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'[TECH] Hobbyest-friendly OBD-II Reader'
2009\06\22@193729 by Forrest Christian

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I have a need for a OBD-II reader this coming weekend and figured I  
would use the need as an excuse to buy a reader I could use to  
experiment with as well, such as playing with building a trip computer  
or fuel economy guage as well.

I know that at least one member on here is in the business of selling  
such devices, so of course I'm interested in those, along with others  
which might be available.  I'm thinking of something with a serial or  
USB interface and also something that has an open interface so it can  
be accessed by my own software.

Since I've never actually played with the obd interface I'm not even  
really sure what to look for, so any ideas would be welcomed.

Thanks!

Forrest

Sent from my iPod

2009\06\23@165421 by Vitaliy

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Forrest Christian wrote:
>I have a need for a OBD-II reader this coming weekend and figured I
> would use the need as an excuse to buy a reader I could use to
> experiment with as well, such as playing with building a trip computer
> or fuel economy guage as well.
>
> I know that at least one member on here is in the business of selling
> such devices, so of course I'm interested in those, along with others
> which might be available.  I'm thinking of something with a serial or
> USB interface and also something that has an open interface so it can
> be accessed by my own software.
>
> Since I've never actually played with the obd interface I'm not even
> really sure what to look for, so any ideas would be welcomed.

The member is probably me, and the company is ScanTool.net
(http://www.scantool.net).

There are many different OBD interfaces out there, but the main difference
is the protocol on the PC side. These are the ones off the top of my head:

http://www.elmelectronics.com
http://obddiagnostics.com/
www.multiplex-engineering.com/interfaces.htm
http://autotap.com/
http://www.obd-2.com/

Everyone else either uses interfaces made by these companies, or makes
"compatibles". For example, Autoenginuity uses interfaces built by
Multiplex.

Our scan tools are based on the ELM327 IC by Elm Electronics. In 2002, we
released open source diagnostic software compatible with the ELM command
set, and today ELM327-based interfaces or ELM327 clones are by far the most
popular PC-based scan tools.

The Autotap protocol looks solid and I've heard good things about it. The
Multiplex protocol is the most awful protocol I've seen.

ELM327 protocol is an adaptation of the AT command set. For example:

ATZ = reset device
ATE1 = echo on
ATI = print ID string

OBD commands are sent without the headers (ELM327 automatically calculates
them). So to get RPM, you would send:

010C

Coolant temperature is 0105, vehicle speed is 010D, and so on. By default,
responses also contain only data (no headers), so you would get back:

// 41 is response to 01, the second byte is the Param ID (PID)
41 0C 01 23    // last two bytes are data, 1/4 RPM per bit
41 05 FF          // last byte is data, 1 degree per bit w/ 40C offset
41 0D FF        // 255 km/hr

If you're going to do any serious development, I would strongly advise you
to get an ECU simulator:

http://www.ecusims.com

Alternatively, you can get an ECU from a junk yard, but varying PIDs would
be problematic.

If you decide that one of our interfaces is a good choice, please drop me an
email and I'll send you a discount code. :)

Vitaliy

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