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 arran
