A process calibrator is formed with functions of a fieldbus communicator. Preferably, functions of a fieldbus communicator support several fieldbus protocols, and it is provided such that the use of it does not hamper the functioning of a process calibrator. This has been achieved by providing the process calibrator with a pair of fieldbus connecting terminals for measuring of the fieldbus signal. The process calibrator comprises a processor, and the pair of fieldbus connecting terminals comprises a signal terminal and a ground terminal. The process calibrator further comprises at least two parallel fieldbus protocol units between the processor and the signal terminal of the pair of fieldbus connecting terminals. Each fieldbus protocol unit is dedicated to its respective fieldbus protocol. Further, the process calibrator comprises a selection unit for selecting a fieldbus protocol unit and a voltage unit for being between the terminals of said pair of fieldbus connecting terminals, which voltage unit is arranged to adjust the voltage and the impedance according to the selected fieldbus protocol unit. The selecting unit is controlled manually through the user interface. The user interface is realized by a touch screen or a combination of a touch screen and keyboard or only by a keyboard.

A field device of a room automation system or of a power management system or of a hazard detection system is disclosed. The field device includes at least one human-machine interface with at least one memory configured to store physical layer data, at least two protocol stacks, wherein the field device is configured to select at least one protocol stack based on the physical layer data stored in the memory of the human-machine interface. The field device also includes at least two physical layers, at least two pairs, wherein each compatible pair includes at least one physical layer and at least one protocol stack. The field device also includes selector switch(es) configured to connect and to disconnect physical layers and protocol stacks, such that a physical layer element and a protocol stack connected or disconnected by the selector switch(es) belong to the same compatible pair.

A process calibrator is formed with a user interface, which guides to choose the right connection arrangements of the process calibrator according to the function of the process calibrator. The user interface shows in the display of the process calibrator the connection arrangements of the process calibrator and indicates the place of the connection arrangement assigned to the selected function visually compared to other connection arrangements. The connection arrangements shown by the user interface in the display are placed similarly to the physical connection arrangements of the process calibrator. As the user interface indicates the connection arrangement that is assigned to the selected function of the process calibrator, it is easy for the user to make the right connection.