Even before the “invention” of board-level buses, the demand for a reduction of cabling weight in avionics and space technology had led to the development of the Military Standard 1553 bus, which can be regarded as the first “real” fieldbus. Released in 1970, it showed many characteristic properties of modern fieldbus systems: serial transmission of control and data information over the same line, master–slave structure, possibility to cover longer distances, and integrated controllers. Later on, similar thoughts (reduction of cabling weight and costs) resulted in the development of several bus systems not only in the automotive industry but also in the automation area. A characteristic property of these fieldbuses is that they were defined in the spirit of classical interfaces, with a focus on the lower two protocol layers, and no or nearly no application-layer definitions. The controller area network (CAN) is a classical example for this type of fieldbus. Sometimes, higher layer definitions were added later to make the system applicable to other areas, too