| 摘要 |
PURPOSE:To decrease the number of a voltage discriminating circuit by giving sequentially a reference voltage having a different voltage and discriminating a voltage signal in response to the serial data of plural bits produced corresponding to each reference voltage from a comparator. CONSTITUTION:A gate pulse is transmitted sequentially from output ports R0- R4 of a processor CPU, a terminal voltage VD at control circuits CT1-CT5 is scanned sequentially and given to a voltage discriminating circuit. A selecting pulse is transmitted from output ports P0-P3 in synchronizing with the gate pulse and the reference voltage is changed into four kinds with resistors R104- R107. In deciding the reference voltage VS in response to the terminal voltage VD when buttons K1-K3 are turned on, a 4-bit serial data is obtained at each period of the gate pulse and this is given sequentially to an input port KD of the CPU, allowing to discriminate transmission information of button telephone sets ET1-ET5. |
