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ELM329P 参数 Datasheet PDF下载

ELM329P图片预览
型号: ELM329P
PDF下载: 下载PDF文件 查看货源
内容描述: CAN解释 [CAN Interpreter]
分类和应用:
文件页数/大小: 76 页 / 353 K
品牌: ELM [ ELM ELECTRONICS ]
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ELM329  
Setting Timeouts - the 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  
your understanding of how the AT ST and AT AT  
settings are used by the ELM329.  
times for you, averages several readings, and then  
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 ELM329 make the adjustments for  
you.  
OK - the ELM329 is able to measure times, and  
suggest a setting for the AT ST time, but the IC still  
has to wait after receiving a reply to see if any more  
are coming. Surely there has to be a way to eliminate  
that final timeout, if you know how many responses to  
expect? There is a way - by telling the ELM329 how  
many messages to receive.  
A typical vehicle request and response is shown in  
the diagram below:  
request is sent  
ELM329  
response  
Vehicle  
ELM waits up  
to 100 msec  
ELM waits 100 msec  
for more responses  
If you wish to make a request, and know how  
many responses there should be, simply add that  
response count as a single digit after your request. For  
example, if you know that two ECUs will respond to an  
01 00 request, then send:  
The ELM329 sends a request then waits up to  
100 msec for a reply (the standard requires 50 msec).  
If no reply arrives in that time, an internal timer stops  
the waiting, and the ELM329 prints ‘NO DATA’. If a  
reply has been received, the ELM329 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). While all replies should be received within 50  
msec, the 100 msec setting ensures that a response is  
not missed.  
As an example, consider a vehicle that responds  
to a query in 10 msec. With the ST timeout set to  
100 msec, the fastest scan rate possible would only be  
about 9 queries per second (it’s 10 + 100 msec per  
response). Changing the ST time to about 40 msec  
would more than double that rate, giving about 20  
queries per second. Clearly, if you were to know how  
long it takes for your vehicle to reply, you would be  
able to improve on the scan rate, by adjusting the ST  
time.  
>01 00 2  
The ELM329 will send the 01 00 request, and will  
return to the prompt state immediately after the second  
response is received (or after the ST timer times out if  
the response does not arrive). In this way, every  
response is shortened by that ST time. This can  
increase the polling rate considerably for most vehicles  
(many users report achieving 50 or more samples per  
second).  
In general, you do not know how many ECUs will  
respond to a request, so this feature is best used by  
software that can query the vehicle to determine the  
number of responses that will be coming, store that  
value, and then use it to set the responses digit for  
subsequent requests.  
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. Even when a response begins,  
different frames within a multi-frame response can  
have very different delays. The physical measurement  
of the time is not easy either - it requires expensive  
test equipment just to make one measurement. To  
help with this, the ELM329 includes a feature called  
‘Adaptive Timing’.  
Adaptive Timing actually measures the response  
ELM329DSB  
Elm Electronics – Circuits for the Hobbyist  
www.elmelectronics.com  
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