J1939 Request and Acknowledgement: PGN 59904 and 59392
Most parameter groups in J1939 are transmitted cyclically: engine speed, temperatures or lamp status arrive unprompted at a fixed rate. Some information, however, is only available on request – the DM2 fault memory, software and component identification, or the address assignment of the devices on the network. For these cases SAE J1939-21 defines two parameter groups: the Request (PGN 59904), used to ask for any PGN, and the Acknowledgement (PGN 59392), which a device sends when it answers with a confirmation instead of data. Both are quick to implement. The rules on when a device must respond and when it must stay silent, however, are a common cause of problems between devices from different manufacturers.