ELM327P datasheet(61/76 Pages) ELM

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ELM327

ELM327DSH

Elm Electronics – Circuits for the Hobbyist

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Setting Timeouts - AT ST and AT AT Commands

Users often ask about how to obtain faster OBD

scanning rates. There is no definite answer for all

vehicles, but the following information may help with

understanding how the settings might apply to your

vehicle.

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 new feature to the v1.2 IC, 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, as of v1.3 of the ELM327.

Instead of sending 01 00 for the above request,

the ELM327 will now 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 that you use this new 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

message, 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.

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