The MC33121 is designed to provide the interface between the 4–wire side of a central office, or PBX, and the 2–wire 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 (4–wire and 2–wire).