ELM327
AT Command Descriptions
The following describes each AT Command that the
current version of the ELM327 supports:
AL
[ Allow Long messages ]
is always as set by AT ST), while AT2 is a more
aggressive version of AT1 (the effect is more
noticeable for very slow connections – you may not
see much difference with faster OBD systems). The
J1939 protocol does not support Adaptive Timing – it
uses fixed timeouts as set in the standard.
BD
AR
[ Automatically set the Receive address ]
Responses from the vehicle will be acknowledged
and displayed by the ELM327, if the internally stored
receive address matches the address that the
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 such things as the contents of
the first header byte, and whether the message uses
physical addressing, functional addressing, or if the
user has set a value with the SR or RA commands.
Auto Receive is turned on by default, and is not
used by the J1939 protocol.
AT0, AT1
and
AT2
[ Adaptive Timing control ]
[ perform an OBD Buffer Dump ]
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).
All messages sent and received by the ELM327
are stored temporarily in a set of twelve memory
storage locations called the OBD Buffer. Occasionally,
it may be of use to view the contents of this buffer,
perhaps to see why an initiation failed, to see the
header bytes in the last message, or just to learn more
of the structure of OBD messages. You can ask at any
time for the contents of this buffer to be ‘dumped’
(ie printed) – when you do, the ELM327 sends a length
byte (representing the length of the message in the
buffer) followed by the contents of all twelve OBD
buffer locations. For example, here’s one ‘dump’:
>AT BD
05 C1 33 F1 3E 23 C4 00 00 10 F8 00 00
The 05 is the length byte - it tells us that only the
following 5 bytes (C1 33 F1 3E and 23) are valid. The
remaining bytes are likely left over from a previous
operation.
The length byte always represents the actual
number of bytes received, whether they fit into the
OBD buffer or not. This may be useful when viewing
long data streams (with AT AL), as it represents the
actual number of bytes received, mod 256. Note that
only the first twelve bytes received are stored in the
buffer.
BI
[ Bypass the Initialization sequence ]
When receiving responses from a vehicle, the
ELM327 has traditionally waited the time set by the
AT ST hh setting for a response. To ensure that the IC
would work with a wide variety of vehicles, the default
value was set to a conservative (slow) value. Although
it was adjustable, many people did not have the
equipment or experience to determine a better value.
The Adaptive Timing feature automatically sets the
timeout value for you, to a value that is based on the
actual response times that your vehicle is responding
in. As conditions such as bus loading, etc. change, the
algorithm learns from them, and makes appropriate
adjustments. Note that it always uses your AT ST hh
setting as the maximum setting, and will never choose
one which is longer.
There are three adaptive timing settings that are
available for use. By default, Adaptive Timing option 1
(AT1) is enabled, and is the recommended setting.
AT0 is used to disable Adaptive timing (so the timeout
ELM327DSH
This command should be used with caution. It
allows an OBD protocol to be made active without
requiring any sort of initiation or handshaking to occur.
The initiation process is normally used to validate the
protocol, and without it, results may be difficult to
predict. It should not be used for routine OBD use, and
has only been provided to allow the construction of
ECU simulators and training demonstrators.
Elm Electronics – Circuits for the Hobbyist
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