| 摘要 |
PURPOSE:To reduce the number of cores of wiring cable by providing an offset adjusting circuit to a PLLIC of a receiving circuit demodulating an information signal extracted from an extension line to attain serial transmission between a master device and a slave device of a key telephone set. CONSTITUTION:In a button telephone set where a call signal, an information signal in the signal form of a frequency shift signal and a power supply current applied from a master device to each slave device are transmitted to an extension line with multiplexing, a reference voltage at a terminal Y of the PLLIC8 is adjusted so as to be lower than an output voltage of a terminal X when no input signal is given by an adjusting resistor VR and so that an output voltage of the terminal X is lowered when an input signal in a frequency higher than f0 in order to prevent malfunction due to synchronizing shift between the master device and each slave device. The serial transmission of the information signal between the master device and each slave device of the key telephone set is attained and the number of cores of wiring cable is reduced. |
