Introduction to On Board Diagnostics (II)

OBD II for L & MD VehiclesSTD ManualClass B Data Communication Network Messages- Message Definition for Three ByteHeaders (SAE J 2178/4):SAE J 2178/4 Message Definition for Three Byte Headers, standard defines the informationcontained in the header and the data fields of non-diagnostic messages for SAE J 1850 datacommunication class B networks. This standard describes and specifies the header fields, datafields, field sizes, scaling, representations, and data positions s used within messages. SAE J 1979standard defines the specifications of emission-related diagnostic message header and datafields which OBD II is mainly concurred with. SAE J 2190 defines other diagnostic data fields.This standard focuses on the message definition for the three byte form of the consulted headerformat. Section 5 of this standard provides the list of functional target addresses or Primary IDsfor all of the functionally addressed messages on J 1850 except type #3, which is Function Read.SAE J 1850 type # 3 messages have a separate address assignment due to absence of secondaryaddressing. Section 6 of the standard shows the valid extended address aassignments from themessage definition tables. Section 7 lists the secondary message definitions. The information inthis standard follows the same format as the Frame IDs for Single e Byte Forms of Headersin SAE J 2178/3 standard described above.