The SAE J1939 documents are intended for light, medium, and heavy-duty vehicles used on or off road as well as appropriate stationary applications which use vehicle derived components (e.g. generator sets). Vehicles of interest include, but are not limited to, on- and off-highway trucks and their trailers, construction equipment, and agricultural equipment and implements.
The purpose of these documents is to provide an open interconnect system for electronic systems. It is the intention of these documents to allow Electronic Control Units to communicate with each other by providing a standard architecture.
This particular document, SAE J1939-21, describes the data link layer using the CAN protocol with 29-bit Identifiers. For SAE J1939 no alternative data link layers are permitted.
Rationale
This SAE Recommended Practice revision incorporates changes to the assigned SAE Reserved Bit to be the Extended Data Page Bit, Acknowledgement sections, the new proprietary A2 PGN (PGN 126720) and includes new SPN assignments in Appendix D.
The 'Reserved' bit of the 29-bit of J1939 header has been defined as the 'Extended Data Page' (EDP) bit. The J1939 Subcommittee has concerns there might be existing and developing components for J1939 that don't verify the Reserved/EDP bit is zero (0) before processing the 29-bit header per the J1939-21 structure. It is important that existing and future J1939 applications examine the values of the EDP and DP bits before interpreting the remaining 24 bits per the PDU Format (PF), PDU Specific (PS), and Source Address (SA) structure specified in J1939-21.
Backward compatibility is maintained during this change by specifying the Extended Data Page bit value of zero for all existing J1939 PGNs. Prior to this functional change, J1939-21 required the Reserved bit to be set to zero (0) in all J1939 PGN Data Frame headers.
The Truck and Bus Control and Communications Network Committee of the Truck & Bus Electrical/Electronic Steering Committee is responsible for developing, reviewing and approving standards, recommended practices and information reports related to communication networks for the exchange of control and diagnostic information among electronic components on or connected to truck and bus vehicles. The objective is the architecture of communication networks offering an open interconnect system that supports communication between loose package components, such as engines and transmissions, which are sourced from many different component suppliers and integrated into vehicle systems. The communication networks are developed for use in on-highway and off-highway trucks and their trailers, buses, construction equipment, and agricultural equipment and implements, and applicable to light-duty, medium-duty, and heavy-duty vehicles used on-road or off-road, and for appropriate stationary applications which use vehicle derived components (e.g. generator sets).
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Data Link Layer
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