A fieldbus is a serial bus system used in machines and systems to connect sensors and actuators (motors) to each other and to one or multiple masters (industrial PCs, PLCs). Fieldbuses make it possible to exchange data between different system components over long distances and under high external load. They operate in master-slave mode: While the master is responsible for controlling processes, the slave handles single subtasks.
Fieldbuses differ in the "physical layer" they use, i.e. the hardware level (e.g. CAN, RS485, Ethernet), and in the protocol level that defines the form of the exchanged information. There are fieldbuses, such as CANopen and DeviceNet, that use the same physical medium (CAN) but operate with different protocols. Equally, there are protocols that can be implemented on two different types of hardware (e.g. Modbus on RS232 and as Modbus/TCP on Ethernet).
The hardware level determines fundamental bus properties, such as cable lengths and transmission capacity. The protocol level determines which standardized messages and functions are available between the master and slave.
Depending on the application, each of the established fieldbuses has its strengths and weaknesses. Selection of a specific fieldbus is frequently determined by the availability of the components for a specific fieldbus interface.