Direct access arrangement
The present invention is directed towards a low cost Data Access Arrangement (DAA) with improved current control for controlling the current drawn from a telephone line pair to drive the light emitting diodes (LEDs) in an opto-isolator utilized with the DAA, and to an enhanced method of sinking DC line current. Also, an improved method is provided to implement line current draw and battery detect features.
1 . A Direct Access Arrangement (DAA) for co-operating with a Central Office (CO) line, said DAA comprising:
a voltage tap coupled to said CO line,
a current source responsive to said voltage tap for generating a current proportional to the voltage on said CO line by a factor of 1/Z L , where Z L represents the impedance of said CO line, and
a resistor coupled to said CO line for sinking said AC current such that said resistor presents said CO line with an input impedance substantially equal to Z L .
2 . The DAA of claim 1 wherein said voltage tap includes a resistor network and said current source includes an operational amplifier and BJT configured as a voltage to current converter.
3 . A Direct Access Arrangement (DAA) for co-operating with a Central Office (CO) line carrying signals each having a DC component and a feedback AC component, said DAA comprising:
an optocoupler powered by said signals, and
a cancellation circuit having a monitor for generating an AC voltage responsive to said AC component, and a current source responsive to said AC voltage for generating a counter AC component equal and opposite to said AC component, said current source being coupled to said CO line such that said counter AC component nullifies power distortion resulting from said AC component.
4 . The DAA of claim 3 wherein said monitor further comprises a voltage tap including a resistor and said optocoupler, and wherein said current source includes an operational amplifier and BJT.
5 . A Direct Access Arrangement (DAA) for co-operating with a Central Office (CO) line carrying signals each having a DC component and an AC component, said DAA comprising:
an optocoupler powered by said signals,
a shunt regulator coupled to said CO line for sinking a portion of said DC component and a portion of said AC component,
a voltage tap coupled to said shunt regulator for generating an AC voltage responsive to said portion of said AC component, and
a voltage controlled current source, responsive to said AC voltage and coupled to said CO line, for generating a further signal for canceling said portion of said AC component.
6 . The DAA of claim 5 wherein said shunt regulator includes a resistor and a zener diode, and wherein said current source includes an operational amplifier and BJT.
7 . A Direct Access Arrangement (DAA) for co-operating with a Central Office (CO) line carrying signals each having a DC component and an AC component, said DAA comprising:
an optocoupler employing said signals for power and responsive to said AC component for signaling, and
a line availability detector, coupled to said optocoupler, said detector generating an indication responsive to the DC component of said signals.
8 . The DAA of claim 1 wherein said DAA additionally comprises a transmitter, and a receiver, said transmitter and receiver having a common transceiving node, and wherein said transmitter and receiver are coupled by a compensating circuit for canceling transmission feedback.