ELM327_13 Datasheet, PDF (31/82 Pages) ELM Electronics – OBD to RS232 Interpreter

ELM327
Talking to the Vehicle (continued)
That is because rpm is sent in increments of 1/4 rpm!
To convert to the actual engine speed, we need to
divide the 6904 by 4. A value of 1726 rpm is much
more reasonable.
Note that these examples asked the vehicle for
information without regard for the type of OBD protocol
that the vehicle uses. This is because the ELM327
takes care of all of the data formatting and translation
for you. Unless you are going to do more advanced
functions, there is really no need to know what the
protocol is.
The above examples showed only a single line of
response for each request, but the replies often
consist of several separate messages, either from
multiple ECUs responding, or from one ECU providing
messages that need to be combined to form one
response (see ‘Multiline Responses’ on page 44). In
order to be adaptable to this variable number of
responses, the ELM327 normally waits to see if any
more are coming. If no response arrives within a
certain time, it assumes that the ECU is finished. This
same timer is also used when waiting for the first
response, and if that never arrives, causes ‘NO DATA’
to be printed.
Version 1.3 of the ELM327 introduced a way to
speed up the retrieval of information, if you know how
many responses to expect. By telling the ELM327 how
many lines of data to receive, it knows when it is
finished, so does not have to go through the final
timeout, waiting for data that is not coming. Simply add
a single hex digit after the OBD request bytes - the
value of the digit providing the maximum number of
responses to obtain, and the ELM327 does the rest.
For example, if you know that there is only one
response coming for the engine temperature request
that was previously discussed, you can now send:
>01 05 1
and the ELM327 will return immediately after obtaining
only one response. This may save a considerable
amount of time, as the default time for the AT ST timer
is 200 msec. (The ELM327 still sets the timer after
sending the request, but that is only in case the single
response does not arrive.)
Some protocols (like J1850 PWM) require an
acknowledgement from the ELM327 for every
message sent. If you provide a number for the
responses that is too small, the ELM327 will return to
the prompt too early, and you may cause bus
congestion while the ECU tries several times to resend
the messages that were not acknowledged. For this
reason, you must know how many responses to
expect before using this feature.
As an example, consider a request for the vehicle
identification number (VIN). This number is 17 digits
long, and typically takes 5 lines of data to be
represented. It is obtained with mode 09, PID 02, and
should be requested with:
>09 02
or with:
>09 02 5
if you know that there are five lines of data coming. If
you should mistakenly send 09 02 1, you might cause
problems.
This ability to specify the number of responses
was really added with the programmer in mind. An
interface routine can determine how many responses
to expect for a specific request, and then store that
information for use with subsequent requests. That
number can then be added to the requests and the
response time can be optimized. For an individual
trying to obtain a few trouble codes, the savings are
not really worth the trouble, and it’s easiest to use the
old way to make a request.
We offer one additional warning when trying to
optimize the speed at which you obtain information
from vehicles. Prior to the APR2002 release of the
J1979 standard, sending J1850 requests more
frequently than every 100 msec was forbidden. With
the APR2002 update, scan tools were allowed to send
the next request without delay if it was determined that
all the responses to the previous request had been
received. Vehicles made prior to this time may not be
able to tolerate requests at too fast a rate, so use
caution with them.
Hopefully this has shown how typical requests are
made using the ELM327. If you are looking for more
information on modes and PIDs, it is available from
the SAE (www.sae.org), from ISO (www.iso.org), or
from various other sources on the web.
ELM327DSI
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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