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Elm 327 Manual

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ELM327
52 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
Setting Timeouts - AT ST and AT AT Commands
Users often ask about how to obtain faster OBD
scanning rates. There is nothing that we can do about
how fast (or slow) a vehicle is to respond, but we can
optimize how the ELM327 handles the responses.
A typical vehicle request and response is shown in
the diagram below:
ELM327
Vehicle
request is sent
response
ELM waits up
to 200 msec
ELM waits 200 msec
for more responses
The ELM327 sends a request then waits up to
200 msec for a reply. If none were to come, an internal
timer would stop the waiting, and the ELM327 would
print ‘NO DATA’.
After each reply has been received, the ELM327
must wait to see if any more replies are coming (and it
uses the same internal timer to stop the waiting if no
more replies arrive). With our initial OBD products (the
ELM320, ELM322 and ELM323) we found that older
vehicles often needed a timeout setting of about 100
msec, and occasionally needed more, so we settled on
a standard default setting of 200 msec.
If a typical vehicle query response time were about
50 msec, and the timeout were set to 200 msec, the
fastest scan rate possible would only be about 4
queries per second. Changing the ST time to about
100 msec would almost double that rate, giving about
7 queries per second. Clearly, if you were to know how
long it takes for your vehicle to reply, you might be
able to improve on the scan rate, by adjusting the ST
time.
It is not easy to tell how fast a vehicle replies to
requests. For one thing, requests all have priorities
assigned, so responses may be fast at some times,
and slower at others. The physical measurement of
the time is not easy either - it requires expensive test
equipment just to make one measurement. To help
with this, we added a feature to the ELM327 called
‘Adaptive Timing’.
Adaptive Timing actually makes the response time
measurements for you, and adjusts the AT ST time to
a value that should work for most situations. It is
enabled by default, but can be disabled with the AT0
command should you not agree with what it is doing
(there is also an AT2 setting that is a little more
aggressive, should you wish to experiment). For 99%
of all vehicles, we recommend that you simply leave
the settings at their default values, and let the ELM327
make the adjustments for you.
Consider the following times taken from a J1850
VPW vehicle, in response to an 01 00 request:
ELM327
Vehicle
58 msec
engine
(ECU 10)
transmission
(ECU 18)
4 msec
The engine controller responds very quickly, but
the transmission takes considerably longer. The
adaptive timing algorithm measures the longer
transmission response times and will use them to set
the timeout, likely to a value in the range of 90 msec.
With a timeout of 90 msec, the maximum scan rate
would be about 6 readings per second.
Surely there has to be a way to eliminate that final
timeout, if you know how many responses to expect?
There is a way – instead of sending 01 00 for the
above request, the ELM327 will also accept 01 00 2.
This tells the IC to send 01 00, then return immediately
after receiving 2 responses. It can not speed up a slow
ECU, but it will eliminate the final delay, as the
ELM327 knows the number of responses to expect.
This one change might give you 10 to 12 responses
per second, instead of the 6 obtained previously.
We do caution you to use this feature carefully. If
you set the last digit to a number that is less than the
actual number of responses, then acknowledgements
that may be required will not be sent, and some
protocols may begin resending the replies, looking for
a response. This will lead to unnecessary network
congestion, which must be avoided. Before using this
feature, always determine the number of responses
that will be coming from the vehicle, and then set the
responses digit to that value.
… 52 …


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