ELM327
AT Command Descriptions (continued)
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.
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
first 5 bytes (ie 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.
AT0, AT1 and AT2
[ Adaptive Timing control ]
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.
BI
[ Bypass the Initialization sequence ]
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.
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.
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 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.
BRD hh
[ try Baud Rate Divisor hh ]
This command is used to change the RS232 baud
rate divisor to the hex value provided by hh, while
under computer control. It is not intended for casual
experimenting - if you wish to change the baud rate
from a terminal program, you should use PP 0C.
Since some interface circuits are not able to
operate at high data rates, the BRD command uses a
sequence of sends and receives to test the interface,
with any failure resulting in a fallback to the previous
baud rate. This allows several baud rates to be tested
and a reliable one chosen for the communications.
The entire process is described in detail in the ‘Using
Higher RS232 Baud Rates’ section, on pages 63 and
64.
BD
[ perform an OBD Buffer Dump ]
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
If successful, the actual baud rate (in kbps) will be
4000 divided by the divisor (hh). The value 00 is not
accepted by the BRD command.
BRT hh
[ set Baud Rate Timeout to hh ]
This command allows the timeout used for the
Baud Rate handshake (ie. AT BRD) to be varied. The
ELM327DSI
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