OBD Adapter Help: Troubleshooting & Setup

Hi, I built the circuit today as described in the technical article. However, I'm still having a small problem: I performed the following tests as described:
Preparation:

Connect pin 5 of the PC connector (GND) to the vehicle's ground connection.
Connect pin 4 of the PC connector (DTR) to the vehicle's 12-volt power line.
Set all trimmers to their maximum setting.
Tests: OK
- Connect pins 3 and 7 of the PC connector (TXD and RTS) to the vehicle's ground rail, and supply 12 volts from the vehicle side.
feed in. OK!!!!

In this case, both the K and L lines must each carry 12 volts. The collector of T3 must not have any voltage. LEDs 1 through 3 are not lit. -OK!!!!

- Instead of connecting pin 7 of the PC connector to ground, apply 12 volts. LEDs 1 and 3 should light up, which corresponds to a specific test state in the LED test menu of VAG-COM.
Then, connect pin 7 to ground again. OK!!!!

- Apply 12 volts to pin 3 of the PC connector (TXD). LED 2 must light up. The collector of transistor T3 must be at 12 volts. This corresponds to the data flow from the PC to the car. OK!!!!!

- Pin 3 der PC-Buchse (TXD) und die K-Line mit Masse verbinden. LED 3 muß leuchten, solange die K-Line mit Masse verbunden ist. Das entspricht dem Datenfluß vom Auto zum PC.????Wenn ich jetzt PIN 3 mit K verbinde (also wieder mit 12V da K doch 12V führt) leuchtet doch wieder die LED 2 wie im vorherigen Schritt. Wenn ich jetzt auch noch Masse mit K verbinde habe ich ja einen Kurzschluss (da K 12V führt und dies gegen Masse)Ists sicher das hier Pin 3 gemeint ist. Weil im vorigen Schritt wird ja Pin3 gegen Plus geschalten und dann soll ja LED 2 leuchten. Warum soll nun wenn ich PIN 3 gegen K schalte (also wieder gegen 12V) auf einmal LED3 leuchten???Und warum wird die 12V führende K leitung gegen Masse kurzgeschalten. Vielleicht kann mir ja jemand persönlich unter [email protected] weiterhelfen wenn dieses thema zu Umfangreich fürs Forum wäre. Vielen dank im Voraus. SchBest regards

mickdoohan wrote:

- Apply 12 volts to pin 3 of the PC connector (TXD). LED 2 must light up. The collector of transistor T3 must be at 12 volts. This corresponds to the data flow from the PC to the car. OK!!!!! - Connect pin 3 of the PC connector (TXD) and the K-line to ground. LED 3 should light up as long as the K-line is connected to ground. This corresponds to the data flow from the car to the PC. If I now connect PIN 3 to K (meaning connecting it to 12V since K carries 12V), then LED 2 will light up again, as in the previous step. If I now also connect ground to K, I will have a short circuit (since K carries 12V and this is connected to ground).

Hi,

The test steps should be designed in such a way that no "intentional short circuits" are created.

Quote:

Is it certain that pin 3 is meant here? Because in the previous step, pin 3 is switched to plus, and then LED 2 should light up. Why, when I switch PIN 3 to K (i.e., back to 12V), should LED 3 suddenly light up??? Pin 3 is not connected directly to ground (GND), but both pins are connected to ground together.

And why is the 12V positive wire shorted to ground

Quote:

? K doesn't directly provide 12 volts, but rather does so through resistors R8 and R9. Using a sufficiently high-impedance measuring device, a voltage of 12 volts is displayed for K. When a capacitor is connected to a resistor, it doesn't cause a short circuit with several amperes; instead, the current flow is approximately 13 volts / 3770 ohms = only 3.45 milliamperes. Is it clear now?

Hi,

The test steps should be designed in such a way that no 'intentional short circuits' are created.
I would also like that:-))

'Zitat:' translates to 'Quote:' in English.
Is it certain that pin 3 is being referred to here? Because in the previous step, pin 3 is connected to the positive voltage, and then LED 2 should light up. Why, when I switch pin 3 to ground (i.e., back to 12V), does LED 3 suddenly light up?

Pin 3 is not connected directly to ground (GND), but both pins are connected to ground together.
I don't quite understand. If PIN 3 is connected to K and K is connected to ground, wouldn't that also connect K to ground???
'Zitat:' translates to 'Quote:' in English.
And why is the 12V control line shorted to ground?


K doesn't directly supply 12 volts, but rather does so through resistors R8 and R9.
When connected to a sufficiently high-impedance measuring device, a voltage of 12 volts is displayed.

When a capacitor is connected to a resistor, it doesn't cause a short circuit with several amperes; instead, the current flow is approximately 13 volts / 3770 ohms = only 3.45 mA.
How are you measuring those 13 volts? If K is connected to ground? What potential are you measuring the K wire against? I don't quite understand.
Is it clear now?
No. Can I reach you by phone? You can confidentially provide me with your number at [email protected] if you wish. In any case, thank you very much. Best regards.

Okay, having a basic understanding of circuit design is definitely an advantage when building the adapter yourself.
It might be a good idea to test the self-built adapter on a PC with a COM port (interface) card installed. So, not on a laptop, and not on a PC with a COM port integrated into the motherboard.

Best regards, Rainer.

Dear Mr. Rainer,

I specifically wanted to avoid this kind of conversation in my initial post. I don't believe it's necessary for me, as an engineer specializing in communication technology, to discuss my circuit design skills with you. Instead, I asked a specific question, and it would be appreciated if you could address that question, as Mr. Uwe kindly did.

Sincerely,

Sehr geehrter Herr Rainer. Genau diesen Dialog wollte ich in meinem ersten Beitrag vermeiden. Ich glaub nicht das ich mich als Ing. für Nachrichtentechnik mit Ihnen über meine Fähigkeiten des Schaltungstechnik unterhalten will, sondern habe eine konkrete Frage gestellt, und es wäre nett wenn auch diesbezüglich geantwortet würde, so wie es freundlicherweise Herr Uwe getan hat. MfG


hehehe

*shakes head in disbelief*

And another one...

Rainer K. wrote:

Okay, having a basic understanding of circuit design is definitely an advantage when building the adapter yourself. It might be a good idea to test the self-built adapter on a PC with a COM port (interface) card installed. So, not on a laptop, and not on a PC with a COM port integrated into the motherboard. Greetings, Rainer

Hi you scooter racers,

The above should be a well-intentioned piece of advice, not a reprimand. It wouldn't be the first time an interface IC has ended up with a small, dark spot in the middle.

Okay, here's the translation:

"Brace yourself: I'm a qualified engineer in communications technology myself. Therefore, I'd be very interested to know your grade in circuit technology and the name of your university or technical college. You certainly wouldn't have earned a degree at the University of Applied Sciences in Coburg."

Best regards, Rainer.

I've unlocked the thread again; the name of the university or technical college would be interesting.

@mickdoohan

Maybe this is interesting for you! It should actually help you!

I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.

I would also be interested in knowing the name of the university of applied sciences/university and your grade in circuit technology or electrical engineering/electronics! I have a hunch, but I'll keep it to myself for now.

- Pin 3 der PC-Buchse (TXD) und die K-Line mit Masse verbinden. LED 3 muß leuchten, solange die K-Line mit Masse verbunden ist. Das entspricht dem Datenfluß vom Auto zum PC.????Wenn ich jetzt PIN 3 mit K verbinde (also wieder mit 12V da K doch 12V führt) leuchtet doch wieder die LED 2 wie im vorherigen Schritt. Wenn ich jetzt auch noch Masse mit K verbinde habe ich ja einen Kurzschluss (da K 12V führt und dies gegen Masse)Ists sicher das hier Pin 3 gemeint ist. Weil im vorigen Schritt wird ja Pin3 gegen Plus geschalten und dann soll ja LED 2 leuchten. Warum soll nun wenn ich PIN 3 gegen K schalte (also wieder gegen 12V) auf einmal LED3 leuchten???Und warum wird die 12V führende K leitung gegen Masse kurzgeschalten. Vielleicht kann mir ja jemand persönlich unter [email protected] weiterhelfen wenn dieses thema zu Umfangreich fürs Forum wäre. Vielen dank im Voraus. SchBest regards


Oh dear... engineers and reading...

You need to ground both pin 3 AND K. It doesn't say you should connect pin 3 to K...

'The 'K' pin and pin 3 have absolutely no direct connection to each other in the circuit; even a grandmother could see that. And ground lines, when connected to ground, are NOT electrically or logically connected to each other!'

'And if you connect pin 3 AND K to ground, LED3 will light up. This is clearly evident from the circuit diagram.'

Here's the translation:

'small tip: familiarize yourself with the function of the K-line. then you'll understand that you can use the vehicle's K-line for whatever you want without damaging anything '

CU Gremlin.

mickdoohan wrote:

How are you measuring those 13 volts? If K is connected to ground? What potential are you measuring the K wire against? I don't quite understand this

.
The measurement is taken with respect to ground, as is common in typical DIY electronics.
I have updated the adapter article accordingly to hopefully eliminate this potential source of misunderstanding in the future.

Hello Ulf,

but if he now connects K with mass, how does he measure the 13V against ground?


Best regards, Rainer.

Rainer K. wrote:

but if he now connects K with mass, how does he measure the 13V against ground?

That will be covered in the next lesson - maybe next year...