Abstract: A data transmission system for interfacing a computer with a plurality of data sources which are in groups, each group at an individual remote location, comprising local interface apparatus for the computer and remote interface apparatus for each remote location, wherein individual sending and receiving means at the local apparatus are provided for connection by a two-wire channel to sending and receiving means at each remote apparatus, wherein the local apparatus includes a buffer for each channel to receive input data words one at a time in bit serial form its respective channel, means for polling each buffer in turn until a buffer having an input data word is located when polling stops, and means responsive to an input by the computer of an input data word from a buffer to clear that buffer, send an acknowledge word in bit serial form to the respective remote location and restart the polling.