[Elaine]

Bellow is the interface I'm using to read the ECU memory.
Here is the full story. I have a '92 model. In my first attempt to read my car's ECU I used the circuit found at alcyone.org.uk. It worked very well with the engine turned off but failed when I turned the engine on. By playing with the potentiometer adjustment I could make it working with the engine on as well. Since the beginning I wasn't happy with the potentiometer idea but having different adjustments for engine on/off was already too much. So I brought my scope and played a bit with the adapter I built. Here are my findings:
- The ECU input looks like a pulled up one. I played with the pull down current and I found that for a good "0" I need to "suck" at least 10 mA.
- The ECU output is a 1Vpp signal (1.2Vpp with the engine off for whatever reason) with a DC component of half of the car battery voltage. The fact that the car battery moves from 12.6V to 13.8V when the engine is on explains why the potentiometer needed to be adjusted again.
- With the engine on, both the 12V and the ECU output lines were affected by some 0.4Vpp noise but it was identical on both lines.

So I decided to change the classic design well known already by powering the analog interface to 12V and make the conversion to TTL after that. I ended up using the LM393 which has an open collector output so the interface to the 5V circuitry becomes a pull up resistor. Moreover, this device doesn't need any additional protection circuitry for this application.
For the RS232 interface I used the MAX231 but any RS232/USB transceiver can be put instead. D4 which is a Ge diode is used to move down the comparator threshold by 0.1V. It doesn't make a big difference, it can be removed. If you want to build this circuit don't use any other diode instead, it will ruin your threshold level. Also, don't use any filtering capacitor in parallel with R6, it won't help. R5 and R6 should have the same value with 1% tolerance.

I'm not sure if my ECU works properly or not (it's the only one I played with), although what I can see on my scope make sense to me. The beauty of this circuit is that it's simple, doesn't create any stress on both the ECU and the adapter itself and doesn't need any tuning, it's plug and play. If you play it right you can get even LM393 free samples....