Abstract: A data communication system is described in which individual pulses or groups of pulses relating to specific items of information are multiplexed together to produce a pulse stream and this pulse stream is continually repeated so as to reduce the significance of errors in transmission which would be corrected automatically in the next transmission of the pulses relating to the incorrectly transmitted data. The system is described with reference to a data processing system for recording details and charges relating to manually connected telephone calls. Two-way communication between data entry and utilization means and a station control unit is provided in which the data rate for communication from the control unit to the data entry and utilization means is four times higher than that for communication from the data entry and utilization means to the control unit.