JPS647794A - Digital-analog conversion circuit - Google Patents

Abstract

PURPOSE:To prevent leakage of a digital signal into analog signal system by providing a gate circuit to the digital signal input/output part of a codec IC that execute the reciprocal exchange between an analog signal and a digital signal, and blocking the inputting of a digital signal to the IC except a speech communication. CONSTITUTION:In a digital electronic exchange system, paying the attention that the probability of the simultaneous use in the plural telephone sets included in the system is 10-30%, the circuit is constituted as follows. Namely, on AND gate 22 is provided to the digital signal input system, and the gate opens only a period when control signals FS are being received to input the signal to the codec IC 13 through a signal line 16. During the period other than above mentioned one, the inputting of various digital signals through a plug part 15a is blocked. That is, a clock signal CLK is prevented from being inputted to the IC 13 at the time except during the time of a speech communication when control signals SB are maintained. Also, a tri-state buffer 24 is so ser as operable only during the input period of a signal FS.