ELM327
Bus Initiation
Both the ISO 9141-2 and ISO 14230-4 (KWP2000)
vehicle’s key to ‘ON’ before attempting to talk to the
vehicle.)
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:
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, as the default
settings work well with most systems.
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
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 that
these are being sent, simply watch the OBD transmit
LED – you will see the periodic ‘blips’ as the ELM327
sends each one.
ISO 14230 (KWP). If you would prefer that a different
message be sent, simply use the Wakeup Message
command to define it.
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 11 22 33 44 55 (with a
checksum byte following). You do not provide the
checksum byte - it is 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
allows for messages of one to six bytes total, not
including the checksum.
The ELM327 normally sends wakeup messages
after 3 seconds of no activity (this time is adjustable
with the AT SW command). This is within the five
second limit set by the standard.
The default content of these messages vary
depending on the protocol - for ISO 9141, the ELM327
will send 68 6A F1 01 00, and it sends C1 33 F1 3E for
ELM327DSI
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
34 of 82
ELM [ ELM ELECTRONICS ]
