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Elm 327 Manual

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63 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
Programming Serial Numbers
Saving a Data Byte
A number of our customers have asked for ways
to uniquely identify a product that uses our ELM327
integrated circuit. While this is often a request for a
means to store a ‘serial number’, people have also
asked for a way to store dates, and version codes, too.
The @2 and @3 commands were created to assist
with this.
If you send the command AT @2 to a new
ELM327 integrated circuit, you will receive an error.
That is, you will see a response that looks like this:
>AT @2
?
In the above dialog, the ELM327 is trying to tell
you that either the chip is very old and does not
support the @2 command, or that nothing has been
programmed into these memory locations yet.
To program characters into the @2 memory, you
must provide exactly 12 characters using the AT @3
command. These characters must be in the ASCII
printable group, in the range from ‘!’ (hex value 21) to
‘_’ (hex value 5F). Typically, an AT @3 command use
would look like:
>AT @3 MYBOARD_9906
OK
This number can never be altered once it is
entered, so you must be sure that you are entering the
values properly. If developing code which does this,
you may find that purchasing an ELM328 IC will save
the expense of trial and error. The ELM328 does not
support OBD protocols however, so is not a viable
option for other uses.
Once the @3 code is set, it will always be
available through the @2 command:
>AT @2
MYBOARD_9906
That’s all there is to using the ELM327 device
identifier.
The ELM327 provides one memory location that
can be used to save any single byte of information.
This location uses special ‘non-volatile’ EEPROM
memory for storage, so your data is not lost, even if
you should turn the power off.
Typically, this memory location is used by the
controlling software to store the state of flags that were
set by vehicle conditions, by hardware configurations,
or by software choices. By storing them in this type of
memory, the settings will be remembered between
uses of the scan tool.
Storing data is easily done with the Save Data
command - for example, to save the value 7F, simply
send:
>AT SD 7F
OK
and data is just as easily retrieved using the Read
Data command:
>AT RD
7F
Since this single byte of data is stored in the
internal EEPROM array, it is subject to the usual limits
of EEPROM technology - unlimited reads, but typically
only about 1 million writes, with a retention time of 40
years (or more). This should not pose any limits to
ELM327 users that we are aware of.
… 63 …


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