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ELM327DSH Elm Electronics – Circuits for the Hobbyist
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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 connection 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 no activity. 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
and from that point forward, every wakeup message
that the ELM327 sends will be as shown. You should
not provide a checksum byte, as it will be automatically
added for you.
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).
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 generally not until a request needs to be sent (you
can force one with the FI and SI commands). If the bus
initiation occurs during an automatic search, 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
the dots. 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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