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.
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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 exisitng 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.