Abstract: Protocol converter device converts between a 4-20 mA analog signal format and a digital Fieldbus signal format. An input circuit is provided, coupled to the transmitters which includes a multiplexer for selecting one of the analog signals from the transmitters of, e.g., three different field devices or from the analog signal converted from a digital signal. After the analog signal is converted to a digital signal, a central processing unit converts the converted digital signal into a digital quantity of a property of interest which may be display and/or transmitted to other devices or a control computer via a Fieldbus. The central processing unit selects which transmitter converted analog signal is to be displayed. The converter circuit can be placed in an environmental hazard resistant housing, to create a protocol conversion unit that can be located in a convenient field or control center location.