Patent ID: 7539299

Claim:
An office-line trunk circuit selectively operating on ground-start and loop-start modes, the trunk circuit comprising: an office bilateral trunk for an office exchange; a tip line and a ring line; and an office-line trunk for a PBX (private branch exchange), the office-line trunk being connected to the office bilateral trunk via the tip and ring lines and being used for both a ground start and a loop start selectively performed by the office bilateral trunk, wherein said office-line trunk comprises: a DC current detection circuit comprising a first photo coupler detecting a DC loop current when a ground is supplied to the tip line and a negative power to the ring line from the office bilateral trunk, and a resistor whose resistance is adjusted so as to prevent the loop start in the office bilateral trunk and allow DC current to be detected in the DC current detection circuit, the first photo coupler detecting the loop current using the resistor; an incoming detection circuit comprising a second photo coupler detecting a ring signal coming from the office bilateral trunk; and a DC sink circuit having comprising a diode bridge circuit connected between the tip and ring lines and configured to have a DC circuit provided with a transistor controlling the DC loop current so that the office bilateral trunk is allowed to be loop-activated and loop-held.