Abstract: A line interface module and selected channel unit modules are replaced in a digital terminal which couples time multiplexed digital communications lines to a plurality of voice signal lines. The line interface unit is provided with means for responding to a controller to insert PCM data into selected time slots of the PCM signal on the digital line and to latch data received on the line. In addition to receiving pulse amplitude modulated signals from a receive unit in the terminal, voice channel units may also receive the PCM code. A state machine in the channel unit recognizes control sequences of insert data. By means of the insert data, the controller may direct provisioning of the channel unit with such adjustments as gain control and equalization. It may also direct the channel unit to switch to a loopback mode of operation for diagnostics. PCM corresponding to tones may also be inserted by the line interface unit to assist in diagnostics.