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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  
Selecting Protocols (continued)  
>AT SP 0  
OK  
this is not what you want, the results may be a little  
frustrating.  
To use your own header (and data) values when  
attempting to connect to an ECU, do not tell the  
ELM329 to use protocol 0. Instead, tell it to either use  
only your target protocol (ie. AT SP n), or else tell it to  
use yours with automatic searches allowed on failure  
(ie AT SP An). Then send your request, with headers  
assigned as required. The ELM329 will then attempt to  
connect using your headers and your data, and only if  
that fails (and you have chosen the protocol with AT  
SP An) will it search using the standard OBD default  
values.  
In general, 99% of all users find that enabling the  
memory (setting pin 5 to 5V) and using the ‘Auto’  
option when searching (you may need to send AT  
SP 0) works very well. After the initial search, the  
protocol used by your vehicle becomes the new  
default, but it is still able to search for another, without  
your having to say AT SP 0 again.  
and when the next OBD command is to be sent, the  
ELM329 will automatically look for one that responds.  
You will see a ‘SEARCHING...’ message, followed by  
a response, after which you can ask the ELM329 what  
protocol it found (by sending AT DP).  
The ELM329 always searches in the order set by  
the protocol numbers (ie 6, 7, 8, etc.). Note that the IC  
only appears to provide some support for protocols 1  
to 5, but it never actually sends messages using them  
- all searches start with protocol 6.  
The automatic search works well with OBDII  
systems, but may not be what you need if you are  
experimenting. During a search, the ELM329 ignores  
any headers that you have previously defined (since  
there is always a chance that your headers may not  
result in a response), and it uses the default OBD  
header values for each protocol. It will also use  
standard requests (ie 01 00) during the searches. If  
OBD Message Formats  
On Board Diagnostics systems are designed to be  
very flexible, providing a means for several devices to  
communicate with one another. In order for messages  
to be sent between devices, it is necessary to add  
information describing the type of information being  
sent, the device that it is being sent to, and perhaps  
which device is doing the sending. Additionally, the  
importance of the message becomes a concern as  
well – crankshaft position information is certainly of  
considerably more importance to a running engine  
than a request for the number of trouble codes stored,  
or the vehicle serial number. So to convey importance,  
messages are also assigned a priority.  
The information describing the priority, the  
intended recipient, and the transmitter are usually  
needed by the recipient even before they know the  
type of request that the message contains. To ensure  
that this information is obtained first, OBD systems  
transmit it at the start (or head) of the message. Since  
these bytes are at the head, they are usually referred  
to as header bytes. Figure 3 below shows a typical  
OBD message structure that is used by the older OBD  
TA  
SA  
priority  
receiver transmitter  
3 header bytes  
up to 7 data bytes  
checksum  
Figure 3. An OBD Message - Initial Protocols  
ELM329DSB  
Elm Electronics – Circuits for the Hobbyist  
32 of 76  
www.elmelectronics.com