Operation Manual

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