The MC33121 is designed to provide the interface between the 4wire side of a central office, or PBX, and the 2wire subscriber line. Interface functions include battery feed, proper loop termination AC impedance, hookswitch detection, adjustable transmit, receive, and transhybrid gains, and single/ double fault indication. Additionally, the MC33121 provides a minimum of 58 dB of longitudinal balance (4wire and 2wire). The transmit and receive signals are referenced to analog ground, while digital signals are referenced to digital ground, easing the interface to codecs, filters, etc. The 2 status outputs (hookswitch and faults) and the Power Down Input are TTL/CMOS compatible. The Power Down Input permits local shutdown of the circuit.