apparatus for facilitating the transport and switching of synchronous and asynchronous digital data signals presented by data terminal or data communication equipments to a voice switching network such as a pbx advantageously without the use of modems . the clock rate of the pbx user &# 39 ; s digital data is multiplied by a variable factor to define a synchronous transmission clock rate on a data link between a pair of interfaces , one locally connecting to the data terminal or data communications equipment and the other appearing as a port of the pbx switching equipment . the higher rate on the data link permits both the digital data as well as a control channel to be provided . variations in the data terminal or data communications equipment clock rate do not affect the control channel . variations in the voice sampling rate of switching modules serving different groups of ports through which the digital data is transported are accommodated .