Re: [opendiag] Re: Anyone use the L9637 ISO 9141 Interface chip from ST?

On Wed, Jul 26, 2000 at 06:15:04AM -0000, blg@... wrote:
> I guess I'm a little confused on this... The car i'm working on is a
> 2000 Audi S4. The ODBII connector only has a single ISO 9141 K line
> for communication, so I assumed I need to use that line to talk VAG
> protocol. I'm I confusing terms here?

ISO9141-2 specifies that the K line communicate with the tester at
10.4kbps. You can't obtain 10.4kbps on a normal RS-232 port, so it is
necessary to have an interface chip that can do some buffering for you.

In the proprietary VAG mode, you can communicate at 9600bps on the
K line. In this mode, your hardware only needs to be concerned with
level shifting between RS-232 (-3V and +3V thresholds) and automotive
voltages (somewhere around 1V and 8V thresholds.) Everything else can
be done in software.

The VAG mode is not strictly OBDII, but as far as I know, it's the
only mode in which you can access proprietary data blocks. (I could
be wrong.)

> There is an additional set of pins on the OBD connector that may be
> what you are talking about. Pins 1 and 13 have pins in them, but I
> could never find a pinout on the web that shows these as other than
> "discretionary". Are these pins for the VAG interface to which you
> are referring?

My memory is fuzzy. Are you sure that 1 and 13 aren't power or ground?
My 98 Passat's connector has two grounds and one power, and a K line.
My GTI also has an L line. I can look it up later if you like.

The same K wire is used for either protocol. My guess is that only the
5bps initalization sequence differs. I have not analyzed the VAG protocol
yet, I'm just using VWtool. Someone else has reported that Ross-Tech's
VAG-COM software also works (which for the time being, is free.)

> This stuff is new to me, so I appreciate your comments.

I don't claim to be an expert, but the schematic on my web page works
and is probably a good starting point for you. It's mostly a clone of
the VWtool interface. In your case, you can use a simpler interface:
since your car doesn't connect the L line to anything, you can leave off
an optoisolator and a few other parts. I'd guess that you could build
the thing for a minimum of $5-$10.

I'm unclear on what your long-term goal is. Did you want something you
would leave running in your car for data logging?

I hope I've been helpful.

Regards,

Jeff

 
Received on Wed Jul 26 07:46:56 2000