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Elm 327 Manual

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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 (but
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 the ‘ON’ position 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 as 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, as the default
settings work well with most systems.
33 of 94
ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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