A bidirectional serial TDM backplanenullUTOPIA interface contains an ATM cell boundary location and transmit flow control mechanism, to provide for the efficient capture and storage of ATM cells from a serial TDM channel. Once stored in a transmit buffer, individual ATM cells are controllably read out for application to a downstream UTOPIA interface. In the upstream direction from the UTOPIA bus toward the serial TDM backplane, ATM cells are stored in a multi-cell receive buffer, so that they may be serialized for application to the TDM backplane. In the absence of ATM data cells to transfer, unfilled timeslots are filled with idle cells to maintain the ATM bus active.