This is an adapter between the J1939 round Deutsch 9-pin connector (test equipment side - mates to the J1939 vehicle) and an OBD2 style 16-pin connector (female vehicle side - mates with an OBD2 test connector). The J1939 +/- data lines are connected to the CAN +/- lines, and the power and ground lines are also connected.

The most common application is to connect a GPS Tracker that is meant for an OBD2 vehicle to a vehicle that has the 9-pin round Deutsch connector. The power lines are appropriately connected to provide power to the tracker.

This adapter will allow an OBD2 scan tool (with the appropriate software) to connect to the round J1939 connector. Bear in mind that most OBD2 software and scanners are based on reading PIDs, which are single parameters. J1939 is a PGN based protocol, where each PGN contains multiple parameters. Your scan tool must be able to handle PGNs if it is to work with this adapter.