple circuit that could make that happen .

Simple One Transistor Circuit:
Aldl has a port for sending data and the RS232 on PC has a receive data pin (RxD) so the easiest way is to connect them directly but since each system uses a different voltage we should put a transistor between them .

The simplest ALDL interface circuit i could find consisted of one transistor and two 10k resistors it simply connected the data pin on the ALDL plug to the RxD(Receive Data) port on the RS232 on the PC through the transistor .
This circuit worked , but for some reason the response was slow , i couldn't make it work with a Usb-to-Serial converter , and on some PCs you can't take power from Pin4 so you have to connect it to 12v external power source (the resistor stays).

Two Transistor Circuit:
Since the first circuit performance wasn't great and the whole thing was messy , instead of refining the first circuit i thought i should try a new design .
This circuit consisted of two transistors one connecting the "ALDL data" to "RS232 Receive" , the other connects the "RS232 Send" to "ALDL Ground" .


The Circuit Diagram :

Parts Needed:

1. 2 2N3904 NPN Transistor
2. 3 10K Resistors
3. 1 1N4148 Diode
4. StripBoard
5. DB9 Connector
6. Car cigarette lighter Plug
   

Here is a picture the resistors , transistors , diode and DB9 connector:


The Strip Board:


The ALDL interface circuit components soldered to board:
...and the ALDL circuit with all the wires connected:
a closer shot of the finished ALDL circuit:

Important definitions:
OBD1 / OBD2 : On-Board Diagnostic system is the ECU's operating system OBD1 was implemented early 1980's to 1996 , OBD2 from the year 1996 to present.
ECU / ECM : Engine Control Unit / Engine Control Module a device used in modern engines to control engine's operation .
ALDL : Assembly Link Diagnostic Link is an interface for communicating with the Ecu developed by GM for pre-OBD2 ECUs.