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SAE J1939 protocol is an automobile engineering society (SAE) definitions, mainly for commercial vehicles, but also used in ships, rail locomotives, agricultural machinery and large engine. In addition, SAE J1939 is used as the international standard NMEA 2000 (maritime) and ISO 11783 (agricultural machinery) basis, so that the protocol stack can also be used for these applications.
In the commercial vehicle sector standard serial protocol developed by SAE has long been used in a single electronic control unit and a transmission device on the communication between the components. Can usually be obtained from serial terminal J1708/ J1587 protocol can be regarded as the precursor based on.
Because of the need to be compatible with J1708/1587 protocol, J1939 for example, must request CAN message identifier from the 11-bits extension to the 29-bits (extended format), and development or protocol CAN module implementation should support this message format.
Because there is a CAN identifier, which can be used as J1708 that will define the rules mapping, communication relationship to CAN. Part of the identifier is used to specify a 8-bit source and an 8-bit destination address (number of nodes).
So the transmission measurements and control data and configuration element to SAE through J1939. In addition, but also be able to read or remove a single element diagnostic data, and calibration of a single control.
In order to make this possible, for all the relevant variables are defined in the commercial vehicle range, resolution, identification number in the SAE J1939 etc.. Most of these on implementation of J1587 technology based on specifications.
Therefore, in the J1939 protocol, not only specify transmission type, message structure and segmentation, flow check, and the content of the message itself is made precise definition. SAE J1939 in the ISO/OSI layer in the model distribution according to the OSI layer model, SAE J1939 is divided into several layers, each layer are respectively corresponding documentation. Similar to all actual field bus protocol, fifth in the SAE J1939 and the 6 layer is not needed, so there is no definition.
SAE J1939 function is divided into the following layers: the first layer (physical layer) electronic interface description in other things and the physical medium; second layers (data link layer) is described by the data communication technology specification of CAN 2.0B based on CAN; the third layer (network layer) describes two network part in bridge message transmission function, and only with the J1939 bridge to realize correlation; layer fourth (transport layer) basically described is for message application mode, confirm the various network services segment transmission and large blocks of data; a seventh layer (application layer) description of the actual data (with a range of network parameters or variables, resolution physical unit, and transmission type). Each message unambiguously corresponds to a number (parameter group number);
Because the network management can be regarded as a separate unit, direct access to the hardware (first layer), so in this layer of the model, the module is used as the right hand side of an independent function block. Network management basically includes automatic assignment or a decision node address (PNP principle). There is no definition node monitor in SAE J1939, so it must be in the application through the cycle of implement.
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