ELM327
Programmable Parameters
The ELM327 contains several memory locations
that retain their data even after power is turned off.
Every time the IC is reset, these locations are read
and used to change the default settings for such things
as whether to display the headers, or how often to
send ‘wakeup’ messages. Because they define the
operation of the ELM327, we refer to these memory
locations as the Programmable Parameters.
All of the Programmable Parameters are easily
altered at any time using a few simple commands.
These commands are standard AT Commands, with
one exception: each one requires a two-step process
to complete. This extra step provides some security
against random inputs that might otherwise cause
changes.
As an example of how to change a Programmable
Parameter, consider PP 01 (shown on page 58) which
sets the default state for the AT H command. If you are
constantly powering your ELM327 and then using
AT H1 to turn the headers on, you may want to change
the default setting, so that the headers are always on
by default. To do this, simply set the value of PP 01 to
00:
enabled (as discussed above), the summary table
would look like this:
>AT PPS
00:FF F 01:00 N 02:FF F 03:32 F
04:01 F 05:FF F 06:F1 F 07:09 F
08:FF F 09:00 F 0A:0A F 0B:FF F
0C:68 F 0D:0D F 0E:9A F 0F:D5 F
10:0D F 11:00 F 12:FF F 13:32 F
14:50 F 15:0A F 16:FF F 17:6D F
18:00 F 19:62 F 1A:FF F 1B:FF F
1C:03 F 1D:0F F 1E:FF F 1F:FF F
20:FF F 21:FF F 22:FF F 23:FF F
24:00 F 25:00 F 26:00 F 27:FF F
28:FF F 29:FF F 2A:38 F 2B:02 F
2C:E0 F 2D:04 F 2E:80 F 2F:0A F
You can see that PP 01 now shows a value of 00,
and it is enabled (oN), while the others are all off.
Another example shows how you might change
the CAN filler byte. Some systems use ‘AA’ as the
value to send for unused CAN data bytes, while the
ELM327 uses ‘00’ by default. To change the ELM327
in order to have it send AAs instead, simply change
PP 26:
>AT PP 01 SV 00
OK
>AT PP 26 SV AA
OK
This changes the value associated with PP 01, but
does not yet enable it. To make the change effective,
you must also type:
>AT PP 26 ON
OK
>AT PP 01 ON
OK
Again, PP 26 is of type ‘D’, so the above change
will not actually take effect until the defaults are
restored, whether it be by an AT D command, or by
resetting the ELM327.
At this point, you have changed the default setting
for AT H1/H0, but you have not changed the current
AT H1/H0 setting. From the ‘Type’ column in the table
on page 58, you can see that PP 01 is a type ‘D’
parameter, so the change only becomes effective the
next time that defaults are restored. This could be from
a reset, a power off/on, or possibly an AT D command.
If you send the command AT D, then you should find
that the headers are now displayed by default.
As there are many Programmable Parameters that
you can alter, it might occasionally be difficult to know
what changes you have made to them. To help with
that, the ELM327 provides a Programmable Parameter
Summary (PPS) command. This command simply
prints a list of the entire range of PPs (whether
currently supported or not), their current value, and
whether they are on/enabled (N), or off/disabled (F).
For a version 2.0 ELM327, with only the headers
The Programmable Parameters are a great way to
customize your ELM327 for your own use, but you
should do so with caution if using commercial
software. Most software expects an ELM327 to
respond in certain ways to commands, and may be
confused if the carriage return character has been
redefined, or if the CAN response shows a data length
code digit, for example. If you make changes, it might
be best to make small changes and then see the effect
of each, so that it is easier to retrace your steps and
‘undo’ what you have done, should you have to. If you
get in too deeply, don’t forget the ‘all off’ command:
>AT PP FF OFF
No matter what software you use, you might get
into more serious trouble, should you change the baud
ELM327DSI
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