ELM327P datasheet(6/51 Pages) ELM

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ELM327

ELM327DSC

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Communicating with the ELM327 (continued)

[ 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 its internally stored

receive

address

matches

the

address

that

the

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.

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.

AT Commands

message sent to the vehicle was in error.

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

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).

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