The CMX624 V.23 / Bell 202 modem is intended for use in any telephone based information and telemetry system with low power requirements. Using FSK signalling, fast call set-up times and robust error resistant transmission can be implemented by efficient low power circuits. The circuit can operate at 1200bps full duplex over a 4-wire circuit or 1200 bps plus low speed data over a 2-wire circuit. Flexible line driver and receive hybrid circuitry are integrated on chip requiring only passive external components to build a 2- or 4wire interface. A low impedance pull down output is provided for a hook relay. Control of the device is via a simple high speed serial bus; this allows easy interfacing to a host Controller. The data transmitted and received by the modem is also transferred over the same high speed serial bus. On-chip programmable Tx and Rx UARTs allow asynchronous data to be simultaneously encoded and decoded. Either UART may be disabled to allow 8-bit raw data to be received or transmitted. Any repetitive 8-bit data pattern can be sent without the controller having to reload data every 8 bits. All 16 DTMF combinations are available along with a single tone 'melody' mode. The ringing, 2100Hz, call progress and data detectors included on the CMX624 make the set-up of a telephone call a simple matter for the host Controller.