Measurement-expressing apparatus

Claim 1

Measurement-expressing apparatus for monitoring the state of each of a plurality n of stations, said apparatus comprising: a plurality n of electrical switches corresponding to the stations, each switch capable of an open state and a closed state, with the state of each switch representing the state of the corresponding station; a plurality n of resistors having different resistances which are binary related to each other, each resistor being connected electrically in parallel with a corresponding switch, and the resultant plurality of parallel combinations of switches and resistors being connected electrically in series; a constant current source connected to the series of parallel combinations to drive a constant current therethrough, current flowing through each resistor whose corresponding switch is open such that a voltage drop is produced thereacross, and current being diverted through switches which are closed; an n-bit analogue-to-digital converter having n outputs and having an input connected to the series of parallel combinations so as to respond to the sum of the voltage drops across those of said resistances whose corresponding switches are not closed, said analogue-to-digital converter having a quantisation period t required for performing a conversion and capable of repetitively operating over successive quantisation periods to sample the input voltage at the beginning of each quantisation period and to provide an n-bit binary-weighted output at the end of each quantisation period; the values of said resistances, the magnitude of the constant current and said analogue-to-digital converter being selected such that each of said n converter outputs corresponds to an individual one of said parallel combinations and the state of each of said n converter outputs represents the state of the corresponding switch; and a plurality n of counting means respectively connected to corresponding ones of said n converter outputs for accumulating, during each of a plurality of consecutive intervals mt, where m is an integer greater than one, the number of signals received from the corresponding converter output indicative of one of the two possible states, whereby the number accumulated in each of said counting means during each interval mt represents the approximate proportion of the interval mt during which the corresponding switch is either open or closed.