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The SAE J1939 protocol is defined by the society of Automotive Engineering (SAE), which is mainly used for commercial vehicles, but also for ships, rail locomotives, agricultural machinery and large engines. In addition, SAE J1939 is used as the basis of NMEA 2000 (maritime) and ISO 11783 (agricultural machinery), so this protocol stack can also be used for these applications.

In commercial vehicles, the standardized serial protocol developed by SAE has been used for a long time for communication between single electronic control unit and components on the transmission device. Based on the J1708 / J1587 protocol which can be obtained from the serial terminal of single chip microcomputer, it can be regarded as the precursor.

Because it needs to be compatible with J1708 / 1587 protocol, for J1939, can message identifier must be extended from 11 bits to 29 bits (extended format), and can module development or protocol implementation should support this message format.

Because of the extended can identifier, it is possible to map the definition rules of the communication relationship to the can as used in J1708. Partial identifiers are used to specify an 8-bit source and an 8-bit destination address (number of nodes).

Therefore, SAE J1939 is able to transmit measurement and control data and configure components. In addition, it can read or delete single component diagnosis data and calibrate single control.

In order to make these possible, all relevant variables of commercial vehicles should be defined in SAE J1939, such as value range, resolution, identification number, etc. These are mostly realized based on J1587 technical specifications.

Therefore, in J1939 protocol, not only the transmission type, message structure and its segments, flow check, etc. are specified, but also the message content itself is precisely defined. The distribution of SAE J1939 in the ISO / OSI layer model according to the OSI layer model, SAE J1939 is divided into several layers, each layer has corresponding documentation. Similar to all actual fieldbus protocols, layers 5 and 6 are not required on SAE J1939, so there is no definition.

The functions of SAE J1939 are divided into the following layers: the first layer (physical layer) describes the electronic interface with physical media in other things; the second layer (data link layer) describes the data communication through can based on can 2.0B technical specifications; the third layer (network layer) mainly describes the functions of the bridge between the two network parts for message transmission, and only with the J1939 bridge implementation is related; the fourth layer (transmission layer) basically describes various network services for message application mode, confirmation transmission and segmented transmission of large data blocks; the seventh layer (application layer) describes the actual data (parameters or network variables with value domain, resolution, physical unit and transmission type). Each message corresponds to a number (number of parameter groups) unambiguously;

Because network management can be regarded as a separate unit, which can reach the hardware (the first layer), in this layer model, the module is an independent function block on the right side. Network management basically includes automatic allocation or determination of node address (plug and play principle). There is no node monitoring defined in SAE J1939, so it must be realized by looping message in application.

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