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Telephone line interface circuit with voltage switching
| Details |
Inventors: Rosenbaum, Stanley D.; Sutherland, Brian A. F. S.; Rosch, Reinhard W.;
Assignee: Northern Telecom Limited (Montreal, CA)
Primary Examiner: Dwyer; James L.
Assistant Examiner: Fournier; Paul A.
Attorney, Agent or Firm: Haley; R. John
A two-wire telephone line interface circuit comprises a driver circuit for supplying direct current on the line, a controlled voltage generator, a sensing circuit for monitoring the line current, a control circuit for controlling an output voltage of the voltage generator, and a switching circuit. The control circuit controls the switching circuit to supply, as a supply voltage for the driver circuit, a battery voltage in an on-hook state of the line or the controlled output voltage of the voltage generator in an off-hook state of the line, this output voltage being controlled to provide off-hook current limiting. The control circuit also controls the switching circuit to selectively supply the controlled output voltage of the voltage generator as a signalling voltage to at least one wire of the line for high voltage signalling, e.g. ringing, on the line. Desirable forms of the switching circuit are described. |
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DETAILED DESCRIPTION According to one aspect of this invention there is provided an interface circuit for a two-wire telephone line, comprising: driver means for supplying direct current on the line; a controlled voltage generator; control means for controlling an output voltage of the voltage generator; and switching means controlled by the control means for deriving a supply voltage for the driver means selectively from either the controlled output voltage of the voltage generator or a source voltage.
Thus the supply voltage for the driver means can be constituted by either the source voltage, which is typically a C.
O.
battery voltage, or the controlled output voltage of the voltage generator.
Power can be conserved in the former case, occurring most of the time while the telephone line is not in use (the on-hook state), by not activating the voltage generator, and in the latter case by controlling the output voltage of the voltage generator to limit off-hook current on the line.
Preferably the switching means is further controlled by the control means for selectively supplying the controlled output voltage of the voltage generator either as a signalling voltage to at least one wire of the line or as the supply voltage for the driver means.
This enables the voltage generator to be used for both off-hook current limiting and supplying high voltage signals, such as ringing signals, to the line.
The switching means conveniently comprises first to sixth switches respectively for selectively connecting the source voltage to a supply voltage line of the driver means, the controlled output voltage to the supply voltage line of the driver means, a first output of the driver means to a first wire of the line, a second output of the driver means to a second wire of the line, the controlled output voltage to the first wire of the line, and the controlled output voltage to the second wire of the line.
In preferred implementations, the third and fifth switches are constituted by first and second contacts of a first relay, the fourth and sixth switches are constituted by first and second contacts of a second relay, each relay has its second contact phased oppositely to the first contact and includes a third contact having the same phase as the first contact, and the third contacts of the two relays are connected in series with one another and together constitute the second switch
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Telecommunications 
