Abstract: Circuitry for automatically and dynamically reconfiguring the channel or port configuration of a modem handling a plurality of ports. A standard signal such as DTR (data terminal ready) is used to provide a code to a master modem indicating the particular port configuration requested by cooperating apparatus. The requested configuration code is continuously compared to a code stored by a shift register controlling the actual modem port configuration. When the requested code changes, an indication of the code change is transmitted to a communicating slave modem and a control signal is sent, after a suitable delay, to the shift register to conform its contents to the newly requested configuration code. During the delay, the slave modem switches to the newly requested configuration. The circuitry follows a defined sequence of configuration switching, dropping channels successively and adding channels by passing through the state where all channels are active.