Since there are a lot of odb tools around, I assume that the odb protocol is published somewhere. I have had a Google, but was on my phone and didn't find much.

Does anyone know if this info is freely available?

ok, so I found that VAG cars (after 1996) use ISO 9141 (or perhaps ISO 14230).

Is this correct?

Dunno if this helps m8 My link

Have a look at the ELM chips, e.g. ELM323, ELM327, a lot of the tools are based on this. It's very easy to work with and it's cheap.

Edited 8 March, 201115 yr by Mute

I've already got a lead which I know works with VCDS Lite. I'm no stranger to writing software with serial port connections so toying with the idea of writing my own tool. I'll open it up and see which chip it uses, as you say, it looks likely to be ELM323 from what I saw earlier today.

What I'm trying to get at is the message format that the software talks to the ELM chip with. I mean, the PC software doesn't know it's not some bog standard RS232 device - the chip does all the interpretation from RS232 <-> ODBII.

I don't think VCDS lite works with ELM chips!

Taking the ELM case, the protocol is clearly documented and *very* easy to speak to. The ELM chip basically presents itself in the same way a modem would.

If you know your ATDT from your ATA you're laughing, it's the same idea.

http://elmelectronics.com/DSheets/ELM327DS.pdf

well, I've taken my cable apart, and no sign of an ELM chip. There's a usb to RS232 UART chip (FT232BL) and a comparator chip (LM393). There's a load of resistors and capacitor, plus some other components labelled F and Q.

So I'm no further to understanding how the serial to odbii takes place. :S

ISO 9141-2 would be the best place to start, but you'd have to pay for it. I have seen a good webpage covering the electrical protocol but I hve no idea where it was. Essentially, if I remember it right, the K-line uses the same signalling as RS232, but with TX and RX tied together on a single bidirectional line and logic levels of 0V and 12V instead of RS232's +15V and -15V. The circuitry in the cable is basically there to buffer (and possibly invert) the 0-12V signals to the 0-5V UART.

With the electrics in place, the communication protocol is straightforward. Here's a nice overview:

http://www.hex.co.za/vaginfo/index.html

and here's a slightly more confusing one from VW

http://www.obdclearinghouse.com/documents/newdocuments//tsb/volkswagen/K-line%20communication%20description_V3%200.pdf

edit: just to clarify, the ELM chips are a full-on microcontroller solution that provide an abstraction away from the protocol in use. This way they can present the same interface for all the common OBD protocols, and support multiple protocols with the same hardware (ELM327). Because of this, they also only implement the basic OBD code and data reading specifications, and don't support proprietary extensions like VAG diagnostics - they won't simply pass raw bytes to the car like a 'dumb' interface does.

Edited 8 March, 201115 yr by RobinSLXi

I think I get how the telegrams are sent and received now. Just the init that I'm not quite clear on; oh well its late.

comments

5 bps address of the Engine Controller 1start,

7data, odd parity, 1stop

this is the sync byte to determine the baudrate

(01010101) 1start, 8data, 1stop

Key word LSB 1start, 7data, odd parity, 1stop

Key word MSB 1start, 7data, odd parity, 1stop

Complement of Key word MSB

So you send a 1 at 5baud with the above framing info? Might make more sense tomorrow!