Abstract: Apparatus (10) for providing isolation and interconnection between the tip (1) and ring (2) conductors of a telephone line and data communications equipment to be connected thereto, such as a modem or other device (12). The apparatus serves also as a circuit for dialing the telephone, responsive to dialing signals applied thereto. All switching is done electronically, eliminating the need for mechanical relays. A diode bridge (18) is connected to the tip (1) and ring (2) conductors of the telephone line; the active circuitry of the interface derives power through the bridge. A switchable constant-current load (48, 20, 22) is placed across the output nodes of the bridge. The load is switched by an opto-isolator (40) which, in turn, is controlled by the line-seizing switch of the data communications equipment. When the line-seizing switch is closed (i.e., the device is off-hook), the opto-isolator (40) causes the switchable load to place a low impedance d.c.