| 摘要 |
PURPOSE:To selectively receive a specific key only in the on-hook state by providing a means turning on/off a line between a keyboard and a key entry section in a circuit outputting a timing signal to the keyboard by means of a control signal. CONSTITUTION:Transfer gates 40-45 of CMOS constitution turned on and off are provided to lines R1-R4 and C1-C4 through which a timing signal is outputted to the keyboard 1. With a signal G at a high level, the gates 40-45 are energized to output clock signals phi1 and phi2, and latch circuits 10-17 are operated by timing signals T1, T3 to accept all the key entires of switches SW1, SW16. With the signal G at a low level, the gates 40-45 go to non- continuity and only the SW4 is accepted. No current flows in depressing other keys and the key entry is not accepted. Thus, the control signal G has only to be in a low level with the on-hook state. Which line of the lines R1-R4 and C1-C4 the transistor gates are to be provided with selected optionally.
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