ELM327P datasheet(11/76 Pages) ELM

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ELM327

ELM327DSH

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AT Command Descriptions

[ Allow Long messages ]

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).

[ 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 ]

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

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.

[ 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 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.

[ Bypass the Initialization sequence ]

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.

The following describes each AT Command that the

current version of the ELM327 supports:

ELM327P Datasheet(PDF) 11 Page - ELM Electronics

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