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ELM327

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Wakeup Messages

After an ISO 9141 or ISO 14230 connection has

been established, there needs to be periodic data

transfers in order to maintain that connection, and

prevent it from ‘going to sleep.’ If normal requests and

responses are being sent, that is usually sufficient, but

the ELM327 occasionally has to create its own

messages to prevent the connecton from timing out.

We term these periodic messages that are created

the ‘Wakeup Messages’, as they keep the connection

alive, and prevent the circuitry from going back to an

idle or sleep mode. (Some texts refer to these

messages simply as ‘idle messages.’) The ELM327

automatically creates and sends these for you if there

appears to be no other activity – there is nothing that

you need do to ensure that they occur. To see these,

once a connection is made, simply monitor the OBD

transmit LED – you will see the periodic ‘blips’ created

when the ELM327 sends one. If you are curious as to

the actual contents of the messages, you can then

perform a Buffer Dump to see the bytes. Note that the

ELM327 never obtains or prints a response to any of

the wakeup messages.

The standards state that if there is no activity at

least every five seconds, the data connection may

close. To ensure that this does not happen, and to

provide some margin, the ELM327 will send a wakeup

message after three seconds of inactivity. This time

interval is fully programmable, should you prefer a

different setting (see the AT SW command).

As with the ELM323, the ELM327 allows users to

change the actual wakeup message that is sent. To do

so, simply send the ELM327 a Wakeup Message

command, telling it what you wish the message to be

changed to. For example, if you would like to send the

data bytes 44 55 with the header bytes set to 11 22

33, simply send the command:

>AT WM 11 22 33 44 55

From that point forward, every wakeup message

that the ELM327 sends will be as shown above.

You can change these as often as you want, the

only restriction being that every time you do, you must

provide the complete message – the header bytes and

the data bytes (the current version of the ELM327 only

allows for messages of one to six bytes in length). You

should not provide a checksum byte, as it will be

automatically added for you.

Bus Initiation

Both the ISO 9141-2 and ISO 14230-4 (KWP2000)

standards require that the vehicle’s OBD bus be

initialized before any communications can take place.

The ISO 9141 standard allows for only a slow (2 to 3

second) initiation process, while ISO 14230 allows for

both a slow method, and a faster alternative.

The ELM327 will perform this bus initiation for you,

but not until your request needs to be sent. If the bus

initiation occurs during the automatic search process,

you will not see any status reporting, but if you have

the Auto option off (and are set to protocols 3, 4, or 5),

then you will see a message similar to this:

BUS INIT: ...

The three dots appear only as the slow initiation

process is carried out – a fast initiation does not show

them. This will be followed by either the expression

‘OK’ to say it was successful, or else an error message

to indicate that there was a problem. (The most

common error encountered is in forgetting to turn the

vehicle’s key to ‘ON’ before attempting to talk to the

vehicle.)

Once the bus has been initiated, communications

must take place regularly (typically at least once every

five seconds), or the bus will revert to a low-power

‘sleep’ mode. If you are not sending data requests

often enough, the ELM327 will generate requests for

you to ensure that the bus stays ‘awake’. You will

never see the responses to these, but you may see

the transmit LED flash periodically when these are

being sent.

By default, the ELM327 ensures that these

‘wakeup’ or ‘idle’ messages are sent every 3 seconds,

but this is adjustable with the AT SW command. The

contents of the wakeup message are also user

programmable with the AT WM command, if you

should wish to change them. Users generally do not

have to change either of the above though, as the

default settings work well with almost all systems.

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