This paper describes the interface among the elements of the PCB modem Vayrs:. The designed ASIC allows the PCB modem to perform an automatic backup process from a Leased Line to the Public-Switched Telephone Network. The PCB modem performs the automatic backup in an autonomous way, controled by a microprocessor. The designed controler is a totally configurable circuit that supports different communication protocols (HDLC, sunchronous character oriented protocol and asynchronous communication) and different operation modes (autobauding, local echo, ..). The greater part of the circuit has been designed by means of automatic synthesis tools, and has been made in a 1 μm CMOS process.