Is listening at the diagnostic port useful?

Hello,

I'm new here and I hope that my specific questions can be answered.

When I listen on the diagnostic port of my car (Audi) without anything previously requesting data, such as VCDS, I shouldn't "hear" anything because the diagnostic port doesn't simply "broadcast" data unless something has been requested. Is this statement generally correct when referring to the CAN bus at the OBD connector?

If the above assumption is correct, then it would make much more sense to read the data from a central location, such as the control unit J533 in the Audi, where I could passively read information like "right turn signal." Of course, I understand that the data isn't being transmitted in plain text over the bus, and that I'll have to figure out most of the functions through trial and error, but it should be possible to do that.

The OBD port is already very conveniently accessible. Does it make sense, and is it even possible, for the underlying control unit to mirror the desired (or even all) data to the port of the OBD connector, so that I can read it from there?

I have VCDS and some other devices that I can freely program. Unfortunately, with VCDS, you can't just passively read data; otherwise, I might have already tried to access the J533. The can tool hardware seems to be capable of passive data reading, but it's probably not offered as a ready-to-use device at the moment.

I would appreciate it if someone could answer my beginner questions.

Regards,
Rolf.

rowi wrote:

If I listen on the diagnostic port of my car (Audi) without, for example, VCDS having previously requested anything, I shouldn't "hear" anything, because the diagnostic port doesn't simply "broadcast" data if nothing has been requested. Is this statement generally correct when I'm referring to the CAN bus at the OBD connector?

Yes, at VAG.
Apparently, other manufacturers also had cars where the CAN bus signal simply reached the diagnostic connector. According to the OBDII standard, additional, non-diagnosis-relevant messages are explicitly permitted.

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Unless the above-mentioned. Assuming the assumption is correct, then it would make much more sense to read data from a central location, such as the control unit J533 in the Audi,

Yes.

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where I could actually passively read information like "turn signal right"?

You will then need to decide which CAN bus you want to listen to. The gateway doesn't translate all messages.

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I have VCDS and some other devices that I can freely program. Unfortunately, it seems that with VCDS, you can't simply read data;

Okay.

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Otherwise, I might have already dared to try the J533. The hardware of the can tool is apparently capable of passively reading data, but it is probably not currently offered as a complete product.

Simply "passively" reading data is not always necessary. If your hardware always sends an acknowledgment for received messages, it usually doesn't cause any problems. However, it might interfere if you're actively looking for an error that a control unit is generating because it's not receiving the acknowledgment.

Thank you for the quick response! That really helps me build understanding.

Okay, I now have my gateway (J533) to which all the buses are connected. The OBD connector is also connected to the gateway. I can reach the gateway at address 19. Now, if I want to see the status of the parking brake (address 53) from another control unit, for example, I would need to be able to tell the gateway at address 19 to forward the information from address 53, if necessary, so that I can access it on the bus without having to repeatedly query it. Is this procedure correct and common practice?

Ultimately, it seems I can access all control units via the OBD socket, as VCDS at least displays information ranging from the reversing camera to the adaptive cruise control. Therefore, I assume that these components can be actively addressed, allowing them to respond. However, if the VCDS signal is missing, the bus will likely not be populated with any further information.

It would be interesting to find a way to tell the gateway (19) that you would like to receive the parking brake information (53) permanently. Or, should I simply query the parking brake directly through the gateway? I understand that something like this might be possible, but not for everything. What exactly can be forwarded then is likely vehicle-specific. Is that correct?

Regards,
Rolf.

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I now have my gateway (J533) to which all the buses are connected. There, the OBD socket is also connected.

The wiring to the OBD socket then represents a separate CAN bus.

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I can reach the gateway at address 19.

Address 19 is the address of the diagnostic gateway. Strictly speaking, it's not even OBD, as it's manufacturer-specific. VCDS can access it because it's specifically designed for VAG vehicles, while a generic OBD tester would not be able to.

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If I want to see, for example, the status of the parking brake (address 53) from another control unit, I would probably need to be able to tell the gateway at address 19 that it should please forward the information from address 53,

No. You directly access address 53 at the OBD connector; the gateway knows which bus the parking brake is connected to and forwards the information.

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if necessary, so that I can have it readily available on the bus, without having to repeatedly ask for it. Is this procedure correct and common?

I can't say for sure whether it's possible to read a measurement value without polling; I haven't yet delved deeply enough into OBD (or manufacturer-specific diagnostics).

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Ultimately, it seems I can access all control units via the OBD socket, as VCDS at least displays information all the way down to the reversing camera or the distance control system. Therefore, I assume that these components can at least be actively addressed, so that they respond. However, if the VCDS impulse is missing, the bus will likely not be further populated with information.

Essentially correct, but for anything that goes beyond OBDII (such as, for example, your parking brake), the protocol is not publicly disclosed.
Furthermore, some control units (I know for sure this happens with some transmissions) disable diagnostic communication above a certain vehicle speed.

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It would be interesting to have a way to tell the gateway (19) that you would like to receive the parking brake information (53) permanently.

Rather, it's about the parking brake itself, as mentioned above.
I can't say whether it's fundamentally possible, but even if it were, it would still be subject to the limitations mentioned above.

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Or should I only ask about it via the gateway when the parking brake is engaged?

Yes, that's generally how diagnosis works.

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I understood that something like that might be possible, but not for everything. What exactly can be forwarded then is probably vehicle-specific. Is that correct?

What the gateway forwards from which CAN bus to which CAN bus depends on...
a) Vehicle
b) Gateway hardware/software status.
c) Encoding in the gateway.
d) If applicable, identify any components installed within the gateway that are running.
Sure, here is the translation of the text from German to English:

"ab."

This is a German word that means "off." It can be used as a verb or an adjective. As a verb, it means "to turn off" or "to go off." As an adjective, it means "off" or "absent."

Here are some examples of how the word "ab" can be used:

* "Ich schalte das Licht ab." (I turn off the light.)
* "Der Bus fährt ab." (The bus is leaving.)
* "Er ist ab." (He is off.)

I hope this helps!

The simplest solution for your parking brake will be:
- listen in on the CAN bus, which also connects to the braking system - there will definitely be the relevant bit(s) in one of the messages.
- listen in on a CAN bus where the desired information (forwarded from the gateway) is likely to be found, for example, the CAN-Kombi (due to...). Warning light.
That's not a diagnosis, of course, but simply listening in on the normal data exchange between the control units.

Best regards,
Ingo

Great! Then I probably need a CAN sniffer like the Lawicel canusb and the Canhack software. I'm still trying to figure out whether the Peak Adapter (around 200 euros) offers more than the Lawicel (around 100 euros).

Regards,
Rolf.

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Great! Then I probably need a CAN bus sniffer like the Lawicel canusb and the Canhack software. I'm still trying to figure out whether the Peak Adapter (around 200 euros) offers more than the Lawicel (around 100 euros).

Peak: Professional-grade product, good driver support even for Windows 7 and older versions. We've been using it in the company for many years, and I now have two for personal use as well.

Lawicel: I have no experience with that.

I have developed a custom program for the Peak adapter that can display the traffic on the engine CAN bus (with data version 5.0.4) in plain text. The program is quite unfinished; I currently don't have any matrices for comfort and infotainment features, and I'm not sure when that will change.
If you're interested, I might also be willing to collaborate on that.


Best regards,
Ingo

Hello,

I would like to revisit this topic.

@Diesel drivers
Thank you for the explanations; they help me understand things much better.

I would like to read CAN messages, and to that end, I have developed a platform based on a BeagleBone Black and a custom-made CAN board.

When connected to the diagnostic port of my Audi A3 8P, manufactured in 2008, I can also see CAN messages when I simultaneously initiate a query using VCDS. Here's an example: the door control module for the passenger side.


(1394588118.531163) can0 300#B1
(1394588118.641177) can0 300#B2
(1394588118.662174) can0 300#223935393830324B
(1394588118.671846) can0 300#2320203030363003
(1394588118.692232) can0 300#2518AA547565722D
(1394588118.721711) can0 300#1820
(1394588118.723733) can0 312#B9
(1394588118.851313) can0 300#1A040106
(1394588118.950924) can0 300#B4
(1394588118.962119) can0 300#2B00115A910E3850
(1394588118.981921) can0 300#1D4B202020FF


'Now, I don't want to always have to request the control units to send me data via the diagnostic interface in diagnostic mode, but rather simply 'listen in'.' That's not so easy to do through the diagnostic port.



What the gateway forwards from which CAN bus to which CAN bus depends on
a) Vehicle
b) Gateway hardware/software status.
c) Encoding in the gateway.
d) If applicable, identify any components installed within the gateway that are running.
ab.

Can I use VCDS to configure the gateway so that it mirrors CAN messages from the Comfort CAN directly onto the diagnostic CAN?

I'm hesitant to remove the casing and connect my sniffer directly to the gateway...