VAGCOM Minimal Adapter

Hello everyone.

A new minimal adapter by Christian Helm has been featured in the diagnostic journal.
The circuit can certainly claim the record for the simplest design – if you remove anything or replace it with a jumper wire, it probably won't work anymore.

Unfortunately, I couldn't find a clear indication of the author in the membership list. Therefore, I would like to ask him the following question:

Your adapter cannot produce negative voltage levels on the RXD line (the minimum is almost GND, or the saturated collector-emitter voltage at T2), although, as far as I know, these are necessary for a clear low-level detection according to the RS-232 standard.

According to this, your adapter is unlikely to function correctly (or at all) on standard COM ports.
Or maybe? Then, please provide clarification regarding the "correct" RS-232 standard .

That's right!

The circuit does not produce a negative voltage level on the RS232 side and therefore does not operate according to the standard.

The fact is that the adapter has already worked with two different laptops and two different cars.

I saw a similar circuit used as a Nokia phone interface on the homepage of a friend. This circuit was also not compliant with regulations in that respect, but it still works.

Best regards,
CH

But I just noticed that the error crept into the RS232 connector.

The following connections to the PC are required:

2 - RX
3 - TX

not the other way around...

I have sent the corrected circuit diagram to webmaster@dieselschrauber.de.

Best regards,
CH

I recreated it.

It doesn't work for my problem with the 1Z.

Perhaps it's related to pins 2 and 3. Sorry about that.

The 1Z circuit has worked for me many times before.

Best regards,
CH

For the 1Z, it would even be possible to omit the transistor and the two resistors for the L-line.

That would really be minimal .

Best regards,
CH

It's not because of that. I immediately saw that something was wrong! But even so, it's still not working. There's no proper communication happening! Maybe it's too minimalist! There was another option, but I'd rather stick with the more complex components!

joergs wrote:

No reasonable communication is happening! Perhaps it's too minimalist!

Hi Jörg,

If you still have that component, please connect a 22k resistor in parallel with the B-E junction of transistor T2.
This shifts the 1-0 detection threshold for K to around 3.5 volts (instead of below 1 volt, as in the circuit, which my AFN engine computer can barely handle - although the other STG units can).

If it works, then, in my opinion, a "proof of concept" for such minimal circuits (including those from Noxon) would be practically demonstrated.

It's likely that some PCs will accept 0 volts on the RXD line as a low signal, while others will require a definite negative voltage.
If your PC belongs to the latter group, then the modification mentioned above will, of course, not help.


@Christian:
The connection between your name and your user ID seemed "too abstract" to me when I was reviewing the list of members – I was only looking for Chris... and Helm .

@ulf

My wife gave me a little bit of freedom this afternoon!

I've soldered in the 22k resistors. The result: no change compared to before. Now, here's the thing: when I connect my DIY control unit to the car (028 906 021 FB), there's communication, but it quickly breaks down! When I connect it to my AWX, there's communication, and data logging is possible for a short period with the engine control unit, but I can't even access any of the other control units! It's certainly possible that this is largely due to my notebook! But even so, I prefer your adapter, or the one from Maik. At least those work right away and are stable!

chrhelm wrote:

The circuit does not produce a negative voltage level on the RS232 side and therefore does not operate according to the standard. The fact is that the adapter has already worked with two different laptops and... 2 different cars work.

Hi Chris,

"If you yourself are saying that your adapter doesn't work according to the standard (in fundamental aspects like signal voltages), and even Jörg, who is certainly an experienced VAGCOM user, is having problems with the wiring... "
...then I wonder how much sense it makes to suggest replicating the circuit.

I believe that even a minimal adapter should still offer acceptable operational reliability and reasonably broad compatibility.
If one wants to move your circuit (according to our current knowledge), it would, in my opinion, require at least...
1. Enable negative low-level signals on RXD.
Set the K-detection threshold close to Ub/2.

With such additions, depending on the approach, one might already be entering the basic concept of my 3.1, if one were to omit the LEDs and the filtering of the supply voltage. . .

By the way: Are you sure that the BC 547 transistors are not reverse-biased? How can your adapter's T1 and T3 components (without a protective diode) safely handle reverse voltage spikes of up to 15 volts that they might experience during operation?

Without wanting to presume to offer advice as a circuit designer, I would like to suggest to you that...
- to include a note in your diagnostic article about potentially likely compatibility issues, or
- To remove your adapter from the product again.

Don't take it personally - especially, my goal is not to force other adapter designers out of business .

- to include a note in your diagnostic article about potentially likely compatibility issues with your plan, or
- To remove your adapter from the item again.

Yes, that's correct.

The circuit is only useful if someone unexpectedly wants to read out their error memory and doesn't have any optocouplers at home (like me ).

The MAX232 also requires some external components and a power supply. I had it, but somehow I lacked the motivation to build such a large circuit.

That's how the minimal circuit came about.
As I said:
-The circuit helped me.
- Works with my 1Z and my brother's 1.8L gasoline-powered Vento (including...). L-Line)
-compatible with Compaq Pentium 75 laptop and Compaq Pentium 266 laptop.

The Nokia Interface by Karsten Beck
is also not compliant with regulations, but it also works http://home.arcor.de/karbeck/mbus.jpg.
More details about that: http://home.arcor.de/karbeck/mbus.pdf

I'll leave the decision to Rainer.
I'll send him an email...

Best regards,
CH

chrhelm wrote:

The circuit is only useful if someone unexpectedly wants to read out their error memory and doesn't have any optocouplers at home (like me

).

Hi Christian,

. . . and if, by chance, his diagnostic PC also works with the minimal adapter.
I suspect that most people don't know that in advance.

Christian,

What was the manufacturing year of the 1Z model that was used for the communication?
I'm using the other minimal interface, and it has worked so far with various cars (Coupe 20V, year 1988; Golf 4 1.6, year 1999; Golf 3 1Z, year 1995; A4 1.8, year 1996; A6 2.6, year 1995...).
'Only with the 1Z and AAT 5-cylinder engines (85 kW), both manufactured in 1992, did nothing work. VAGCOM detected the baud rates, but it couldn't synchronize.'
Mario.

Quote:

chrhelm: For the 1Z circuit, it would even be possible to omit the transistor and the two resistors for the L-line.

Why? What is different about the 1Z?[/quote]

Why? What is different about the 1Z?

He only has a K-line, not an L-line.

What do K and L actually stand for, and what is the difference between K, K+L, K+K, and K+K+L (although it's probably only one of the last two variations that exists, right?)?