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. Onchip 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. In many data collection and telemetry systems low power consumption is important. The CMX624 features a 'Zero Power' standby mode. Whilst in standby the ring detector continues to operate and will supply the host Controller with an interrupt when line reversal or ringing is detected. The CMX624 can operate on a supply voltage between 3.0V and 5.5V across the full temperature range of -40C to +85C. The CMX624 is pin compatible with the CMX644A V22 and Bell 212A modem also from CML.