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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  
Monitoring the Bus  
Some vehicles use the OBD bus for information  
you are looking at a J1939 network, simply tell the  
ELM329 to set the protocol to A (AT SP A), or if you  
have an 11bit, 500kbps ISO15765 system, tell it  
AT SP 6. The SP command description (page 21)  
gives a list of all the protocols and their numbers.  
If the ‘Monitor All’ command provides too much  
information (it does for most CAN systems), then you  
should restrict the range of data that is to be displayed.  
The best way to do this is with the CAN Receive  
Address command (AT CRA).  
transfer during normal vehicle operation, passing a  
great deal of information over it. A lot can be learned if  
you have the good fortune to connect to one of these  
vehicles, and are able to decipher the contents of the  
messages.  
To see how your vehicle uses the OBD bus, you  
can enter the ELM329’s ‘Monitor All’ mode, by sending  
the command AT MA from your terminal program. This  
will cause the IC to display any information that it sees  
on the OBD bus, regardless of transmitter or receiver  
addresses (it will show all). Note that the ELM329  
remains silent while monitoring, so periodic ‘wakeup’  
messages are not sent, and the CAN module does not  
acknowledge messages (unless CAN Silent Monitoring  
has been turned off).  
The monitoring mode can be stopped at any time  
by putting a logic low level on the RTS pin, or by  
sending a single RS232 character to the ELM329. Any  
convenient character can be used to interrupt the IC  
as there are no restrictions on whether it is printable,  
etc. Note that any character that you send will be  
discarded, and will have no effect on any subsequent  
commands. The time it takes to respond to such an  
interrupt will depend on what the ELM329 is doing at  
the time. The IC will always finish a task that is in  
progress (printing a line, for example) before printing  
‘STOPPED’ and returning to wait for your input, so it is  
best to wait for the prompt character (‘>’) to be sent, or  
the Busy line to go low, before beginning to send a  
new command.  
Unexpected results occasionally occur if you have  
the automatic protocol search feature enabled, and  
you tell the ELM329 to begin monitoring. If the bus is  
quiet, the ELM329 will begin searching for an active  
protocol, but it may find something that you were not  
expecting. The ELM329 may stop searching at one  
protocol if the baud rate matches, or it might stop at  
multiples of the actual baud rate (and then report  
receive errors). If you are testing ‘on the bench’, the IC  
might not even find any protocol if the silent mode is  
enabled (it is by default). Be aware however, that we  
do not advise setting the silent mode off (AT CSM0)  
while searching for a protocol to monitor, as the  
ELM329 may incorrectly interact with the CAN  
network, and cause problems. In the extreme case,  
the ELM329 might even have internal problems and  
report an ERR94.  
Perhaps you have an 11 bit system, and only want  
to see messages that begin with 7. To do that, simply  
type:  
>AT CRA 7XX  
and follow it with an AT MA command. From that point  
on, all messages that begin with 7 will be displayed  
(the X’s say that you do not care what those other  
digits are).  
Similarly, you might be working with a 29 bit CAN  
system, and want to see all messages from the  
engine. If the engine uses address 10, then simply  
type:  
>AT CRA XX XX XX 10  
and follow it with an AT MA command. From that point  
on, only messages with IDs that end in ‘10’ will be  
displayed.  
Note that the CRA filter (as well as the CF and CM  
one) will reduce the amount of information seen with  
the AT MA command, but there may still be times  
when the rate that the information is generated by the  
vehicle far exceeds that which can be handled by the  
PC connection. In these cases, the internal memory  
(or ‘buffer’) fills up more quickly than it is being  
emptied, and you will see a BUFFER FULL error  
message. If this is happening, you may wish to  
consider increasing the baud rate of your connection  
(see page 44).  
Another way to reduce ‘BUFFER FULL’ errors is  
by reducing the number of characters that are put into  
the buffer. You can use AT S0 to eliminate space  
characters, turn off formatting (AT CAF0) to eliminate  
‘DATA ERROR’ reports, or possibly turn off the  
headers (AT H0) to eliminate those bytes.  
As a final note, the ELM329 can be set to begin  
‘monitoring all’ automatically after power on, if PP 00 is  
set to the value 00 and is enabled. This only causes  
an AT MA to be sent, however - there is no facility to  
automatically provide filtering of the information.  
When monitoring, it is always best if you can  
select the protocol for the ELM329. If you know that  
ELM329DSB  
Elm Electronics – Circuits for the Hobbyist  
www.elmelectronics.com  
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