Building ELM323 interface

**bes51659** · 07-27-2008, 05:55 AM

I want to include OBDII interface on my pcb I will put all kind a "glue" functionality for my BMW E46. It supports ISO-9141-2. I will use the ELM 323 chip. Doing this I can get access OBD with a single wire rather than all sorts of connectors.

Purpose is to pickup vehicle speed and direction to my integrated GPS module, and also to give access to interface from PC where I probably will use dashcommand.

I found this link in another thread. From that PID list it seems like I cannot get direction :-(
Q1) Is this correct?

Assuming I've got dashcommand or alike running on the PC I expect to be able to evesdrop on communication and fish the speed. But then I need to know protocol format.

Q2) Anyone's got the protocol of ISO-9141-2? Or just the format of the speed message maybe?

Any opinions are welcome

**malcom2073** · 07-27-2008, 08:58 AM

Q1: Correct

Q2: With an ELM323 if you send 010D it will likely return speed, in the format "41 0D 0F"

where 41 is the mode (01), 0D is the request (speed), and 0F is your speed (in km/h).

For other requests, you need to use the calculations listed on the wiki page you linked to figure out what it means.

**bes51659** · 07-29-2008, 06:40 AM

Thanks malcom

So I have to send my own request, I cannot just evesdrop... Will this interfere with requests from the PC? Will I have to build a gateway to make it work?

**malcom2073** · 07-29-2008, 05:33 PM

I know on CAN cars you can evesdrop, but regular OBD2 you cannot, as you are usually the only other device on the bus other than the ECU, nothing to listen to.

On a side note, you cannot "listen" over the ELM device while sending requests if you happen to have a CAN bus to listen to.