System for selectively addressing electrical control signals from a control unit to a plurality of remote units

A system comprising

• a plurality of remote units connected to a common two-wire transmission line with each unit having at least one controlled device,
• a control unit including d.c. voltage supply means connected to said line to feed a constant d.c. supply voltage to the remote units, and means for selectively addressing through said line electrical coded control signals to the remote units by amplitude-modulating the value of said d.c. supply voltage to activate a selected one of said controlled devices;
• each remote unit comprising
• decoder means connected to said line for decoding the control signals from the control unit, and
• comparator means arranged to output an activation signal when the signals from the decoder means correspond to an address associated to a controlled device of the remote unit;
• wherein said controlled device of at least one remote unit is a sensor means arranged to monitor a physical quantity and responsive to said activation signals being fed thereto to output to the control unit, via said line, information signals indicative of the value of said monitored quantity, said sensor means including
• a sensing element arranged to monitor said physical quantity and output electrical signals indicative of its value,
• a signal converter means arranged to convert the electrical signals output by the sensing elements into frequency modulated electrical signals,
• current modulator means connected to receive said frequency modulated signals from the converter means and connectable to the said line for the purpose of modulating the current flowing in said line in dependence on said frequency modulated signals,
• switch means connected between the current modulator means and said line, and controlled by the comparator means to connect the the modulator means to said line when the comparator means output the activation signal,
• the control unit being provided with frequency demodulator means connected to the said line and arranged to detect the frequency modulation of the current flowing in the line.