A method of data transmission over an internal bus of a workstation, effecting communication of an interface coupler and a telephone line and devices adapted for various communication functions. The method employs the synchronization signal of a voice network to define 125-microsecond communication frames. Included in each 125-microsecond frame thus defined are a maximum of eight variable-length subframes corresponding to a maximum of eight devices. The first two bytes of each subframe are header bytes. Coded in the header bytes of each subframe are a device destination address, and length codes indicating the number of bytes of data included in the subframe and the number of active bits in the last byte.