MAX232 Adapter: Survey & Diagnostics

Hello,
Since I've already shipped out several kits for my diagnostic adapter, and I'm sure many more have been assembled, I'd like to ask how people's experiences have been with it. On which vehicles did it work without problems, where were there issues, and what were they? Perhaps we can derive some improvements from that. I have a Golf 3 TDI, manufactured in June 1994, with engine code 1Z. On this car, I can read the engine control unit, the ABS control unit, and the airbag control unit without any problems. My car doesn't have any other control units .

Then, go ahead and type

Regards,
Maik.

hi maik

Passat Variant, year of manufacture 1994, 1Z. It's running quite well. However, lately, and rarely, there are 'too many communication errors'. But it's the same problem with the engine control unit (ECU), with the glowing plug indicator blinking and the engine briefly revving high when using the original ECU (028 906 021 B).
see also: https://community.dieselschrauber.org/en/viewtopic.php?t=1907
It is very likely, however, that this is not an adapter problem!

Absolutely no problems with a different engine control unit (028 906 021 FB)!

It has been the only adapter that has worked with my vehicle so far!


Hi Jörg.

Hello.

Maik's adapter is certainly easier to use and more reliable in communication than adapters with optocouplers that are not properly configured.

But surely the developers of the Opto-adapters must have had some reason for doing so??

I've also devised a design for an adapter on paper that doesn't use an optocoupler – in its simplest form, using 3 transistors, including an L-line driver, and components that you might even still find in a poorly stocked electronics store.

It becomes problematic, in my opinion, but possibly , when such adapters are not the only connection between the car and the PC, especially when, for example, a laptop is powered from the car's electrical system.

For example, if the ground connection in the cigarette lighter socket is faulty, significant voltage differences can arise between the laptop and the car, which the adapter then tries to compensate for. This is aside from the fact that the signal voltages will be shifted, which can lead to communication errors, something that adapters with optocouplers completely ignore.

Maik's adapter still includes a current-limiting resistor (R1 = 100 Ohms) that limits the current flowing through pin 5 of the COM port to approximately 120 mA in extreme cases.

If an adapter provides a near-zero Ohm connection between pin 5 of the COM port and pin 4 of the OBD connector, then the operating current of a laptop powered from the vehicle's electrical system may flow through the COM port if there is a poor ground connection in the power cable , and I suspect that most laptops, with their delicate circuit traces, are not designed to handle 3 Amps or more.
Then the COM port is probably usually broken }.

I believe that considerations of this nature or similar ones are the basis for the development of galvanically isolated adapters.

However, if someone only powers their laptop in the car using the battery, I believe they can't really go wrong by using adapters without galvanic isolation.

I'll also be assembling and testing my simple adapter soon, and of course, I'll include a surge protector for emergencies.

If it works, is there any interest in another plan for an adapter "from the spare parts bin"?

Hello Maik,

I painstakingly soldered the kit together, but the double-sided PCB is quite small (but beautiful), and you have to solder from the component side. After borrowing a tiny 12V soldering iron, it went better... then I soldered the LEDs in the wrong way, and I probably fried the output transistors when testing with +12V (applying +12V to the K and L lines). I set the potentiometers to the middle position and then tested it in the car: I immediately gained access to the engine control unit, navigation system (MCD), central locking, and climate control. I didn't try the others. Then I took a test drive and logged data. I was able to log 2 groups with a total of 8 data points. With 3 groups and 12 data points, I got synchronization errors, and the connection suddenly disappeared. It was also difficult to work with a second application while data was being transmitted (Win98). I don't think the problem is with the adapter itself.
The car is a Golf IV with a 90 horsepower ALH engine, manufactured in 07/00, with 98,500 km on the odometer. The entire car is in good condition and has no defects (at least, none that I can attribute to the car).
However, it seems that the air mass limit at maximum power is not quite being reached.

'I ran the whole thing using an IBM Thinkpad 600E, along with the original 220V power adapter and a 12/220V/150W voltage converter in the car. The power adapter is 100% galvanically isolated (between input and output); only the ground wire is galvanically connected to the output circuit, but it's not connected, so I don't have to worry about significant leakage currents.'
I'll leave the AC component to the 100-ohm resistor; it won't be much.

'I think Maik's adapter is well-designed, with a simple circuit and reliable operation thanks to the MAX232. However, building it yourself isn't quite as easy; it's better to buy a pre-built module from him.'

Best regards, Gerd.

Hello Maik,

'I've already responded before, but hopefully more interested people will see this: The adapter worked perfectly right away. I had a short circuit on the PCB (thanks to EAGLE), and the assembly was tricky due to the ground planes. During a test drive with a Ford Galaxy (AFN), I logged data from two groups and located a faulty mass airflow sensor.'

I have a noname laptop with a 500 MHz processor running Windows 98, and it's currently running on battery power.

In my 1997 Golf III with a 90 HP engine, not even the warning light for the fuel supply is illuminating. I would like to read out the airbag control unit because the warning light for the airbag system is constantly on.

Question - this probably has nothing to do with your adapter: Is this normal - do I need to supply 12V externally?

Best regards,

Helmut.

Hello Helmut,
The red LED should light up as soon as the OBD2 connector is plugged into the car. It's possible that the fuse for the OBD2 power supply has blown. As far as I know, it's the same fuse as the one for the cigarette lighter.

Regards,
Maik.

Hello,

'During testing, I noticed that the voltage at pins 6 and 7 of the MAX232 IC is not being reached. The expected values are -10V DC at pin 2, and -10V at pin 7 for a logical '1', and +10V for a logical '0'. However, I was only able to measure approximately 0.6V for the negative level.'

It was due to the 4 capacitors C5, C10, C12, and C13 (10000nF).
sollten sein lt. Data sheet:
MAX232 = 1000nF
MAX232A = 100nF
ST232 (compatible with MAX232A) = 100nF.

After I replaced the existing capacitors, the voltages were correct.
However, the adapter also worked with the oversized connectors. According to the V24 specification, the voltage levels for logical 0 should be +3 to +12V, and for logical 1, they should be -3 to -12V.
By the way, these are voltage levels, and therefore, directly controlling current-consuming optocouplers in that location is also a violation .

Hello HGK,
Ash on my head. I probably didn't read the datasheet correctly. Thank you for the correction.

Regards,
Maik.

Hello Maik,

Congratulations on your adapter - it's working perfectly!
Since my circuit board doesn't have through-hole connections, I had to solder on both sides. It might look terrible, but it works.
I used the ST232 instead of the MAX232, along with the C code you suggested. I set the two trimmer resistors to the middle position and then tried to connect it. The connector I ordered from OBD-2.de (which only took 2 days) was not suitable for my vehicle. On the Passat 3B, the connector can only be plugged into the OBD-2 socket with considerable force. The connector housing is much too thick and therefore only fits very tightly through the plastic frame above the socket. I then simply removed the pins from the connector housing and plugged them directly into the socket. Also, this connector does not have any strain relief.

Hardware: Laptop with a 500 MHz processor and Windows XP, Volkswagen Passat 3B TDI with an 85 kW PD engine.
Software: vag-com 2.082

Best regards,

fred.labosch

Hello,
I have received more circuit boards. If you are interested, please send me an email.

Regards,
Maik.

Hello Maik!

I can only agree -- Your adapter works with VAG-COM for me.

So far, absolutely no problems!

No corrections or adjustments were needed, it was truly "plug and play"...

Passat, 1Z, first registration 08/95. Notebook: 120MHz, 48MB RAM, battery operation.

Regards,

JRoth.

'Yes, I can only agree. It works without any problems in my Golf 4 ALH, manufactured in 1998.'

Hi Maik,
The adapter worked with the OBD interface on the first try.
The connection was established immediately after being connected to my Golf 4 (manufactured in 2001).
I just read the error memory of all available control units for testing purposes (everything was okay).

For the Potis, I left the middle setting as is, because you unfortunately don't provide an optimal setting.

The circuit board was difficult to solder due to the very small solder pads.
The most problematic aspect is the solder pads located under the SUB-D interfaces. It's impossible to solder on the component side because the connector body covers the solder pads. However, I found through-hole pins (0.8mm outer diameter) that were perfectly suitable.

If you could provide some information on the optimal settings for the potentiometers, I would be completely satisfied (e.g., measurement values?).

Harry.

Hi Harry,
You can leave the potentiometers in the middle position if the communication is working correctly. If you are experiencing communication problems, you can try rotating the potentiometers to improve the connection quality. I will be releasing a new version without potentiometers soon.

Regards,
Maik.

Hello Maik,

I've also built this adapter (several times) based on your design, and so far I haven't had any problems. It's a really useful tool. I've tested it on: Golf 4, Golf 3, and Audi A4s.

I've created a single-sided layout for your circuit diagram, which is approximately 4x6cm in size (the SubD connectors are not mounted directly on the board, but are connected with cables, as there are only 4 cables per side). It fits perfectly into a Conrad enclosure with dimensions of 100x50x25.
If you're interested in offering the layout of the adapter instructions for download, you are welcome to do so.

Where do you get your double-sided PCBs made? Isn't it incredibly expensive?

greetings
andi

Hi Andi,
I'm glad everything is working so well.

The circuit boards are self-etched and not through-hole plated. You have to solder on both sides.

Regards,
Maik.