ELM327
Communicating with the ELM327 (continued)
message sent to the vehicle was in error.
case-sensitive, so ‘ATZ’ is equivalent to ‘atz’, and to
‘AtZ’. Also, it ignores space characters and all control
characters (tab, linefeed, etc.) in the input, so they can
be inserted anywhere to improve readability. Another
feature is that sending only a single carriage return
character will always repeat the last command (making
it easier to request updates on dynamic data such as
engine rpm).
While processing OBD commands, the ELM327
will continually monitor for an RTS input, or an RS232
input. Either one will interrupt the IC, quickly returning
control to the user, and possibly aborting any initiation,
etc. that was in progress. If you desire to interrupt the
ELM327, that’s fine, but for normal orderly data
transfer, users should always wait for either the prompt
character (‘>’ or hex 3E), or a low level on the Busy
output before beginning to send the next command.
Finally, it should be noted that the ELM327 is not
AT Commands
Several parameters within the ELM327 can be
adjusted in order to modify its behaviour. These do not
normally have to be changed before attempting to talk
to the vehicle, but occasionally the user may wish to
customize these settings – for example by turning the
character echo off, adjusting a timeout value, or
changing the header bytes. In order to do this, internal
‘AT’ commands must be issued.
Those familiar with PC modems will immediately
recognize AT commands as a standard way in which
modems are internally configured. The ELM327 uses
essentially the same method, always watching the
data sent by the PC, looking for messages that begin
with the character ‘A’ followed by the character ‘T’. If
found, the next characters will be interpreted as
internal configuration or ‘AT’ commands, and will be
executed upon receipt of a terminating carriage return
character. The ELM327 will usually reply with the
characters ‘OK’ on the successful completion of a
command, so the user knows that it has been
executed.
Some of the following commands allow passing
numbers as arguments in order to set the internal
values. These will always be hexadecimal numbers
which must generally be provided in pairs. The
hexadecimal conversion chart in the OBD Commands
section may prove useful if you wish to interpret the
values. Also, one should be aware that for the on/off
types of commands, the second character is the
number 1 or 0, the universal terms for on and off.
The following is a description of all of the AT
commands that are recognized by the current version
of the ELM327. Since there are many, a summary
page is provided after this section.
AL
[ Allow Long messages ]
message is being sent to. With the auto receive mode
in effect, the value used for the receive address will be
chosen based on the current header bytes, and will
automatically be updated whenever the header bytes
are changed.
The value that is used for the receive address is
determined based on the contents of the first header
byte. If it shows that the message uses physical
addressing, the third header byte of the header is used
for the receive address, otherwise (for functional
addressing) the second header byte, increased in
value by 1, will be used. Auto Receive is turned on by
default, and is not used by the J1939 formatting.
The standard OBDII protocols restrict the number
of data bytes in a message to seven, which the
ELM327 normally does as well (for both send and
receive). If AL is selected, the ELM327 will allow long
sends (eight data bytes) and long receives (unlimited
in number). The default is AL off (and NL selected).
AR
[ Automatically set the Receive address ]
Responses from the vehicle will be acknowledged
and displayed by the ELM327, if its internally stored
receive address matches the address that the
ELM327DSC
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