An ISDN interface unit interconnects a data processing unit, an ISDN, and an analog telephone and includes a DMA request signal generation circuit for generating a DMA request signal from an 8 kHz clock signal from an ISDN line. In accordance with the request signal, PCM coded data input from the ISDN line is transferred, using DMA, to a memory of the data processing unit or vice versa, or the ISDN interface unit includes a code conversion circuit, data stored in the data processing unit are converted to nonlinear PCM coded data and transmitted to the ISDN line or vice versa. Further, the ISDN interface unit includes communication functions for the digital signal and for the voice band signal, a control channel signal sent from the ISDN line is interpreted, and one of the communication functions can be selected.