Abstract: A field apparatus which allows for migration from an analog signal transmission system to a field bus transmission system without a need to change the configuration of the field apparatus and performs a self diagnosis. The field apparatus includes a receiving unit which receives a field bus oriented signal from another apparatus connected to the transmission line, an analog signal transmitting unit which transmits a measured magnitude of a measurement object through the transmission line as an analog signal, a field bus signal transmitting unit which transmits a measured magnitude of a measurement object through the transmission line as a field bus oriented signal, and a switch which selects either the analog signal transmitting unit or the field bus signal transmitting unit which transmits an analog signal or a field bus oriented signal respectively through the transmission line.

Abstract: A field bus system may be used in which transmission ability of the system can be maintained even if communication error occurs due to noises or failure of a transmission line. The system can be easily shifted at a lower cost from a conventional system to the field bus system without degrading the high reliability thereof. The transmission line is constituted by a multiple-cable transmission line having at least three transmission cables. An external power supply supplies power to field devices through a pair of transmission cables of the multiple-cable transmission line. The field devices are connected to the transmission cables through a transmission line switching unit constituted by a plurality of rectifier elements, so that the field devices are supplied with current flowing in one predetermined direction when any of the pairs of transmission cables is selected.