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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  
SAE J1939 Messages  
The SAE J1939 CAN standard is being used by  
many types of heavy machinery – trucks, buses, and  
agricultural equipment, to name a few. It uses the  
familiar CAN (ISO 11898) physical interface, and  
defines its own format for data transfer (which is very  
similar to the ISO 15765 standard that is used for  
automobiles).  
define their own proprietary PGNs.  
The ID portion of a J1939 CAN frame is always 29  
bits in length. It provides information as to the type of  
message that is being sent, the priority of the  
message, the device address that is sending it, and  
the intended recipient. Information within the ID bits is  
divided roughly into byte size pieces as follows:  
The following will discuss a little of how data is  
transferred using the J1939 standard. Considerably  
more information is provided in the Society of  
Automotive Engineers (SAE) standards documents, so  
if you are going to be doing a lot of work with J1939, it  
may be wise to purchase copies of them. At minimum,  
the J1939-73 diagnostics, the J1939-21 data transfer,  
and the J1939-71 vehicle application documents  
should be purchased. Another great reference for this  
work is the HS-J1939 two book set, also available from  
the SAE.  
The current version of the J1939 standard allows  
only one data rate (250 kbps), but work is underway to  
amend the standard so that an alternate rate of  
500 kbps will also be allowed. For the purpose of this  
discussion, the data rate is not important - it is the  
format of the information that we will discuss.  
All CAN messages are sent in ‘frames’, which are  
data structures that have ID bits and data bytes, as  
well as checksums and other items. Many of the J1939  
frames are sent with eight data bytes, although there is  
no requirement to do so (unlike ISO 15765, which  
must always send eight data bytes in each frame). If a  
J1939 message is eight bytes or less, it will be sent in  
one frame, while longer messages are sent using  
multiple frames, just like ISO 15765. When sending  
multiple frames, a single data byte is used to assign a  
‘sequence number’, which helps in determining if a  
frame is missing, as well as in the reassembly of the  
received message. Sequence numbers always start  
with 01, so there is a maximum of 255 frames in a  
message, or 1785 bytes.  
3 bits 2 bits  
Priority  
8 bits  
8 bits  
8 bits  
PDU  
Format  
Destination  
Address  
Source  
Address  
PDU1 Format  
The data structure formed by the 29 bit ID, and the  
associated data bytes is called a Protocol Data Unit, or  
PDU. When the ID bits have a destination address  
specified, as is shown above, it is said to be a PDU1  
Format message.  
The two bits shown between the Priority and the  
PDU Format are known as the Extended Data Page  
(EDP), and the Data Page (DP) bits. For J1939, EDP  
must always be set to ‘0’, while the DP bit is used to  
extend the range of values that the PDU Format may  
have. While the DP bit is typically ‘0’ now, that may not  
be true in the future.  
Not all J1939 information is sent to a specific  
address. In fact, one of the unique features of this  
standard is that there is a large amount of information  
that is being continually broadcast over the network,  
with receivers using it as they see fit. In this way,  
multiple devices requiring the same information do not  
have to make multiple requests to obtain it, information  
is provided at regular time intervals, and bus loading is  
reduced.  
If information is being broadcast over the network  
to no particular address, then the Destination Address  
field is not required. The eight bits can be put to better  
use, possibly by extending the PDU Format field. This  
is what is done for a PDU2 Format frame, as shown  
here:  
One major feature of the J1939 standard is its very  
orderly, well defined data structures. Related data is  
defined and specified in what are called ‘parameter  
3 bits 2 bits  
Priority  
8 bits  
8 bits  
8 bits  
groups’. Each parameter group is assigned  
a
PDU  
Format  
Group  
Extension  
Source  
Address  
‘parameter group number’, or PGN, that uniquely  
defines that packet of information. Often, the  
parameter groups consist of eight bytes of data (which  
is convenient for CAN messages), but they are not  
restricted to this. Many of the PGNs, and the data  
within them (the SPNs) are defined in the J1939-71  
document, and manufacturers also have the ability to  
PDU2 Format  
So how does one know if they are looking at a  
PDU1 Format frame that contains an address, or a  
PDU2 Format frame that does not? The secret lies in  
ELM329DSB  
Elm Electronics – Circuits for the Hobbyist  
www.elmelectronics.com  
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