The CMX644A V.22 modem is intended for use in any telephone based information and telemetry system with low power requirements. Using V.22 signalling, fast call set up times and robust error resistant transmission can be implemented by efficient low power circuits. The circuit can operate at 1200b/s full duplex over 2-or 4wire circuits. Control of the device is via a simple high speed serial bus and data may be optionally formatted by the on-chip UART. 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. In addition to V.22, support is included to meet the Bell 212A standard. The integrated DTMF encoder can be used as part of the dial out function. All 16 DTMF combinations are available along with a single tone `melody' mode. The answer tone generator/detector and call progress tone detectors included on the CMX644A make the setup of a telephone call a simple matter for the host Controller. In many data collection and telemetry systems low power consumption is important. The CMX644A features a `Zero Power' standby mode. Whilst in standby, the device can still detect a ringing voltage or line voltage reversal. The CMX644A can operate on a supply voltage between 3.0V and 5.5V across the full temperature range of -40C to +85C. A low impedance pull down output is provided for a hook relay. The CMX644A is pin compatible with the CMX624 V23/Bell 202 modem also from CML.