Circuitry to facilitate testing of serial interfaces is described. An object is scanned in a spatially restricted effective zone of a scanning beam field and the size of the object in terms of the measured dimension is deduced by the detection of one or more interruptions in the scanning beams. In various embodiments, if it is determined that the existing pool has crossed below a minimal threshold, an overflow is obtained. Additionally, the protocol emulator includes a set of event keywords defining each state keyword representative of inputs that the protocol state may receive while being associated with the particular one of the state keywords. The method includes the steps of opening the purge valve at the same time that the fuel gas is supplied to the fuel cell and replacing the nitrogen gas that originates in the air and is present in the fuel gas circulation path by fuel gas; and closing the purge valve after the nitrogen gas in the fuel gas circulation path has been replaced by the fuel gas.