Serial bus systems designed for industrial real-time data communication are called fieldbus systems. It provides communication between a bus master and remote slave stations, or distributed actors/sensors in order to read and write process data. Sensor/actor fieldbus systems are designed for low latency and low transmission volumes. Their packet sizes are in general in the range of up to 200-300 bytes. The transmission speed can be chosen in a wide range from e.g. 12Mbit/s down to 1200bit/s. The useable segment length depends on the transmission speed, transmission media, and the topology of the individual fieldbus. In general, higher transmission speed decreases the segment length of bus oriented fieldbuses to a length in the range of 50 to 100 meters; low transmission speed allows a segment length in the range of a few kilometers. This flexibility in transmission speed is a big advantage against Ethernet based networks. Ethernet based fieldbuses are working with 100Mb/s or 1GB/s Ethernet media with a full packet size as defined by Ethernet. Pure sensor/actor networks of Ethernet based fieldbusses - like VARAN - are using a limited packet size of e.g. 128 bytes.
Fieldbuses with a ring topology like INTERBUS, EtherCAT, and SERCOS are working often with a physical extension in the range of kilometers because of the signal refreshing which is done by the fieldbus I/O modules. Fiber optical transmission medias allow in general extended segment lengths and increased noise immunity. The cable infrastructure of a fieldbus is much leaner than a comparable centralized point-to-point infrastructure. Our Fieldbus Comparison shows a collection of the most important fieldbuses, their core attributes and their relationship to standards. Today fieldbuses are the core of every industrial distributed control and IO system.