The MC145442 and MC145443 silicongate CMOS singlechip lowspeed modems contain a complete frequency shift keying (FSK) modulator, demodulator, and filter. These devices are with CCITT V.21 (MC145442) and Bell 103 (MC145443) specifications. Both devices provide fullduplex or halfduplex 300baud data communication over a pair of telephone lines. They also include a carrier detect circuit for the demodulator section and a duplexer circuit for direct operation on a telephone line through a simple transformer.