VAGCOM Adapter: The Essential Guide

Hello everyone.

After JörgS managed to connect his Problem-1Z using opto-adapters,

(Post from February 28th, in https://community.dieselschrauber.org/en/viewtopic.php?p=12622

My assumption about VAGCOM and the required adapter quality seems to be confirmed (in this case).
In simple terms, the transmission requirements of VAGCOM, as far as I'm concerned, are essentially at the level of a Trabant, which can easily be achieved with "slow" optocouplers.

Using adapters without optocouplers (like the newer Kauf adapters) for VAGCOM is roughly equivalent to removing the seatbelts from a Trabant and putting on racing slicks.
"This Trabant version will be sold with the following disclaimer: 'With these tires, you can drive over 240 km/h. Furthermore, we can offer you the car, even with VR tires, at a slightly lower price without seatbelts. And since seatbelts are actually never needed, they will no longer be installed in the future.'"


@Maik: No offense. I know that you designed your adapter as a reliable alternative, whereas some of my opto-build suggestions barely reached Trabant-level quality .

I also don't want to completely rule out the possibility that there may be (rare) isolated cases of unfortunate combinations of diagnostic interfaces, background software, and vehicles, where even good optoelectronic circuits may fail, and only adapters like yours can still enable reliable communication.

And I also know that, for example, with the Opto-Adapter 2.4, the connection can be temporarily interrupted due to significant fluctuations in the on-board voltage (e.g., when starting the engine). This could be remedied with a fixed voltage regulator, which would, of course, require a more complex design.

By the way, it's also possible to make Opto-Adapters even faster than the 2.4 version – Jörg actually built a "internal" version for me that was about 3 times faster, but even in his case, it didn't yield better results.
So, he essentially got HR tires for his Trabant for free .

@ulf

You're basically right! The 2.5 adapter doesn't offer any significant advantage compared to the 2.4 adapter, or only a negligible one.
Furthermore, it is undeniable that adapter operating times and signal distortions likely have a significant impact on communication! What made me suspicious was the fact that the original minimal adapter also works quite well with modified VAG-COM settings! I say 'quite well' because communication interruptions are occasionally observed/were observed during driving.

However, the adapter is only one cause. The main problem is the difference in bit lengths of the control unit! As I mentioned yesterday, there should be a way to either use a microcontroller in the adapter to shape the signals so that VAG-COM can understand them, or to use a synchronization clock to make the control unit or the RS232 synchronize with the other device, or to access the communication through software adaptation.

It's a shame that I won't be able to work on my 1Z as much starting at the end of March! Then, most likely, a Passat with an AWX engine will be here!

by the way: does a Passat from 2000 (already the new model) actually have 2 K-lines, or just 1?

@maik:
How can we potentially enlarge the holes in your adapter to fit two K-line connectors?

joergs wrote:

Furthermore, it is undeniable that adapter runtimes and signal distortions likely have a huge impact on communication!

Hi Jörg,

If the errors reach extreme proportions, then you are probably right.
A previous opto-adapter I designed (which didn't yet have trimmers on the opto inputs) caused problems because it shortened the bit lengths from the car to the PC by approximately 30 microseconds.
That seems to be the approximate threshold value where the problem occurs (at least in my tractor).

Below these deviations, I can freely adjust my adapter, and VAGCOM continues to run without interruption.

Quote:

@maik: How can we possibly enlarge the hole in your adapter to fit two K-line connections?

Although I am not Maik, I believe that the rectifier concept from my proposal for 2 K-Lines should also be applicable to Maik's adapter .

Sure, here is the translation of the text from German to English:

"Addendum:"

Quote:

What made me suspicious is the fact that the original minimal adapter also works quite well with modified VAG-COM settings! It works quite well because, during the drive, there are/were occasional communication interruptions.

Probably, the bit length errors of the minimal adapter in conjunction with your PC and car are usually just below the error threshold, and in conjunction with fluctuations in the on-board voltage, the critical limit is sometimes exceeded .

Hello,

I once created a circuit diagram using a MAX232 and an optocoupler.
Maik's circuit diagram has been slightly expanded.
Could you please take a look at this?

http://www.alexweiss.gmxhome.de

The DC/DC converter is still in progress, I'm working on it at the moment, but it should be finished by the end of this week.
I wanted to use a 6N136 optocoupler, not a 6N135 as shown in the schematic.

I would appreciate it if someone could offer some feedback, pointing out what's good and what's not.
I hope I can get the first working prototype board completed this weekend.


Best regards,

Nicklas wrote:

Hello, I once created a circuit diagram using a MAX232 and an optocoupler. Maik's circuit diagram has been slightly expanded. Could you please take a look at this? http://www.alexweiss.gmxhome.de

Hi Nick,

The image in the direct link is too blurry, and the Eagle schematic is essentially just Maik's design.

You can omit the trimmer resistors on the K-line circuit. Use a 2.2k or 3.3k resistor as a pull-up resistor, and for the 1-0 detection of the K-line, use a 100k and a 33k resistor (with the diode placed before the transistor base).

I would also use this opportunity to configure the LEDs so that each only displays the bits sent by the car.
Based on Maik's circuit, it is possible to achieve this by simply rearranging some of the connections.
But I don't want to take away all of your work .

Hello,

I've actually uploaded the wrong Eagle file; I'll correct it later tonight when I'm home.
I can probably remove the trimmer; it shouldn't be a problem, and it will free up more space on the PCB.
I've been thinking about the LED thing, and it should work, but I'll also need a PNP transistor.
I'll update the circuit diagram again and upload it tonight.

Best regards,

[quote='ulf']Hello everyone.

After JörgS managed to connect his Problem-1Z using opto-adapters,

(Post from February 28th, in https://community.dieselschrauber.org/en/viewtopic.php?p=12622

My assumption about VAGCOM and the required adapter quality seems to be confirmed (in this case).
In simple terms, the transmission requirements of VAGCOM, as far as I'm concerned, are essentially at the level of a Trabant, which can easily be achieved with 'slow' optocouplers.

---------------------
'In 1998, I also recreated the simple interface from Jeff's homepage using Siemens' SFH 615 optocouplers. It worked perfectly right away. My car is a Galaxy TDI with a 1Z engine and still has the MSA12 engine control unit. I can access all the control units. I've used the interface on maybe 20 or 25 vehicles - only one Seat Ibiza from 1999 refused to cooperate.'
I might be observing a different problem: when it gets very cold, the operating point of the transistors drifts so far that no connection can be established (even with the charger connected to the battery).
Best regards, Lukas.[/quote]

Hello,

I can't imagine that the operating point of the transistors is the cause of the problem. The transistors are only being used as digital switches.

Sure, here is the translation of the text from German to English:

'Hello'

Nicklas wrote:

I can't imagine that the operating point of the transistors is to blame. The transistors are only used as digital switches.

Hi Nick,

Take a look at the biasing of the "sensor transistor" for the K-line: 150 k from K to the base, and that's it.
That means the on-board electronics must draw less than 0.6 volts to recognize a bit.
I can certainly imagine that this delicate balance is temperature-dependent.

Hello everyone.
To anyone currently struggling with an OBD-2 interface for VAG.COM, I recommend searching on eBay under 'OBD2.'
You can get those things complete with plugs and cables for 55 or 60 euros.
I think building it yourself almost isn't worth it anymore.

I ordered it two weeks ago. This part works perfectly with my 1999 Sharan (AFN) using VAG.COM 208.2.

(No! I'm not the seller, I don't get any commission, and I don't want to do any advertising. Just a tip.)

Regards,

Reinhard.

Hi,

The transistor already has a pull-down resistor at its base, so it only needs to be pulled up. Of course, this isn't for reading a bit, but rather for a static state, which the 4.7k resistor achieves. In the control unit, there's only a high-impedance pull-up resistor and a series resistor (30-80 ohms) to protect against short circuits. Older control units often have a simple open-collector configuration (an NPN transistor), while newer ones use an ASIC (e.g., ATM46), which can easily pull the level down against the 4.7k resistor. The open-collector configuration in the car might cause more issues, but it should generally be manageable. I've also had ECUs in climate chambers at -40°C, and I've never had any problems with the K-line.
The value of the base resistor, whether it's high or low impedance, doesn't matter because the BC337 has an hfe of 600, and it only needs to pull down 10kΩ and 2 TTL inputs, which draws approximately 0.5mA, and then the transistor itself draws 8.3 * 10^-7 A.

I've updated the circuit diagram and corrected the download link.
For those who want to take another look.

http://www.alexweiss.gmxhome.de

Regards,

P.S. Building your own adapters is more fun than buying them.

Nicklas wrote:

Hi, The transistor already has a pull-down resistor at the base, so it only needs to be pulled up, of course, not to read a bit, but only statically, which is what the 4.7k resistor does (in the control unit, there is only a high-impedance pull-up resistor and a series resistor (30-80 ohms) to protect against short circuits). . .

Hi Nick,

Um . . . are we talking about the same circuit?

I'm referring to adapters like the one from Noxon, which Lukas essentially recreated, and which served as the basis for the "minimalist design" described in the adapter technical article.
And in there, for example, there isn't a single 4.7k resistor.