| 摘要 |
A send circuit (12) in a PABX (1) delivers an attenuated non-speech digital signal to a telephone (2) on the c wire of a telephone line (3). The send circuit modulates the DC voltage on the c wire and comprises a resistor R3 connected to a negatively biasing voltage of -46 volts through a switching transistor TR1, the base of which receives the digital signal from a microprocessor (8) of the PABX through a resister R4. The resister R3 is sized so that when the transistor TR1 connects the resister R3 to the negative biasing voltage, the negative voltage on the c wire is pulled down by 50 millivolts from approximately -6 volts to approximately -6.050 volts. The non-speech digital output signal on the pin (10) of the microprocessor (8) operates between a low of -46 volts and a high of -41 volts. <IMAGE> |
