Telephone line interface circuit

DETAILED DESCRIPTION This invention provides a telephone line interface circuit for connecting telephone equipment provided with internal speech lines having predetermined DC impedance to external telephone lines, comprising rectifying means connected with the telephone lines for rectifying the DC voltage coming from the telephone lines; dial pulse sending means which makes connection and disconnection between the output terminals of the rectifying circuit and the internal speech lines for sending dial pulses to said telephone lines; and polarity inversion detecting means which discriminates the voltage relationship between the telephone lines and the internal speech lines for detecting the polarity inversion of the DC voltage coming from the telephone lines.
The voltage polarity of the internal speech lines inside the telephone equipment is constant by the effect of the rectifying circuit regardless of the voltage polarity of the telephone lines, and therefore the direction of the loop current flowing through the internal speech line is also constant.
Because the dial pulse sending circuit is provided between this internal speech line and the rectifying circuit, the direction of the loop current flowing through the dial pulse sending circuit is always constant, thus allowing use of a unidirectional semiconductor switching device as the dial pulse sending circuit.
As the typical unidirectional switching device is a MOS transistor of small size and low cost.
The circuit to detect the polarity inversion of telephone lines is connected between the telephone lines and the internal speech lines.
Therefore, at the time of dial break, that is, when the dial pulse sending circuit is open, the impedance of the telephone equipment viewed from the telephone line becomes infinite regardless of the impedance of the polarity inversion detecting circuit.
Therefore the telephone office can sufficiently recognize the dial pulses.