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.