VAGCOM Adapter: Fast Sync Compatibility?

Hi everyone,

Perhaps we can still design a cost-effective and universally functional DIY adapter based on our collective experience.

As I mentioned before, my draft on Rainer's website works perfectly for me, but apparently not for other colleagues .

Achim got it working by replacing the R3 resistor with an 1.8kOhm resistor - that's already a first step towards improvement.

I'm almost afraid that with a universal adapter, you might have to connect a 1k potentiometer in series with R3 (1.5k) in order to be able to adjust the adapter to the parameters of the PC interface.

Here's one more question based on your experiences, mainly with other adapters: What message do you receive during the adapter test?

"I'm getting an 'OK' message, but with a limitation."
"Fast Sync Compatible: NO"

Does anyone have an adapter where this message says "YES"?

If so, and if you are familiar with the circuit: How does this circuit differ from my design?

By the way, my diagnostic PC is running at a clock speed of 200 MHz.

Hello Ulf!


I have now found the problem: The sub-notebook I was using (I no longer remember the brand and model) did not work with either the schematic from Jeff Noxon or your schematic. My PC works perfectly with both circuits. (About 6 hours of work wasted because of the stupid COM port on the notebook). Furthermore, the notebook still didn't work with R3=1K Ohm.

I'll probably continue using Jeff's interface since it's already soldered onto a grid array. I have your schematic, but because it's more complex, I've built it on a breadboard. I have to admit, though, that I especially like your LEDs.

Now, I just need the full version of VAG-COM. Let's see if there's anything available somewhere at the "student price" .

Best regards, Georg.

Okay, getting to the actual topic of this thread:

Somewhere in the vastness of the Ross-Tech forum, I read a statement from Uwe Ross stating that the "Fast-Synch" feature was his secret. The circuit itself recognizes the time intervals between the pulses on the K-line (like the VAG-Tester probably does) and thus determines the correct baud rate. If the adapter is not "Fast-Synch" compatible, the software tries out common baud rates based on a predefined pattern until the communication works.

Best regards, Georg.