The MC145442B and MC145443B silicongate CMOS singlechip low speed modems contain a complete frequency shift keying (FSK) modulator, demodulator, and filter. These devices are compatible with CCITT V.21 (MC145442B) and Bell 103 (MC145443B) 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.