Fieldbus isn’t really one thing but more of a collection of things.
Fieldbus is a group of protocols that are used in the industrial arena.
The Fieldbus protocols have been standardized as IEC 61158.
Basically, Fieldbus works on a network that permits various topologies such as the ring, branch, star, and daisy chain.
Prior to Fieldbus, industrial controller systems were connected using RS232 serial communications. As you know, serial communications allowed only two devices to communicate.
Whereas today, the Fieldbus connections are more closely compared to the typical Ethernet connections where you can connect multiple field devices to a single connection point that would then connect to the controller.
However, Fieldbus is not a connection type, per se, but instead, a description used to indicate a group of protocols.
There are several protocols in the group such as ControlNet in the Allen Bradley family, Modbus, Profibus, EtherCAT, HART, CIP, and many more.
A primary benefit of Fieldbus is for field connected devices. For instance, consider you have several field devices that are quite a distance from your controller.
With Fieldbus devices such as sensors, motors, lamps, switches, these devices are connected to an “I/O data block”, which is then connected to a “field distribution device”, which is connected to a Fieldbus power supply, then finally connected back to the Programmable Logic controller.