Direct inward-outward dialing trunk circuit

A trunk circuit for interfacing a private branch exchange system having a plurality of extension subscribers and a central office line of a central office network, said trunk circuit comprising:

• a tip and ring terminals connected to the central office line and a power source;
• first and second transformers having a primary coil respectively connected to the tip and ring terminals and a secondary coil connected to the private branch exchange system for impedance matching, and for enabling formation of a speech path between a central office line subscriber and a corresponding extension subscriber of the private branch exchange system;
• tone signal detection means connected to the private branch exchange system, for detecting a dial tone generated in response to one of incoming and outgoing telephone call;
• direct inward-outward dialing detection means connected to the tip and ring terminals, for detecting a polarity of a voltage potential at the tip and ring terminals and the current flowing through the tip and ring terminals to determine reception of one of the incoming and outgoing telephone calls for enabling formation of the speech path between the central office line subscriber and the corresponding extension subscriber;
• a switch connected to the primary coil of the first transformer, for selectively enabling the current to flow between the tip and ring terminals at the central office line through said direct inward-outward dialing detection means via one of a low impedance loop and a high impedance loop, wherein the current passing through the low impedance loop from the tip terminal to the ring terminal at the central office line alerts one of the central office network and the private branch exchange system to form the speech path between the central office line subscriber and the corresponding extension subscriber in response to one of incoming and outgoing calls, and wherein the current passing through the high impedance loop from the ring terminal to the tip terminal at the central office line enables generation of a ringing tone to a calling subscriber from one of the central office line and the corresponding extension line;
• and
• a relay driver for controlling operation of said switch to select one of the low impedance loop and the high impedance loop for the current to flow through said direct inward-outward dialing detection means between the tip terminal and the ring terminal at the central office line.