In the heavy duty vehicle industry, data links allow electronic controllers on a vehicle to interact efficiently to achieve the vehicle's mission. The interactions include sharing sensor data and calculated information, allowing subsystems to influence each other's operation, and communicating subsystem operation state and subsystem configuration. Data links also provide a means for onboard and offboard diagnostic work, including electronic controller identification, reading and clearing diagnostic information, recalibration, and configuration of the controller for the specific vehicle application. The Truck and Bus Control and Communications Network Subcommittee began development of SAE Recommended Practice 51939 to satisfy these needs. This paper presents the serial data communications that have been adopted in the published and draft SAE Truck and Bus J1939 documents: J1939, 51939101, 51939111, 51939121, J1939131, J1939171, J1939173 and J1939181. It contains a comparison to the previous generation of data link standards used in the heavy duty vehicle industry: J1708, 51587 and 51922. Examples of control, information sharing, multiplexing, diagnostics and proprietary messaging are provided.