| 摘要 |
For dial pulse generation in subscriber devices, semiconductor switches are used to an increasing extent instead of relays. In switching systems which operate with initiation currents, the so-called initiation resistor must be disposed upstream of the dial transistors and must be connected to both wires of the subscriber line, and this connection must be interrupted during dialling for the sake of a high-impedance internal resistance of the subscriber device. To provide polarity-independent dial pulse generation and to prevent an interruption of the connection between the initiation resistor and the two wires of the subscriber line in the dialling condition, two self-conducting field effect transistors, which feed the identification current to an initiation resistor connected downstream, are disposed symmetrically in relation to the two wires of the subscriber line. When DC loop current is connected, the two field effect transistors are switched by means of two switching transistors, which are disposed symmetrically in relation to one another, in time with the dial pulses generated by a dial pulse circuit.
|
