The MC145446A is a silicon gate CMOS frequency shift keying (FSK) modem intended for use with telemeter systems or remote control systems over the telephone network. It replaces the MC145446. This device is compatible with CCITT V.21 and contains the entire circuit that provides a fullduplex or halfduplex 300baud data communication over a pair of telephone lines. This device also includes the DTMF generator/receiver and call progress tone detector (CPTD). The differential line driver has the capability of driving 0 dBm into a 600 load with a single 5 V power supply. The transmit level is controlled by the programmable attenuator in 1 dB steps. This device also includes a serial control interface and internal control and status registers that permit a CPU to exercise the following builtin features: