The FX604 is a low power CMOS integrated circuit for the reception or transmission of asynchronous 1200bits/sec data in accordance with CCITT V.23 and ETSI specifications. It is also capable of generating the 75bits/sec 'back channel'. The device incorporates an optional Tx and Rx data retiming function that removes the need for a UART in the associated mC when operating at 1200bits/sec. The device can disable the back channel or be operated so only the mark or space tone is produced. An optional line equaliser is incorporated into the receive path, this is controlled by an external logic level. The FX604 may be used in a wide range of telephone telemetry systems.