Although the term “fieldbus” appeared only about 25 years ago, the basic idea of field-level networks is much older, and the actual roots of modern fieldbus technology are diverse [1]. Both classical electrical engineering and computer science have contributed their share to the evolution. One foundation of automation data transfer has to be seen in the classic telex networks and also in data transmission standards for telephone lines. Large distances called for serial data transmission, and many of these comparatively early standards still exist, such as V.21 (data transmission over telephone lines) and X.21 (data transmission over special data lines). Various protocols were defined, mostly rather simple, because of the limited computing power of the devices available at that time. With improved microprocessors, telephone systems gradually changed from analog to digital. This opened the possibility to transfer large amounts of data from one point to another. Together with an improved physical layer, the first really powerful data transmission protocols were defined, such as X.25 or SS7.