The Digital Annex describes five different calculation rules for the checksums, two of which are based on a typical CRC (Cyclic Redundancy Counter) calculation with a length of 8 bits. If, however, the message counter is not incremented for a subsequent transfer or the checksum does not match the payload data, the receiver can assume that the sender is in an error state. As the checksum is calculated from the payload data, the checksum also changes as a result of each correct transfer, regardless of whether or not other signals have changed. The message counter is increased by one each time a message is transferred.
The message counter and checksum are part of the payload data of a PG. The J1939 Digital Annex specifies checksums for over 200 messages and message counters for the detection of fault conditions of the ECUs involved. SAE J1939 Digital Annex – Messages With Integrated Checksum and Counter
Uniform error codes – so-called Diagnostic Trouble Codes (DTC) – contain the faulty SPN, the error pattern, and the frequency of occurrence of an error. In parallel with that, errors can be read out with a tool via the diagnostic connector.
Errors that occur are evaluated directly in the network and visually displayed, if necessary. In contrast to the UDS diagnostics in which services have to be actively initiated via a software tool, J1939 ECUs also send diagnostic messages independently during standard operation. They also comply with the EU Directives and with the “California Code of Regulation” for Onboard Diagnostics (OBD II), as well as HD OBD (Heavy Duty OBD) and WWH (World Wide Harmonized) OBD (ISO 27145). The PGNs designated as a Diagnostic Message (DM) largely fulfill the scope of functions of the UDS diagnostics (Unified Diagnostic Service). The J1939 diagnostic interface defines a standard diagnostic connector as well as a set of PGNs for handling different diagnostic services.
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