JPS5499504A - Privacy control system for key telephone system - Google Patents

Abstract

PURPOSE:To ensure the privacy action through a simple constitution by producing the privacy signal featuring the equal timing to the key telephone set in the main device as well as to simplify greatly the control circuit part of the key telephone system. CONSTITUTION:In case key telephone set 19 gives transmission to main wire 1, button 19S equivalent to off-hooked main wire 1 is pushed. Thus, hook switch detector circuit 61 applies the voltage to balanced crosspoint 551 and 552 via transformer 49. At the same time, the button signal applies the crosspoint driving signal of the line side to crosspoint group 200 via up-control signal detector circuit 57 and also secures logical product 81 with output 700 of privacy signal producing circuit 600 through logical sum gate 82. In this case, if output 700 features signal ''0'' (privacy state) at timing T1 equivalent to main wire 1, no output is produced for logical product gate 81 and accordingly crosspoint 551 does not close. In other words, the privacy function can be realized with just one circuit of gate 81 for the key telephone system as a whole by producing the privacy signal of the same timing within the main device.