Bedienungsanleitung Elm 327 (Seite 32 von 94) (Englisch)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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 42). 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.

There is a way to speed up the retrieval of

information, if you know how many responses will be

sent. 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 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 just

make a request, without regard to how many

responses are expected.

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.

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